Part 2 Geotechnical Evaluations

5.0  PURPOSE AND SCOPE

5.1  Purpose of Geotechnical Evaluations

Part 2 of this report presents a geotechnical engineering evaluation of the landslides that have occurred throughout the City of Seattle (City).  In Part 3, engineering evaluations will be specifically related to three study areas:  1) West Seattle, 2) Magnolia/Queen Anne, and 3) Madrona.  In Part 4, engineering evaluations will be related to additional study areas:  Northwest Seattle, Northeast Seattle, Capitol Hill, and South Seattle.  Based on these citywide and study area evaluations, typical measures will be presented to improve stability and reduce the risk of future landslides.  In addition to preventive measures, remedial schemes will also be presented for landslides after they occur.  Most of the stability improvements presented can be both preventive and remedial.

The purpose for our studies and recommendations regarding stability improvement is to inform both the public and representatives of the City of the factors that cause landslides and the steps that could be taken to improve stability.  It is important for the City to protect utilities, drainage features, streets, and other City facilities; however, landslides do not obey property boundaries.  Therefore, measures will be presented that could be made by the City and/or adjacent property owners to improve the stability of an entire landslide or unstable slope.

5.2  Scope of Geotechnical Evaluations

In order to meet the purpose described above, the following engineering evaluations have been made:

     We studied the landslide history described previously in this report (Part 1), with respect to topography, geologic and groundwater conditions, slide types, timing, City locations, and causes.  This study has provided background for making engineering evaluations.
     For each type of landslide or potential landslide, we developed stability improvement measures consisting of surface and subsurface drainage, grading, and/or structures.  Typical measures that could be applied citywide are presented in this report section (Part 2).
     We developed unit cost estimates for the typical measures applicable to citywide stability improvements (Part 2).  These cost estimates can be extrapolated to provide budget figures for the stability improvements recommended in Parts 3 and 4 of this report. 
     We studied the general implications of City utilities and streets as related to instability (Part 2).
     For the original three selected study areas (West Seattle, Magnolia/Queen Anne, and Madrona), we conducted detailed studies of the types and causes of landslides, including the effects of City utilities and streets, and we provide recommendations and cost estimate information for stability improvements.  The results of these studies are presented in Part 3 of this report.
     For the four additional study areas (Northwest Seattle, Northeast Seattle, Capitol Hill, and South Seattle), we conducted studies generally similar to those accomplished for Part 3, and we provide recommendations and cost estimate information for stability improvements.  The results are presented in Part 4 of this report.

6.0  TYPICAL IMPROVEMENTS RELATED TO LANDSLIDE TYPE

As presented in Part 1 of this report, most of the landslides were found to fit into four generalized types:  1) High Bluff Peeloff, 2) Groundwater Blowout, 3) Deep-Seated, and 4) Shallow Colluvial (includes landslides that involve fill material).  There are various combinations of these generalized landslide types, as one type of mechanism may lead to another.  This section describes approaches for repairing slopes with these types of landslides, improving the stability of slopes that could be affected by landslides, and reducing the hazard from debris flows to properties below landslides.

6.1  Geologic Conditions that Contribute to Landsliding and Instability

Part 1 of this report provides a detailed description of the geologic and hydrologic conditions that contribute to landsliding in the City.  In general, the following factors affect the stability of a slope: topography, subsurface conditions, surface and groundwater conditions, and external loads, such as structures. 

Factors that commonly trigger landslides include:

        Increased groundwater levels and surface runoff
        Removing support at the toe of the slope by erosion or by excavation
       Changes in the soil strength
        Loading the head of the slope with debris from another landslide or with manmade fills
        Seismic loading

Groundwater contributes to landslides in several ways.  When saturated, a potential landslide block has more weight because of the water, which results in a larger driving force.  Groundwater moving through the soil exerts seepage forces that further reduce stability.  Finally, the presence of groundwater reduces the strength of the soil on a potential slide plane.  Freezing weather can be an important process in reducing slope stability because frozen soil can impede groundwater seepage.  When seepage is impeded at the surface, groundwater levels can build to cause an unstable condition.

Surface storm water runoff can reduce slope stability by infiltrating into the near-surface soils at critical locations, and by causing erosion.  Where groundwater emerges at the surface, resulting in a spring or seep, the runoff can cause surficial erosion that can undermine and/or oversteepen a slope.  Undermining and/or oversteepening and the consequent loss of support at the toe of the slope can trigger a landslide.

Prior to construction of seawalls along Puget Sound, the base of the bluffs and slopes were subject to continual shoreline erosion and oversteepening at the toe of the slope.  Once undercut, the lower part of the slope would slide, thereby undercutting the slope at higher elevations.  With the construction of seawalls and other shoreline protection measures, erosion has been arrested or greatly reduced.  However, these slopes have not necessarily achieved a stable configuration, so landsliding may continue for the foreseeable future.

Development activities can result in undercutting and oversteepening slopes.  This was more prevalent prior to modern building codes, such as the Department of Design Construction and Land Use (DCLU) Director's Rules 3-93, 3-94, and 3-97, regarding development in geologic hazard areas.  Therefore, many oversteepened and improperly sloped or retained older cuts and excavations remain in the City, some of which contribute to instability.  In general, modern cuts and excavations made under the guidance of a competent geotechnical engineer have achieved a suitable degree of stability.

Seasonal variations in moisture and temperature, combined with plant growth and decay, animal burrowing, and soil creep tend to reduce the strength of soil over time.  This process particularly affects colluvial soil and glacially overridden soils that are exposed at the surface by an excavation or by a landslide.

Loads placed on or near the top of a marginally stable slope typically reduce the slope stability.  These loads can be caused by debris from a landslide that occurred upslope, manmade fills or loads from structures.  Modern fills that are designed and constructed under the direction of a geotechnical engineer generally are suitably stable.  However, many older fills were built without proper subgrade preparation, adequate drainage or compaction.  These fills include material that was loose-dumped without compaction in ravines and on slopes and loose sidecast road fills.  A number of recent and older landslides in Seattle involve old uncontrolled fill material.  Structures, heavy equipment, and material stockpiles that are built or placed on a steep slope or near the top of a steep slope can contribute to instability.

Many of the steep bluffs and slopes are susceptible to earthquake-triggered landslides.  Both the 1949 Olympia and 1965 Seattle-Tacoma earthquakes caused a total of at least 41 landslides in the Puget Sound region.  Recent geological and seismological research findings indicate that many of the large ancient landslides identified in the bluffs along Puget Sound were triggered by large prehistoric earthquakes.

6.2  Typical Approaches to Improve Stability

For each type of landslide, we evaluated potential stability improvements, which could be preventive and/or remedial.  In general, the methods for achieving suitable stability for a site or project include:  1) avoiding the slope and 2) improving stability by reducing the forces that cause movement, increasing the forces that resist movement, or a combination of the two.  These methods for improvement measures fall into several generalized categories, as presented in the following table.

In some cases, it may be difficult or not practical to improve the stability of a landslide; but structures, streets, and/or utilities located below the landslide could be damaged by slide debris.  These circumstances could occur where a high bluff is present or where a potential landslide is on another property.  In such cases, the areas below the landslide can be protected from slide debris with catchment or diversion structures designed for impact forces.

The following sections describe how these improvement measures or combination of measures could be applied to the four generalized landslide types described previously.  For each landslide type, we present several sketches that diagrammatically show typical applications of different measures to improve stability.  They are not intended to show all types of stability improvements, nor design details for improving stability of a slope or a landslide.  Subsequent sections provide details regarding the common improvement measures.  The details include a description of how each measure improves slope stability and general design requirements and details.

6.3  High Bluff Peeloff Landslides

The main factors that lead to high bluff peeloff landslides are nearly vertical slopes, groundwater seepage, and surface water runoff.  In many cases, little can be done to prevent these landslides because of their height, steepness, and inaccessibility.  The nearly vertical bluffs typically were formed by coastal erosion.  Although this erosion may have been arrested or slowed by recent shoreline protection, the slopes have not achieved a stable slope through erosion and landsliding.

Figure 2-1 (sheet 1   sheet2   sheet 3) shows simplified sketches of a high bluff peeloff landslide, together with several alternatives for reducing the likelihood of a high bluff peel off landslide, and protecting a structure, street, and/or utility below the bluff, as shown.  Unless the bluff is low or otherwise accessible, remedial measures to reduce the likelihood of a landslide typically are limited to surface and groundwater improvements at the top of the bluff, as shown.  Where structures, streets, and/or utilities are located below a bluff and in the likely landslide runout zone, measures can be taken to reduce damage from the landslide debris.  These include building sufficiently far from the bluff that landslide debris should not affect the structure, building catchment, or diversion structures, and removing trees that likely would be incorporated into the landslide debris.

In cases where the bluff is accessible and/or where the consequences of a landslide are high, the slope could be retained with a wall.  In general, this type of repair is costly and may not be economical or practical to build.  Suitable wall types depend on the height of the bluff, and the topography and geologic conditions of the slope below the bluff.  These wall types could include soil nails with reinforced shotcrete, as shown on Figure 2-1, Sheet 2 of 3, and soldier pile walls with lagging if the slope is low (Sheet 3 of 3).  For a soil nail wall, reinforcing elements would be required into the bluff face.  For a soldier pile wall with lagging, the soldier piles could be tied back or cantilevered.  Installation of these soil nails or anchors would be expensive and access to the bluff would pose safety concerns for the workers.

We recommend removing hazard trees, other large vegetation, or structures that likely would be incorporated into a high bluff peeloff landslide.  Such objects incorporated into landslide debris have damaged structures located below bluffs.  Trees that are isolated or subjected to high winds that accelerate over bluffs are more likely to be uprooted.  An uprooted tree can initiate a high bluff peeloff landslide.

6.4  Groundwater Blowout Landslides

Groundwater blowout landslides occur where a relatively permeable soil overlies a less permeable soil, resulting in perched groundwater and seepage towards the slope face.  The high groundwater levels and seepage towards the slope face result in destabilizing seepage pressure and reduced soil strength.  Seeps and springs that form where groundwater exits the slope face often cause erosion that can undermine and oversteepen a slope further reducing the stability.

Figure 2-2 (sheet 1   sheet 2) shows four simplified sketches of a groundwater blowout landslide, together with several alternatives for reducing the likelihood of a landslide.  Because the primary driving force is groundwater seepage, suitable remedial measures usually include drainage to lower the groundwater level and to control seepage at the slope face.  Drainage measures usually are most effective when they intercept groundwater at the contact between the relatively permeable soil and the underlying less permeable soil.

Sketch A on Figure 2-2 (Sheet 1) shows the application of an interceptor trench subdrain and a springhead drain.  Both improve stability by lowering the groundwater level in a landslide or potentially unstable slope, thereby reducing the driving forces and increasing the soil strength.  The springhead drain is used to collect water that emerges from the slope in a concentrated area, thereby reducing erosion potential and improving stability.  Trench subdrains generally are applicable to slopes where the contact with the underlying low permeability material is relatively shallow.  An interceptor trench subdrain is installed across the slope to intercept the groundwater before it reaches the slope face.  Sketch B shows another type of trench subdrain, called a finger drain.  It is similar in construction to an interceptor trench subdrain, except that it is installed along the slope fall line (perpendicular to slope contours).

Sketch C on Figure 2-2 (Sheet 2) shows two alternatives for drilled drains:  horizontal drains and directionally drilled drains.  Drilled drains are typically used to improve stability of slopes and landslides where the groundwater cannot be intercepted with trench subdrains, or where it is not practical to excavate trench subdrains.  Drilled drains are commonly used to improve the stability of large deep-seated landslides.  Horizontal drains are drilled from the slope face, which limits their application to sites that have suitable access near the toe of the landslide mass.  Directionally drilled drains usually are installed from the top of the slope and can be aimed to intercept a specific zone where the drainage is needed.  Vertical wells (not shown) can be used in special cases; however, their suitable application is limited.  Vertical wells require continual pumping to maintain lower groundwater levels.  As such, they incur the cost of electricity and are subject to power outages during critical rainy periods.

A replacement earth buttress is sometimes used to improve a marginally stable slope and more commonly to repair a landslide that has already occurred.  As shown by Sketch D, the landslide mass or potentially unstable soil is removed and replaced with a well drained fill material.  In some cases, the excavated soil can be recompacted to form the earth buttress, while in others a suitable imported backfill is compacted to form the earth buttress.  In either case, an effective drainage layer and subdrain should be constructed under the earth buttress.

6.5  Deep-Seated Landslides

As described in Section 4.1.3, deep-seated landslides can occur in a variety of geologic settings.  Most deep-seated landslides consist of a relatively large block of soil that may remain partially intact as it slides downhill on an arc- or wedge-shaped failure surface.  The size of a deep-seated landslide can vary from a single backyard to a city block or more.  Groundwater usually contributes to deep-seated landslides, although the source of the groundwater may not be clearly related to a contact between relatively high and low permeability soils.  Because of the varied geologic and hydrologic conditions that contribute to deep-seated landslides, the alternatives for repairs are equally varied.

Figure 2-3 (sheet 1   sheet 2   sheet 3) shows five simplified sketches of a deep-seated landslide, together with several alternatives for improving the stability.  Depending on the soil types and groundwater levels, the various schemes for dewatering that are shown on Figure 2-2 for the groundwater blowout landslide may be applicable.  If the landslide failure surface is too deep to drain with trench subdrains, horizontal or directionally drilled drains could be used.

Deep-seated landslides often can be repaired by adding weight to the toe of the landslide and/or removing material from the upper, driving part of the landslide.  Sketches A through F show various schemes for improving the stability of a deep-seated landslide or potential landslide.

Sketch A on Figure 2-3 (Sheet 1) shows an earth buttress constructed at the toe of an existing or potential landslide to add weight to the lower, resisting part of the landslide.  In general, earth buttress, counterweights and toe berms should include a drainage layer beneath the main fill to reduce the potential for destabilizing groundwater conditions.  Sketch B shows a similar situation, except that the toe of the fill is retained to accommodate a street, property line, or other construction-access limitations.  The wall at the toe of the retained earth buttress fill could be a reinforced soil wall, as shown on the Sketch, or another type of wall.  Both in situ and gravity wall types, as defined in the table with Section 6.2, could be suitable, with the choice of wall type depending on the geometry of the earth buttress fill.

Sketch C on Figure 2-3 (Sheet 2) shows an example where soil is removed from the upper, driving part of the landslide and removed from the site.  The slope behind the landslide or marginally stable slope should be graded so it will be stable.  Alternatively, a retaining wall could be used to reduce the loss of level ground above the landslide.  In this instance, a soldier pile or other in situ wall probably would be most effective, although other types could be used if a suitable foundation can be provided.  Sketch D shows an example where the stability is improved by a combination of removing soil from the upper driving part of the landslide and adding an earth buttress fill to the lower resisting part of the landslide.  For the example shown, the excavation and fill quantities are approximately equal, so that most material would be derived and disposed of on site.  Retaining walls could be used at the top or at the toe of the landslide to keep within rights-of-way or natural grade limitations.

Another alternative that reduces the driving weight is shown on Sketch C.  After removing soil from the upper driving portion of a deep-seated landslide, lightweight fill material could be placed to restore grades.  Wood chips, cinders, and expanded polystyrene (geofoam) are frequently used in this type of application.

Sketch A on Figure 2-3 (Sheet 1) also shows the use of an in situ wall to retain soil in the upper driving part of a marginally stable slope or landslide that has had incipient failure or small displacements.  The piles also provide reinforcement across the landslide failure surface.  In situ walls include soldier pile walls with lagging, secant piles and tangent piles (details given in Section 7.0).  This type of repair is appropriate when other less expensive alternatives, such as drainage and regrading are not practical because of site limitations.  Sketch E on Figure 2-3 (Sheet 3) shows an example where an in situ wall is used to retain the slope above the scarp after a landslide occurs.  In this example, the wall retains the ground above the landslide and prevents progressive upslope failure.  However, the ground below the scarp is not restored nor is the stability improved.  Sketch F shows an example where a drained earth buttress is built at the bottom of the slope to improve stability and a slope fill is built to restore the grades.  A reinforced soil wall or other type of gravity wall could be used to steepen the toe of the drained earth buttress if needed for property or site access limitations.

6.6  Shallow Colluvial Landslides

As described previously, colluvium is present on most slopes in Seattle.  Because of the typically shallow depth, colluvium is particularly susceptible to rapid saturation from infiltration of surface runoff, direct infiltration of precipitation, groundwater seepage, discharge from pipes, or a combination of these sources of water.  Shallow colluvial landslides are the most common type of landslide in Seattle.

Figure 2-4 (sheet 1   sheet 2) shows simplified sketches of a shallow colluvial landslide, together with several alternatives for reducing the likelihood of a landslide.  They also show alternatives for protecting a structure, street, and/or utility below the slope, if it is not practical to stabilize the slope.

Sketch A Figure 2-4 (Sheet 1) shows a combination of surface drainage, subsurface drainage, removing hazards from the site, and a catchment wall.  Depending on the circumstances, these measures could be used individually or in combination.  As discussed previously, colluvium is particularly sensitive to rapid saturation from both surface water runoff and groundwater seepage.  Therefore, all storm water runoff from roof drains, paved surfaces, foundation drains, etc., should be collected into a tightline and discharged to a suitable location.  Ideally, storm drainage collected from the top of the slope should discharge to a storm sewer located at the top and back from the edge of the slope.  If this is not possible, a tightline should convey this storm water runoff to the bottom of the slope or to a storm sewer located downslope.  It is usually advisable that the tightline not be buried, to prevent breakage from soil creep or landslide movement.

Trench subdrains are often effective for providing subsurface drainage in colluvium.  The depth to the underlying glacial soil is usually within the reach of a backhoe or even hand-excavated trenches.  Both interceptor trench subdrains, as shown on Sketch A, and finger drains, as shown on Sketch B of Figure 2-2 (Sheet 1), can be effective.  Drilled drains usually are not practical, except where site access precludes construction other than directionally drilled drains.

In some cases, it is not practical or possible to improve the stability of the slope.  A catchment wall, as shown on Sketch A of Figure 2-4 (Sheet 1), can protect structures, streets, and/or utilities below the landslide.  As stated previously, we recommend removing trees, other large vegetation, or structures that likely would be incorporated in a landslide.  Trees and other large debris incorporated in a debris flow can cause as much or more damage than the moving soil.  It is also possible to use the catchment wall as an in situ wall designed to retain potential sliding soil.

Sketch B shows a retaining wall to provide support for a marginally stable slope, or to allow an excavation at the toe of a slope.  The type of wall will depend on the site conditions and access limitations.  An in situ wall type, as shown on the sketch, often is needed to construct the wall within property lines.  In addition, such walls can be built before making the excavation, so the slope is continually supported.  Other improvements that could be made include an earth buttress and subsurface drainage.

Sketches C and D, on Figure 2-4 (Sheet 2), show two schemes for repairing a shallow colluvial landslide.  Sketch C shows a case where site access limitations prevent extensive work on the slope above a bench where a structure, street, and/or utility is located.  If springs or seeps are present on the slope, springhead drains could be installed to promote good drainage.  Finger drains could be installed in the remaining colluvium at the base of the slope, as shown on Sketch B of Figure 2-2 (Sheet 1).

Sketch D shows a repair where the slope is restored by removing the landslide debris and unstable colluvium, and placing a well-drained structural fill or reinforced soil slope.  Depending on the required slope, desired use, and property limitations, a gravity wall or other type of wall could be constructed at the toe of the new fill.

Landslides involving fill material typically are similar to shallow colluvial landslides.  Therefore, the repairs are also similar.  Figure 2-5 shows two sketches of stability improvements for landslides involving fill material that was placed at the top of a slope, or of a sidecast road fill.  Both sketches show one or more walls to restore a level area damaged by a landslide.  The sketches show that a gravity wall (e.g., reinforced soil, concrete cantilever, crib, etc.) or an in situ wall (e.g., soldier pile) could be effective.  The number, type, and location(s) of the walls would depend on the slope geometry, the final grades desired, site access, and other site limitations.  The fill material and underlying loose colluvium could also be replaced with a drained earth buttress, as shown on Sketch D, Figure 2-2 (Sheet 2), or a reinforced soil slope.

7.0  DETAILS REGARDING IMPROVEMENTS

Many different engineered systems are currently used to mitigate landslides in the Seattle area.  Sometimes a single system is enough to provide the necessary level of slope improvement or property protection.  Often a combination of several mitigation systems is required to adequately increase the stability of a landslide or a marginally stable slope.  Site accessibility and the mitigation scheme required to improve stability are the primary factors that govern the total cost of a slope stability improvement project.  Sections 6.1 and 6.2 describe the geologic conditions that contribute to instability and typical approaches to improving stability.  Sections 6.3 through 6.6 show typical applications of these typical approaches to improving stability for each of the four landslide types.  The following subsections discuss details regarding mitigation measures that are commonly used in the Seattle area.  These include typical details for the following types of improvement measures:

Note that final design details are not provided in this report.  Final details and the selection of the appropriate improvement measure or measures should be developed by a geotechnical engineer experienced in landslide repairs and based on site-specific explorations and engineering evaluations.

7.1  Surface Water Improvements

As described in Section 6.1, surface water runoff can contribute to landsliding by causing surficial erosion and/or rapid saturation of the ground.  Surface water improvements generally are the least costly measures that can be implemented to reduce landslide potential or mitigate existing instability.  These improvements can be effective where storm water runoff, including water from streets, other paved areas, and roofs flows onto or near steep slopes and potential landslide areas.  In most cases, surface water improvements consist of capturing storm water runoff and redirecting it away from sensitive slope areas.  Storm water can be captured in appropriately located ditches, swales, roof drains, curbs, and catch basins.  Once collected, the runoff should be conveyed in a tightline (an unperforated pipe) to a suitable discharge location.  A suitable discharge location includes a storm sewer with adequate capacity or the bottom of a slope.  Where storm water runoff is discharged to the ground surface at the bottom of a slope, appropriate erosion control measures should be placed at the discharge point.

Runoff from roofs should be collected by gutters and conveyed with downspouts to a catch basin or other structure that permits periodic cleaning.  From the catch basin, the runoff should be conveyed in a tightline to a suitable discharge location.  The downspouts at some homes discharge into footing subdrain pipes.  We strongly recommend against this practice.  It introduces surface water rapidly into the ground, which can trigger landslides and can cause foundation settlement.  In addition, poorly drained foundations are often the cause of wet basements.

Where surface water runoff occurs toward a potential landslide slope, we recommend constructing a paved or lined swale near the slope top to intercept runoff.  The adjacent ground should be graded to drain into the paved swale.  The swale should be sufficiently large to convey the design storm with some blockage from leaves, ice, and other debris.  Water in the swale should be collected at a catch basin and then conveyed to a suitable discharge location.

Surface water management on roads and other City-owned pavement surfaces located on or near the top of the slope is discussed in detail in Section 9.  In general, the concepts and need for controlling surface water runoff are the same for any property; however, City property does have some special implications because of streets that create large areas of low permeability surface that can generate considerable storm water runoff.

7.1.1   Tightlines

Tightline systems are an integral part of a surface water system.  As such, it is essential that all tightline pipe systems are properly designed and durable.  The primary functional design requirements include the inlet, pipe capacity (pipe size and slope), and outlet.  An important design factor in landslide areas is that the tightline might be subjected to landslide ground motion.  The location and type of inlet will depend on the storm water collection system.  However, it should include some provision for preventing debris from entering the tightline and for periodic cleaning.  For example, a catch basin allows large debris to settle before the water enters the tightline and provides for periodic cleaning. 

The pipe size is a function of the anticipated runoff discharge and the pipe slope.  All pipes should be continuously graded to prevent settlement accumulation in the pipe that could eventually block the pipe or reduce its capacity.

Tightlines can consist of a variety of materials including different types of plastic and metal pipe.  Each one should be selected based on the particular project requirements.  Tightlines that extend across steep terrain and unstable slopes should consist of durable plastic pipe, such as high-density polyethylene (HDPE).  The joints should be durable and able to carry axial loads and accommodate flexural deformation of the pipe.  Welded or through-bolted, flanged joints are examples of suitable joints.  The pipe should be installed on the surface with an anchor system to prevent the pipe from being pulled apart by soil creep or by a landslide and to allow regular inspection.  Figure 2-6 (sheet 1   sheet 2) shows examples of tightline anchoring systems. 

Tightlines may consist of less expensive, jointed, flexible, corrugated plastic pipe in less critical stability areas, and depending on other project requirements.  In addition, tightlines may be buried in stable areas. 

As described previously, all tightlines should discharge to a suitable location.  They should not be allowed to discharge at the top of a slope, directly into or onto a slope, or onto a mid-slope bench.

7.1.2   Surface Water Systems - Maintenance

All surface water systems should be regularly checked and maintained.  Ideally, the City and residents on or near slopes should collectively implement and maintain the drainage features described above.  We recommend designing maintenance programs in landslide prone-areas that use a partnership between the City and residents.  The City could set up a regular maintenance program to:

    Clean catch basins and storm water runoff systems on a regular basis.  Initially, the City could establish a frequent maintenance schedule in landslide hazard areas based on previous experience and current schedules.  However, a record-keeping system should be implemented to identify an appropriate schedule for specific locations, and typical storm events that could block catch basins.
    Inspect streets in landslide-prone areas for significant cracking and surface wear.  Repair significant cracks, as appropriate, where found to be needed to reduce storm water infiltration.
    Provide residents in landslide prone areas with information on measures they should implement to reduce surface water runoff and infiltration.  These measures should include the issuance of publications such as the Washington Department of Ecology "Surface Water and Groundwater on Coastal Bluffs" (1995), and free inspection programs that might be similar in organization and implementation to the energy audits provided by Seattle City Light.
    In many cases, residents on steep properties cannot discharge storm water runoff into a storm sewer, either because there is no nearby storm sewer or because the grades are inappropriate.  Depending on the property ownership, installing a tightline to the bottom of the slope may be an alternative.  The City could work with these residents and their neighbors to identify storm water disposal alternatives.  These might include installing tightlines that cross more than one property (including City property) to convey runoff to the bottom of the slope or to another suitable discharge point below the property.
    Form "Landslide Block Watch Groups," in which groups of residents would regularly inspect City storm water catch basins for debris.  This group could perform some surficial cleaning when necessary and could alert the City when additional work is needed.  The groups could provide annual or more frequent reminders to their neighborhood when regular maintenance, such as cleaning gutters and drains, should be performed.

Landslide Block Watch Groups could also assist the City with disseminating information on reducing residential surface water runoff and infiltration.  This could include providing a liaison with City personnel, meeting with new homeowners, and identifying and helping to resolve neighborhood storm water runoff problems. 

7.2   Groundwater Improvements

Intercepting groundwater upslope and from within the slope can reduce landslide potential and improve stability of existing landslides.  Groundwater improvements can be effective on many slopes, and when used appropriately, they are often the most cost-effective approach.  The primary goal is to remove groundwater in areas where groundwater reduces stability by adding weight to potentially unstable soils, causes seepage forces, and reduces the soil strength.  However, capturing water flowing within the ground requires some different methods compared to surface water improvements.  Common groundwater improvement methods include:

        Interceptor trench subdrains and finger drains
        Springhead drains
       Drainage blankets
        Drilled drains

All groundwater improvement schemes should be designed based on the site-specific subsurface conditions.  To perform effectively, the system must lower the groundwater level near the landslide failure surface, which requires an understanding of the soil and groundwater conditions that cause instability at the location.  The following sections provide a description of common groundwater improvements with some typical design details and requirements.

7.2.1   Interceptor Trench Subdrains and Finger Drains

Trench subdrains are relatively narrow trenches that contain a drainage pipe and permeable backfill.  Figure 2-7 (sheet 1,  sheet 2) shows typical trench subdrain cross-sections.  Groundwater preferentially flows into the permeable trench backfill and then into the drainage pipe at the bottom of the trench.  From there, it is conveyed into a tightline that discharges the groundwater to a suitable discharge location.  Trench subdrains are most effective when they penetrate at least 1foot below the contact between the layer being drained and an underlying clay, silt, or less permeable layer.  This contact is commonly also at or close to the slide plane.  Two basic types of trench subdrains include interceptor trench subdrains and finger drains.  An interceptor trench subdrain is usually oriented across the slope (parallel to the contours) to intercept groundwater as it flows downslope.  Finger drains are frequently used to lower the water level within an active landslide mass by extending a trench subdrain from the toe of the slope up into the landslide debris.  Other than their orientation (perpendicular to the contours), finger drains are constructed in the same manner as trench subdrains.

Trench Excavation

Most trench subdrains are excavated using a backhoe or a track-mounted excavator.  Therefore, the practical depth for most trench subdrains is about 15 feet or less.  Track-mounted excavators are available that can excavate 20 feet deep or more.  However, deep trenches are often difficult and expensive to excavate because of the shoring required to maintain stable trench sideslopes.  Where groundwater is shallow and site access is limited, hand dug trench subdrains may be practical.

The depth of the trench is generally determined by the maximum practicable depth of the excavating equipment, site conditions, shoring requirements, and other project limitations.  As mentioned previously, trench subdrains are most effective when they penetrate though the layer being drained and at least 1 foot into an underlying less permeable soil. 

Excavating open trenches in marginally stable soil is often difficult because of groundwater infiltration and the tendency for the trench sidewalls to collapse.  Where practical, we recommend beginning the excavation at the outfall and proceeding upslope to allow water to drain away from the advancing trench excavation.  It may be necessary to periodically stop work for a day or more to let the site drain before advancing the trench.

Drainage Pipe

The drainage pipe in an interceptor trench subdrain typically consists of a 6-inch (minimum) diameter slotted or perforated plastic pipe.  The slots or perforations in the drainage pipe allow water to enter the pipe.  However, the drainage pipe only conveys water when the groundwater level rises higher than the pipe invert.  When lower water levels are present in portions of a trench subdrain system, the water flows through the surrounding permeable trench backfill.  Therefore, the pipe may not need to be placed at precise grades.  The pipe should be graded to drain continuously with no sags or depressions where water could infiltrate into the subgrade.

The drainage pipe at the bottom of the trench may consist of rigid or flexible and perforated or slotted pipe.  Each type has its advantages and disad4vantages.  The rigid pipe is more durable and less susceptible to crushing during installation.  However, a worker must be present in the trench to fit the pieces of pipe together and to prepare the bedding gravel.  Having workers in a trench generally requires shoring, which results in a higher cost and a longer time for construction.  The other alternative, flexible plastic pipe, is easily crushed if workers are not careful or if it is not properly bedded.  The primary advantage of the flexible pipe is that it can be lowered down into a deep, unshored trench excavation without workers being in the trench.

The size of the perforations or slots should be compatible with the drainage backfill material around the pipe and the anticipated groundwater flow rates into the pipe.  The following paragraph describes filter requirements for perforations and slots in more detail.

Trench Backfill

The trench backfill material depends on the anticipated groundwater inflow and the grain size of the surrounding soil.  The backfill should be sufficiently permeable so that water easily flows from the surrounding soil into the trench subdrain.  However, the backfill should also act as a filter to prevent migration of the surrounding soil into the subdrain trench.  This migration process, known as piping, can eventually plug the subdrain pipe, the drainage backfill or both.  In some cases, extensive piping can cause extensive settlement around the trench subdrain from the ground lost by piping.  The backfill must also be compatible with the perforated or slotted drainage pipe.  If the openings are too small, water cannot enter the pipe fast enough.  However, if the openings are too large, the backfill will enter and plug the pipe.  To meet the requirements of adequate permeability and filter characteristics, the backfill material should be designed for each specific situation.  The following paragraphs describe examples of typical backfill materials that have been successfully used in Seattle soils.

In the example shown on Figure 2-7, Sheet 1 of 2, the perforated or slotted pipe at the bottom of the trench is bedded in washed pea gravel to provide a highly permeable material directly around the pipe.  The pea gravel should be underlain by a geotextile where it rests on silt or clay soil.  The remaining backfill can be less permeable, because the groundwater is flowing across a larger area.  Therefore, a clean drainage sand or sand and gravel often is appropriate, as shown on the figure.  Modified City Type 26 aggregate (Seattle Standard Specifications, 1989, Section 9-03.16) provides adequate permeability for many applications.  It is also an adequate filter material for many Seattle soils.  The example shown on Figure 2-7, Sheet 2 of 2, shows a slotted pipe with drainage sand and gravel used for both bedding and backfill.

Backfill is placed in the trench in layers and either tamped with a backhoe or system­atically compacted.  Generally, if the trench subdrain is located in a landscape area or an unused portion of property, the backfill can be moderately tamped in place with the backhoe bucket to reduce subsequent settlement.  However, backfill in trench subdrains located where subsequent settlement of the backfill is not appropriate should be placed and compacted as structural fill material. 

Tightline Connections

At the end of the trench subdrain and/or prior to daylighting the drainage pipe on the slope, the slotted or perforated pipe should connect to a tightline pipe.  At this transition, the subdrain trench should be filled with concrete or clay to force water from the permeable subdrain trench backfill into the slotted or perforated pipe.  Figure 2-8 shows an example of a drainage dam constructed with concrete or compacted clay.  From the concrete or clay dam, the tightline should extend to a suitable discharge location.

Trench Cover

The upper 12 to 18 inches of the trench subdrain should be backfilled with a relatively low permeability material to prevent direct infiltration of surface water.  Often the soil excavated from the trench is adequate because it should have a similar or lower permeability than the surrounding soil when recompacted in the trench.  However, if the trench backfill must be compacted as structural fill, the trench excavation spoils may be too wet to achieve sufficient compaction without some drying and aeration. 

In non-structural areas, where compaction is moderate, the backfill should be mounded slightly over the trench to prevent low areas from forming when the trench backfill settles.  The surface should be graded to prevent water from ponding near the trench subdrain.

A paved or lined swale installed in conjunction with an interceptor trench subdrain can be used to limit infiltration into the trench subdrain.  This remediation tactic is commonly used to control both surface and groundwater near the crest of a slope or close to the edge of a bluff, as shown on Figure 2-1, Sheets 1 and 2 of 3.

Geosynthetic Applications

Geosynthetic materials have been used in several trench subdrain applications.  These include geotextiles that provide a filter for trench backfill materials and composite drainage materials that form both the drainage material and filter material.

Geotextiles can be used to separate the permeable trench backfill from the surrounding soil and prevent migration of fines into the trench subdrain.  For this alternative, the drainage backfill could be a coarse-grained permeable soil that is a poor filter for the surrounding soil, such as uniformly graded gravel.  The geotextile would be selected based on its ability to pass water and its filter characteristics to prevent migration of fines from the surrounding soil.  The geotechnical engineer should specify this application on a case-by-case basis after careful consideration of the soil conditions.  Note that geotextiles are made for many purposes.  Therefore, not all geotextiles are appropriate for this application.  In deep trenches where shoring boxes are required, the use of a filter geotextile can increase the time and labor costs of the project.  Refer to Figure 2-7, Sheet 1 of 2, for the use of a geotextile to separate pea gravel from on-site soil.

One common misuse of geotextiles is wrapping the fabric directly around the perforated subdrain pipe.  Because of the small area of the fabric around the pipe, it can quickly clog with fines, effectively blocking groundwater flow into the pipe.

7.2.2   Springhead Drains

Springhead drains are installed to intercept point-source springs, seeps, and shallow water-bearing zones in slopes or on existing landslides.  They reduce the possibility for surficial erosion that can reduce stability by undercutting and oversteepening a slope.  In addition, they reduce the amount of groundwater that can seep into the surficial colluvial and fill soils, which are often particularly susceptible to landsliding when saturated. 

Springhead drains are placed at the point where springs and seeps emanate from the slope, to direct water through pipes to the base of the slope.  Springhead drains have filter soils placed at the beginning of the drainpipe to reduce the potential of piping (migration) of soils into the springhead system.  Figure 2-9 shows an example of a typical springhead drain installation.

The installation generally begins with an excavation to expose the seepage zone.  The size of the excavation depends on the lateral extent of the seep or spring and on the practical size of a springhead drain.  Difficult access on steep, wet slopes may require making excavations using hand tools.  The excavation should extend at least 1 foot deeper than the seepage level to form a collection pool.  A perforated or slotted 4-inch (minimum) diameter pipe is placed in the excavation perpendicular to the direction of the slope with the pipe ends capped.  The pipe is connected to a tightline pipe and a dam of sandbags, concrete, or clay is placed around the connection to seal the leaks and force water into the tightline pipe.  The installation must be completed in such a way that the entire seepage zone is backfilled with a free-draining aggregate that is sufficiently permeable to accommodate the anticipated seepage.  Often the perforated or slotted pipe is backfilled with pea gravel or other more permeable clean granular aggregate to accommodate the increased flow rates as the collected seepage is concentrated near the collector pipe.  Drainage sand and gravel, such as Seattle Type 26 Aggregate, may be suitable for the remainder of the backfill in the seepage zone.  The selection of pipe diameter, perforation or slot size, and backfill materials depends on the amount of seepage and the grain size of the surrounding soil.  As described in Section 7.2.1, the backfill material(s) must be adequate filters for the surrounding soil to prevent piping.

7.2.3   Drainage Blankets

When fills are constructed on a slope, a drainage blanket should be placed between the fill and the prepared subgrade surface to intercept seepage from the underlying soil and to improve drainage of water that infiltrates from the surface.  Fills where a drainage blanket should be considered include toe buttresses, embankment fills, and slope fills placed to restore grades.  A drainage blanket consists of a permeable layer of soil that is placed over the prepared subgrade before a fill is placed.  Because it is designed to transmit groundwater, a drainage blanket should be designed as a filter for the subgrade and fill soils.  Otherwise, piping of fines could plug the filter blanket and/or cause loss of ground.

The drainage blanket should be designed so it is capable of conveying the maximum anticipated seepage and infiltration water without saturating its full thickness.  Figure 2-10 shows an example of a drainage blanket placed beneath an earth buttress fill.  The design elements that need to be evaluated for each site include:

    The anticipated groundwater seepage and surface water infiltration rates.
     Permeability of the drainage blanket material and its thickness.
     The maximum distance to an interceptor trench subdrain or outlet.
    Seals to prevent direct surface water infiltration.
     If build on steep slopes, the drainage blanket should be built in benches or steps that penetrate into the natural slope.  The drainage blanket should be continuous across the benches and should be graded to drain continuously.

7.2.4   Drilled Drains

Drilled drains consist of generally small-diameter drainpipes installed in drilled holes to a water-bearing soil layer.  They are used to lower the groundwater level in a landslide or marginally stable slope where the depth to groundwater is too deep for dewatering using trench subdrains.  The main advantage of drilled drains is that they can be installed at virtually any depth.  Limitations include relatively high cost and the ability to intercept a sufficient amount of the permeable water-bearing zones to effectively lower the groundwater level.  A thorough understanding of the subsurface soil and groundwater conditions is essential in planning a dewatering system using drilled drains.  A geotechnical engineer and a hydrogeologist should explore the subsurface conditions, evaluate groundwater flow, and perform slope stability studies to develop an optimum drain configuration.  The hydrogeologist should design the most appropriate drain spacing, well diameter, and well screen size.  Pumping tests or other aquifer tests are commonly required to evaluate the effectiveness of proposed drilled drains.  If drilled drains are selected as an element in improving the stability of a slope, groundwater monitoring wells should also be installed and monitored before and after drain construction to verify that the drains are achieving the degree of lowering in the groundwater levels desired.  These groundwater monitoring wells can also be used to monitor the effectiveness of the system over time.

The three general categories of drilled drains include nearly horizontal drains (commonly called horizontal drains), directionally drilled drains, and vertical drains or wells.  Horizontal and directionally drilled drains capture groundwater and drain it away from a sensitive slope area with gravity flow.  Vertically drilled drains typically require pumping to remove groundwater, although gravity drainage is possible in certain circumstances, as subsequently described.  Figure 2-11 shows a schematic of the various types of drains.  If drilled drains are suitable, the site access limitations, the subsurface conditions, and construction costs typically dictate which system is feasible for a particular site.

Horizontal Drains

Horizontal drains are installed by drilling a nearly horizontal boring from a point at the bottom of a slope.  Therefore, access to the bottom of the slope for a large, track-mounted vehicle must be possible for this option.  Typically, two or more horizontal drains are radially drilled from one or more points to intercept the water-bearing stratum.  The drilled holes extend as far into the hillside as necessary to intercept and lower the groundwater level.  The length of drilled drains can be 200 feet or more.  They are drilled straight at a constant upward inclination of 2 to 10 degrees from the horizontal, depending on the site access and elevation of the water-bearing zone.  The installation technique generally consists of drilling a subhorizontal boring and concurrently placing a steel casing into the hillside.  A slotted or screened plastic pipe is then placed inside the casing, which is then withdrawn leaving the plastic pipe in-place.  A tightline pipe is attached to the end of the plastic pipe and a low permeability plug installed to force the water into the tightline for conveying the discharge water to a suitable location.  Each pipe is generally fitted with individual valves for shutting-off and cleaning-out. 

Directional Drains

Directional drains are similar to horizontal drains except that they are typically drilled from the top of the slope using a remotely guided drill to intercept a water-bearing soil layer at a predetermined location.  Once the drilled hole reaches the target water-bearing layer, the drill bit continues until it exits the slope at the desired collection point.  From there, the water is conveyed in a tightline to a suitable discharge location.  The advantage to directionally drilled drains is that access to the bottom of the slope for heavy equipment is not needed.  For many landslides or marginally stable slopes, access is not otherwise practical.

The drill rig is typically set up some distance away from the top of the landslide, with an initial drilling inclination on the order of 20 degrees from the horizontal.  The position of the drill bit is monitored using an electronic tracking device.  The drilling assembly can be steered using a specially tooled drill bit to direct it to the desired dewatering zone and exit point.  The allowable radius of curvature of the drill steel limits the amount of steering.  Once the hole is completed, it can be reamed if a larger diameter is needed.  A slotted or screened plastic pipe, usually 2- or 4-inch-diameter polyvinyl chloride (PVC), is pulled through the drill hole from bottom to top to complete the drain.  The discharge is captured at the lower end in a tightline pipe system and conveyed to a suitable discharge location.  The upper end of the pipe is capped and encased in a monument at the surface to allow access for maintenance and cleaning. 

Vertical Drains

Vertical drains consist of vertically drilled bore holes that extend into or through a water-bearing soil layer and remove the water either by constant pumping or in certain circumstances by gravity flow.  Pumped vertical drains are essentially water wells and, as such, are designed and built in the same manner as water wells.  Typically, the boring for a well is drilled through the permeable unit where dewatering is planned and into the underlying low permeability soil layer.  The well consists of a screened section of well casing that extends through the permeable saturated soil and solid casing extending to the surface.  A sand pack is placed between the screen and the native soil to increase the effective diameter of the well and to form a filter between the surrounding soil and the screened well casing.  The filter prevents the well from piping fines from the surrounding soil that could cause loss of ground and impair the capacity of the well.  Water is removed from the well using a submersible pump that is controlled with a switch activated by rising water level or hydrostatic pressure.  Several different pumping system configurations are available.  Dewatering wells can intercept water-bearing units that otherwise are not accessible by other types of subsurface drainage.  However, they require continual pumping and maintenance, which can be costly.  In addition, a reliable power source is essential because of the likelihood of power outages during wet stormy periods.  Backup power systems require frequent maintenance and testing to ensure that they will function when the normal power system is interrupted.

A vertical gravity drain is installed in a similar manner as the pumping well, but instead of using a pump to remove water, the well drains groundwater to an underlying layer of permeable soil.  This type of system requires specific subsurface conditions to be practical.  These are:

The water-bearing layer that is reducing the slope stability must overlie a lower permeability layer (aquitard).

The aquitard must be underlain by a zone of permeable soil (e.g., sand or gravel).

The lower permeable zone must be below the level of slope instability.

The lower permeable zone must be able to drain the upper water-bearing layer, i.e., it must have sufficient permeability, thickness, and there must be a sufficiently large hydraulic gradient.

The design of this type of system requires detailed knowledge of the hydraulic characteristics of the entire system.  A hydrogeologist is typically required to evaluate the soil parameters and design the well system.  Another concern associated with vertical gravity drainage is the potential for cross contamination between upper and lower aquifers.  If the groundwater in the upper soil layer is contaminated, vertical drainage into an underlying aquifer would be prohibited by environmental regulations.  Even if the upper aquifer is not contaminated, the Washington State Department of Ecology or other local environmental regulatory agencies may require work to demonstrate that the underlying aquifer would not be degraded.

7.2.5   Other Subsurface Drainage Systems

Numerous other subsurface drainage systems have been used to lower groundwater levels in landslides and in marginally stable slopes.  These other systems typically are appropriate for specific subsurface geologic and groundwater conditions and are not widely applicable.  Some systems have largely been replaced because of technological advances.  For example, during the Depression, several U.S. Works Progress Administration (WPA) projects were undertaken to install drainage in landslide areas.  In many cases, the drainage consisted of hand-excavated tunnels that were subsequently backfilled with drain pipe and sand and gravel.  Today, many of these drains would be installed by horizontal or directional drilling.  Still drainage tunnels have specific, if limited, use for subsurface drainage.  Other subsurface drainage systems include: electro-osmosis, vacuum dewatering, and siphoning.  Because of the limited and site specific applications, these methods are not discussed in this report.

7.2.6   Monitoring and Maintaining Subsurface Drainage Systems

Subsurface drainage systems are only effective if they lower the groundwater level at least to the level assumed for design and if they maintain the lowered groundwater level.  Therefore, we recommend performing regular maintenance and installing a monitoring system so that the effectiveness of a subsurface drainage system can be monitored.  The type of monitoring depends on the site conditions, the type or types of subsurface drainage system(s) used, and the degree of reliability required.  Maintenance includes clearing vegetation from outlet pipes and tightlines, inspecting and repairing damage to surface installations, removing accumulated sediment from catch basins, and jetting pipes to remove sediment and encrustation.

Subsurface drainage systems are usually monitored by measuring the groundwater level in one or more monitoring wells and measuring the discharge rates from drain outlets.  The continuity of a drain line can also be evaluated by adding water at an uphill cleanout location and observing the flow at a downhill discharge location.  However, this type of test should only be performed during the dry summer season.  The groundwater level in monitoring wells can show that the drainage system is lowering the groundwater to the levels assumed in design.  Monitoring wells should be installed before the subsurface drainage system is installed to establish pre-construction groundwater level(s).  Often the monitoring wells that were installed during the initial site explorations can be used for long-term monitoring.  After the subsurface groundwater drainage system is installed, the groundwater levels should be monitored on a regular basis to evaluate the performance of the drainage system, including its response to seasonal rainfall events.  The measurement and data recording interval should be determined for each site.  Depending on the complexity and criticality of the subsurface drainage system, an automated data recording system may be justified.  Once the groundwater response to seasonal and rainfall events is established, groundwater level monitoring should be conducted at least once on an annual basis thereafter.  Unanticipated changes in groundwater levels typically show the need for cleaning or other maintenance.  The discharge rates from subsurface drains should also be measured and recorded.  Declines in the discharge rate may indicate buildups of encrustation or sediment that reduce the effectiveness of the system.

Subsurface drainage systems require regular maintenance to perform as designed.  Maintenance should start by designing surface installations that are protected from damage.  For wells, this could be accomplished by installing guard posts and steel monuments to prevent vandalism and accidental damage.  All surface installations, such as wells, drains, and tightlines, should be placed in locations where they can be easily found.  Vegetation around these installations should be regularly trimmed to allow inspection for deterioration, breaks, leaks, and other damage.  If groundwater monitoring and/or discharge rates indicate a decline in the performance of the subsurface drainage system, it should be cleaned by flushing, jetting, or other appropriate means.

7.3  Retaining Structures

Structures can be built to increase the stability of marginally stable slopes or existing landslides by:

    Retaining fills that add weight to the resisting part of the landslide
    Retain part of the driving forces
    Transfer driving forces into stable ground
    Increase the resisting forces of the soil along the failure surface
    Retain oversteepened scarp areas to prevent progressive upslope landsliding

Structures are also used to limit the runout of debris and to protect specific areas or structures upslope or downslope of the landslide.  Debris catchment and diversion structures are described in Section 7.6.  The appropriate type of structure for a given landslide or marginally stable slope will depend on many factors such as:

    Access to the site
    Stability of the slope
    Magnitude of slide forces
    Availability and cost of materials
    Future risk to life and safety
    Intended use of the stabilized slide zone

This section describes in situ and gravity walls.  In situ walls include various types of pile walls, such as cantilevered or tied-back soldier piles with lagging, tangent piles, secant piles, and sheet pile walls.  Gravity walls include concrete cantilever walls, mass concrete walls, crib and gabion walls.  Reinforced soil walls and slopes are described in Section 7.4.

7.3.1   In Situ Walls

In situ walls are structures that are built in place, without removing large volumes of soil to form a footing.  They are well suited for many landslide repairs where access limitations or stability concerns prevent excavations needed to construct other wall types or to place earth buttress fills.  They can also be used for catchment and/or diversion walls to reduce the debris flow hazard to structures below landslide-prone slopes.  In general, this type of wall is built with piles, drilled shafts, and/or anchors.  The common types of in situ walls include soldier pile walls with lagging, secant pile walls, tangent pile walls, and sheet pile walls.

Pile walls and tieback installations require a specialty contractor with equipment capable of drilling deep shafts and installing tiebacks into a hillside.  Because landslide zones frequently consist of soft, unstable ground with uneven terrain and limited access, it is sometimes necessary to construct pile walls with limited-access drilling or pile driving equipment.  As shown in Section 9.0, the equipment and labor costs for this type of wall construction are relatively high.

In situ walls are typically built with drilled shafts or driven piles.  Drilled shafts are cast-in-place concrete structures that usually contain a steel rebar cage or a steel H-beam for reinforcement.  Drilled shafts for retaining walls have diameters that typically range from about 2 feet to more than 4 feet.  In situ walls can also be constructed of driven piles including steel pipe piles, timber piles, steel H-beam piles, and sheet piles.  Wood or concrete lagging typically is placed between the piles when they are spaced more than 2 to 4 feet apart to retain the soil between piles.  Lagging is not necessary when the piles touch (tangent pile) or intersect (secant pile wall). 

An in situ wall can be designed as a cantilever structure when the height of the cantilevered portion of the pile or drilled shaft is relatively low (typically less than about 15 feet) and the active (driving) earth pressures are moderate.  A cantilevered pile structure resists lateral movement acting on the upper part of the pile (e.g., in the landslide mass) by transmitting the lateral loads into the portion of the pile embedded in hard or dense soil.  The cantilever is the height of the pile or shaft that is above the slide plane or competent soil.  Pile walls that are higher or must retain large landslide forces could also contain tieback anchors to further resist sliding forces.  A tieback anchor consists of single or multiple steel wires, strands, or bars that are installed at a shallow inclination from the face of the pile, through the landslide mass, and into undisturbed soil.  The tieback is anchored into stable, dense or hard soils with a cement grout or a mechanical end such as a helical plate or a swivel plate that expands into the soil when pulled.  The tieback anchor is usually post tensioned, although not always.  They are typically load tested.  These anchors transmit sliding forces exerted by the landslide mass into the underlying stable soil.

Soldier Pile Walls

Figure 2-12 (sheet 1   sheet 2    sheet 3) shows an example of a soldier pile wall.  Most soldier pile walls in Seattle consist of steel H-beams with wood or concrete lagging between the piles.  The piles are placed in predrilled holes, which are then backfilled with concrete.  The diameter of the predrilled hole depends on the size of the steel H-beam, but typically is about 2 feet.  Piles walls can also be constructed with augercast piles and driven piles, including timber piles; however, driven piles are more difficult to align to facilitate construction of the lagging.  In addition, it can be difficult to drive the piles to the required depth in some soils.  Other materials can be used for piles, including steel pipes.  The distance between the piles depends on a number of factors, including the lateral forces acting on the wall, the size of the piles, the height of the piles, and if tieback anchors are used.  Typically, soldier piles are spaced 4 to 10 feet apart.  Larger, deep-seated landslides typically involve driving forces that are relatively high, and resisting (passive) earth pressures that are low or subject to reduction due to potential slope movement downslope.  Consequently, pile walls for landslide mitigation frequently require use of tiebacks.  Wood or concrete lagging is placed between the piles to retain the soil between piles.

Figure 2-12, Sheet 1 of 3, shows typical earth pressure diagrams for cantilevered soldier piles walls or walls with a single row of tiebacks, and also for multiple rows of tiebacks.  The actual design earth pressures will depend on the wall height, soil type(s), slopes above and below the wall, and groundwater.  These factors typically are different at every site.

The presence of groundwater behind any wall causes large destabilizing forces.  Therefore, proper wall drainage is necessary to maintain stability.  Figure 2-12, Sheets 2 of 3 and 3 of 3, show two examples of drainage.  In the second, drainage is achieved by backfilling behind the wall with a well-drained fill material that is similar to a trench subdrain.  If it is not practical to excavate behind the wall to install drainage, then a drainage board can be placed on the retained side of the lagging, as shown on Sheet 2 of 3.  The drainage board should discharge into a tightline pipe that is sloped to drain and that discharges to a suitable location.

Other In Situ Walls

As described previously, walls can also be constructed using secant piles, tangent piles, and sheet piles.  The design of these wall types is similar to the design of a soldier pile and lagging wall, except that lagging is not used.  Drainage is important for all in situ wall types.

Occasionally, the stability of a landslide is improved using discrete piles and/or anchors rather than constructing a wall.  The concepts are similar, in that the landslide forces acting on the piles or anchors are transmitted into stable soil that is present beneath the landslide.  Instead of a linear wall that may also retain a change in grade, discrete piles and/or anchors are typically installed solely to increase the forces resisting sliding.  They are commonly installed on a grid or along one or more lines across the landslide.  Anchors that are installed as discrete retention elements in a landslide usually require a concrete reaction block to transfer the forces exerted by the sliding mass into the anchor.  The size of the concrete reaction block depends on the size of the anchor and the soil properties of the landslide mass.

7.3.2   Gravity Walls

Gravity walls are structures that resist sliding forces with their weight and internal stability alone.  They include walls constructed with mass concrete, concrete cantilever walls, rock-filled gabion baskets, rock-filled or soil-filled concrete, metal or timber cribs (Figure 2-13), and interlocking concrete blocks (Figure 2-14), which are commonly known as ecology blocks.  Gravity walls resist sliding forces by the friction developed along the bottom of the wall and passive resistance where the wall is embedded into dense or hard soil.  They resist rotation and overturning by being constructed at a batter, i.e., leaning towards the retained landslide mass and/or by having sufficient mass.  Gravity walls are typically keyed into stable foundation soils at the toe of a small landslide and then constructed up to the appropriate height to resist slide forces.

Gravity walls are commonly appropriate to provide toe support for landslides where the horizontal slide forces are relatively small.  Another common application includes a wall to retain an earth buttress fill where property limitations require a steep fill.  Gravity walls are generally less costly than in situ walls (such as soldier pile and lagging walls) and often can be constructed in difficult access areas.  It is important that gravity walls be properly constructed by qualified contractors using proper methods and materials to achieve the required internal shear strength while also being flexible and tolerant of deflections.

7.4  Soil Reinforcement

Soil can be reinforced by adding materials that have high shear strength.  When reinforced, soil can be built into steep slopes and walls.  Soil reinforcement has a number of applications for landslide repairs and for improving the stability of marginally stable slopes.  These include reinforced soil walls, in situ reinforcement, and replacement of the landslide mass with a stronger material. 

7.4.1   Reinforced Soil Walls

Reinforced soil walls, which are also referred to as mechanically stabilized earth (MSE) walls, consist of compacted soil with intervening layers of manmade material that is placed to improve the shear strength of the soil mass.  Adding such internal reinforcement to a soil fill provides shearing resistance against landslide forces.  Most reinforced soil walls are inherently flexible because they do not contain rigid elements such as concrete.  Therefore, they can tolerate relatively large settlements of the foundation soils while retaining their structural integrity.  Typical reinforcing materials consist of synthetic polymer materials (geotextiles and geogrids), welded wire fabric, or metallic strips.  The geotextiles are generally the least costly material; however, geogrids and metallic strips can provide higher strengths that may be required for some projects.  A sketch of a typical geotextile wall section is shown on Figure 2-15.  Figure 2-16 presents an illustration of a typical geogrid-reinforced soil wall.

The finished face of a reinforced soil wall can be vertical or sloped.  Vertical wall faces must be finished with erosion resistant facing such as sprayed-on concrete (shotcrete) or concrete masonry units (CMU) blocks.  There are large varieties of CMU blocks locally available that are suitable for use in conjunction with soil reinforcing.  Depending on the slope, sloping wall faces may be planted for vegetative erosion resistant facing.  Figure 2-17 shows an example of a reinforced soil slope.

The design of reinforced soil walls for slope stabilization is based on the external loading demands due to the slide mass behind the wall and the internal strength capabilities of the wall and strength of underlying soils.  Geotechnical parameters for the foundation soil, the reinforced fill soil, and the retained soil must be evaluated and incorporated into the design.  External as well as internal stability analyses must be completed to arrive at an appropriate wall dimension and reinforcement design.  As shown on Figures 2-15, 2-16, and 2-17, reinforced soil structures must have adequate drainage.

7.4.2   Soil Nail Walls

Soil nails and reinforced shotcrete can be placed on the exposed slope soils to increase the stability of marginally stable slopes and bluffs.  Soil nails consist of metal bars that are installed to reinforce the native soil.  They reinforce the soil by the frictional resistance that occurs between the relatively inextensible bars and the adjacent soil.  In many respects, soil reinforced with soil nails is similar to other types of reinforced soil, except that the soil is reinforced in situ.  Unlike post-tensioned tieback anchors, soil nails do not exert an external force on the retained face.

Typically, soil nails are installed either by driving a steel bar into the soil, or by grouting a bar in a 4- to 6-inch-diameter drilled hole.  For most applications in the Puget Sound area, nails are installed by grouting bars into pre-drilled holes.  The annulus between the soil nail and borehole is then backfilled with cement grout.  The nails are typically installed at a slight angle (5 to 20 degrees) from horizontal.  The nail spacing depends on the soil characteristics, but typically varies between about 4 and 6 feet, measured center to center.  Their lengths are determined based on the estimated thickness of the failure wedge, the height of the bluff, and the engineering characteristics of the soils.  Typically, the length varies between about 3/4 and 1 1/4 times the bluff wall height.

Reinforced shotcrete is placed in between the soil nails and on the exposed soil slope surface to protect the face from erosion and to prevent it from raveling, as shown on Figure 2-1 (Sheet 2 of 3).  Shotcrete consists of concrete that is sprayed onto the surface.  The shotcrete is typically reinforced with wire mesh that is installed before the shotcrete is applied.  The soil nails and shotcrete act in concert to form a reinforced membrane.  Provisions must be included for drainage behind a soil nail wall.  The base of the reinforced shotcrete should be protected from erosion at the toe.  As shown on Figure 2-1 (Sheet 2 of 3), such erosion control measures might include embedding the shotcrete into competent material, constructing a toe drain, and providing a lined drainage swale. 

7.5  Grading

Grading improvements to mitigate landslides involve making changes to surface topographic features on, above, or below slopes.  Grading includes any changes made to the ground surface by excavating, filling, or a combination of excavating and filling.  These changes can be made to accomplish one or more of the following:

     Improve drainage to reduce the amount of rainfall infiltration and runoff on or above a landslide or marginally stable slope.
     Decrease the driving weight of a landslide mass.
     Increase the weight of soil that acts to resist sliding.
     Increase the soil strength to resist sliding.
     Remove unstable soil.

Before any grading is accomplished, a geotechnical slope stability study that includes subsurface explorations should be accomplished to evaluate the effects of the proposed grading.  The geotechnical study should consider the effects of the proposed construction on the landslide or marginally stable slope and the slopes above, below, and adjacent to the proposed construction.  For example, if the proposed stability improvements include excavating soil from the top of a landslide, the geotechnical study should include stability analyses of the slope(s) remaining above the proposed excavation.  In an urban environment, grading improvements are commonly limited to relatively small changes in line and grade.  Large-scale excavations and fills often are not practical because of property limitations and high property values.

7.5.1   Drainage Improvements

Drainage can be improved by grading the ground surface to direct water away from a slope or other areas where infiltration can reduce stability.  Section 7.1 presents related surface water drainage improvements.  Grading to improve surface water drainage is commonly accomplished at the top of the slope to prevent overland surface runoff from flowing onto the slope.  Grading can also improve drainage for water that accumulates in closed swales, ditches, ponds, and other low lying areas, which would otherwise infiltrate and raise the groundwater level.  Section 6.1 describes how groundwater can adversely affect slope stability. 

Landslide surfaces are commonly irregular with sag ponds (depressions in a landslide surface that fill with water) and other poorly drained areas.  Therefore, after a landslide occurs, grading to smooth the surface can promote stability by improving runoff and reducing the opportunity for rapid infiltration.  Old landslide surfaces can also be regraded to a generally smooth constant slope that promotes runoff.  If regrading the entire slope or hummock portions of the slope is not practical, specific areas of ponded water can be drained with ditches and tightlines.  Slope stability analyses should be accomplished before any grading occurs to evaluate the effects, if any, from changes in the slope geometry.

Water should not be allowed to flow over the top of a slope onto a landslide area or marginally stable slope.  In addition, poorly drained areas near the top of a slope should be regraded to prevent ponding.  Ideally, the tops of slopes should graded so that the ground surface slopes away from the landslide zone or marginally stable slopes.  Alternatively, runoff can be directed into a drainage swale or ditch that is parallel to the top of the slope.  The swale or ditch should be continuously graded to prevent ponding and infiltration.  Where a drainage swale or ditch cannot be directed to a suitable discharge location, the water should be collected in a catch basin.  A tightline should convey the water from the catch basin to a suitable discharge location.  In permeable soil, swales and ditches should be lined with asphalt or compacted silt and clay to reduce infiltration. 

Infiltration into permeable soil can be reduced by constructing a low-permeability cover that promotes runoff to a suitable location.  This type of improvement would be practical mainly in areas where grading or other surface drainage improvements are not practical.  In most applications, a low-permeability cover could be constructed by compacting 2 feet of clay soil.  Whenever grading or land clearing occurs on a steep slope, the disturbed ground surface should be compacted to promote runoff and reduce infiltration.  Dense vegetative cover reduces erosion potential and also reduces infiltration by increasing evapotranspiration.  Therefore, we recommend reestablishing suitable vegetation after clearing or regrading a slope or area adjacent to a slope.

7.5.2   Decrease Driving Weight

Grading can be accomplished to remove weight from the portion of a slope that provides a driving force for a landslide or marginally stable slope.  Usually, the driving portion of a landslide is the upper steep portion of the slope.  Therefore, the driving weight can usually be reduced by flattening the slope and/or removing soil from the top of the slope.  If changes in line and grade are not acceptable, the driving weight can be reduced by replacing soil that is causing a driving force with a lightweight fill material.

When permanent removal of soil weight at the top of a slope is a viable alternative, conventional earthwork equipment can be used to excavate and haul soil from the site.  The soil should be excavated in a manner that improves stability.  Temporary soil stockpiles should not be allowed on or adjacent to the slope.  The final surface should be graded to a smooth and stable configuration that also promotes runoff to a suitable location.  The final surface should be seeded and/or planted to provide permanent erosion control.

Lightweight fills include materials such as fly ash, bottom ash, expanded polystyrene (geofoam), sawdust, wood chips, cinders, and cellular concrete.  The particular lightweight fill material selected for a given application depends on the required fill characteristics, availability, and project budget.  Fill materials such as sawdust can result in substantial settlement over the life of a project.  Expanded polystyrene is commonly used because of its very light weight, strength, and workability.  However, it is soluble in gasoline such that expanded polystyrene fills must be protected from fuel spills.  If the unit weight of a lightweight fill material is less than that of water, the fill can float if not weighted down when high groundwater conditions occur or if the site floods.  Chipped tires have been used for several lightweight road fills around the country; however, chipped tires can combust in situ, depending on the fill thickness and other environmental conditions.  Chipped tire fills that have "burned" resulted in the loss of the fill and also caused soil and groundwater contamination.  We, therefore, recommend against using chipped tires as fill.

Soil and vegetative debris derived from clearing and grading activities should not be sidecast over the top edge of a slope.  This practice tends to load the top of slopes and is a cause of many landslides in the City.

7.5.3   Increase Resisting Weight

Grading to increase resistance to driving forces in landslides generally involves placing a fill near the toe of the landslide.  Buttresses, counterweight fills, and toe berms improve the stability by their dead weight in the resisting part of the landslide.  The dead weight over the toe of a landslide increases the shear strength of the soils along the slide plane.  Depending on their geometry, fills placed near the toe of the landslide can extend the length of the landslide failure surface and additional shear strength from the new fill can improve stability.  Buttresses are typically keyed into underlying dense or hard soil to increase sliding resistance, while toe berms (also called counterweight fills) not so keyed still improve stability by the increased dead weight.

The above-described fills can be constructed with any type of inorganic soil fill, provided the new fill itself is stable.  In the Seattle area, fine-grained soils can be difficult to compact if the water content is too high and during wet weather.  Often relatively clean crushed rock or sand and gravel is used to facilitate construction; however, other fill materials can be used provided they can be compacted to a relatively dense condition. Fill slopes generally are built at 2 Horizontal to 1 Vertical (2H:1V) or flatter for constructability and to facilitate maintenance.  Steeper slopes usually require reinforcement or a retaining structure.  Sometimes, crushed rock slopes are constructed to 1.5H:1V.

The height, width, and length of the buttress or toe berm will depend on the size of the landslide and the forces involved.  The design should evaluate the effect of the proposed fill on improving the stability of the landslide or unstable slope.  It should also consider the stability of the fill itself.  The fill should be stable with respect to sliding, overturning, and bearing failure of the underlying soils.  As mentioned above, buttresses are typically keyed into the underlying dense or hard soil to provide sliding resistance.  The fill should have a subdrain system that includes a drainage blanket beneath the fill, unless the entire fill is pervious, as well as interceptor subdrains.  The placement of subdrains depends on the amount and location of groundwater seepage expected, including groundwater encountered during construction.  Therefore, it is advisable to have the geotechnical engineer provide recommendations for additional drainage based on the seepage conditions exposed during excavation.

Subgrade preparation on active landslides requires care and planning to avoid reactivating or accelerating the landslide movement.  In particular, excavations made at the toe of a landslide to place a fill keyed into stable subgrade material or to replace landslide debris with structural fill can remove a substantial portion of the resisting landslide mass.  Therefore, it may be necessary to complete earthwork in relatively small sections.  Each section should be backfilled with structural fill material before excavating the adjacent section.  The area of each section depends on the site-specific conditions.  Therefore, earthwork should be monitored on a full-time basis by a geotechnical engineer who can provide field recommendations if unanticipated movements occur. 

7.5.4  Increase Soil Strength

The stability of a slope can be increased by replacing the soil that is marginally stable or that has already slid with a relatively strong fill material.  Strong fill materials include well-compacted sand and gravel, gravel, quarry spalls, and riprap.  Typically, angular aggregate has higher shear strength than well-rounded aggregate.  Regardless of the fill material, the fill should be well drained.  A drainage blanket and associated subdrains should be incorporated into the design as appropriate.

Figure 2-18 shows an example of a typical replacement fill buttress.  In the example, the majority of the landslide debris is removed and replaced with a stronger, granular backfill material.  The replacement fill material consists of a well-graded sand and gravel or crushed rock that meets the gradation for Seattle Type No. 17 aggregate.  When compacted, this fill material provides relatively high shear strength.  In the example, approximately half of the failure surface is replaced with the stronger fill material.  The drainage layer shown on the figure consists of drainage sand and gravel that would be an effective filter for the granular backfill and the native soil that underlies the fill.  The drainage prevents groundwater from saturating the fill material and, consequently, reducing the shear strength along potential failure surfaces.  The fill must be embedded below the ground surface and to a sufficient depth so that a new failure surface will not develop below the replacement fill buttress.

Fills constructed to increase the resisting weight typically change the surface lines and grades in a manner that tends to lengthen potential failure surfaces.  Therefore, if the likely failure surface following the repair extends through the new fill, the resistance to sliding can be increased by using a relatively strong soil for the fill material.

7.5.5  Remove Unstable Soil

Landslides that involve uncontrolled fill material or other loose or soft soil over a hard or dense substrate can often be repaired by removing all or part of the unstable soil.  The soil should be excavated in a manner that improves stability.  The final surface should be graded to a stable configuration that promotes surface water runoff.  Following final grading, the surface should be revegetated to reduce erosion and surface water infiltration.  In an urban environment, removal of unstable soil commonly is limited to old uncontrolled fills that tend to destabilize a slope.

7.6  Catchment or Diversion Structures

A catchment or diversion structure can be used to limit runout of debris and protect specific areas downslope of potential landslides.  Catchment structures consist of a barrier to stop and contain landslide debris.  Diversion structures are not intended to stop a debris flow, but to divert it away from a specific area.  In either case, once a landslide begins, the landslide debris must accumulate somewhere.  Catchment and diversion structures only change the location where the landslide debris is deposited.  Following a landslide, catchment areas must be cleaned to prevent a future landslide from overtopping the catchment structure.

Catchment structures are typically built to protect a specific structure or road and are oriented across the slope, i.e., at right angles to the landslide debris path.  Catchment structures must be designed to withstand the impact and contain the volume of the landslide debris.  Therefore, their design requires a site-specific study to determine the likely size of the landslide, the zone of debris runout, and the velocity of the landslide debris at the desired catchment location.  Catchment walls can consist of structures normally used for retaining walls provided they can be free standing.  Examples include soldier piles and lagging, concrete cantilever walls, gabion baskets, and riprap or soil berms.  The wall should include a drainage layer on the upslope side to promote dewatering of the landslide debris after it is deposited.  This drainage will reduce the static loads on the wall, and will facilitate excavation of the landslide debris from behind the wall.  Because the landslide debris must be removed following every landslide, the wall design should include permanent access for earthmoving equipment after the wall is completed.  Depending on the location of a catchment wall, it could also be designed to provide support at the toe of a slope.

Diversion structures are not intended to stop landslide debris, but rather to direct the debris away from specific areas or structures.  They can consist of walls, berms, or grading to direct landslide debris to an undeveloped area.  The design criteria include the anticipated size of the landslide, its flow velocity, and radius of any curves in the diversion structure.  The diversion structure must be sufficiently high to prevent overtopping and structurally capable of withstanding impact loads.  Landslide debris should not be diverted without permission onto adjacent property.  If berms are used, they should be sufficiently armored to prevent erosion and breaching.  Permanent access to the depositional area should be provided to remove the landslide debris following landslides.  Diversion structures are often successfully applied in areas where the debris can be diverted into natural channels.  However, before this strategy is employed, the effects of additional sediment loading in streams that could receive the landslide debris should be evaluated.  The Endangered Species Act and other habitat restrictions could prevent or limit this type of diversion.  This scheme is not advisable unless there are no other options available.

Section 7.3 provides recommendations for selecting and designing specific types of retaining walls.  These recommendations are generally applicable for designing catchment and diversion structures, with the additional requirement of the impact loads imposed by a debris flow on the wall.  Figure 2-19 shows a typical pressure diagram for designing a catchment wall that includes the impact loads exerted on the wall by a debris flow.

7.7  Vegetation

Vegetative cover can contribute to the stability of steep slopes by reducing erosion, reducing direct infiltration from rainfall, and increasing the strength of the near-surface soil.  Dense vegetation intercepts direct rainfall before raindrops impact the soil surface, thereby reducing or eliminating rainsplash erosion.  With dense vegetative cover and thick forest litter, the likelihood of overland flow (sheetwash) is also reduced or eliminated.  If overland flow does occur, the flow velocity will be reduced by the vegetation.  Without overland flow or with reduced flow velocities, surficial erosion will be eliminated or reduced.

Thick vegetation, forest litter, and thick organic soil horizons typically retain moisture from direct precipitation.  After a rainstorm, plant leaves retain water that is available for evaporation back into the atmosphere.  The plants also transpire water that is absorbed by root systems.  Water that does not runoff or return to the atmosphere by evapotranspiration eventually infiltrates into the ground.  However, thick forest litter, organic soil, and heavy vegetation root systems can reduce the rate at which excess water is released into the groundwater.

Root systems can increase the strength of the soil they penetrate.  The increased strength occurs as an apparent cohesion, but it does not appear to affect the angle of internal friction of the soil.  The amount of apparent cohesion depends on the plant type and density.  The increase in apparent cohesion that results from root strength ranges from about 20 to 250 pounds per square foot (Turner and Schuster, 1996).  The effects of root reinforcement are limited to relatively shallow soil.  Therefore, root reinforcement can help reduce the likelihood of shallow landslides, but will provide little improvement on slopes where deep-seated landslides are likely.

Certain types of vegetation can have an adverse effect on slope stability.  Unstable trees can initiate a landslide if they are toppled during high wind conditions.  Therefore, trees that pose a safety hazard (rotting, dying, or excessively leaning trees) should be removed from tops of bluffs and on slopes; however, stumps should be maintained.  Slopes vegetated with dense, low-lying, deeply-rooted plants or shrubbery provide better protection from erosion and shallow landsliding than shallow-rooted vegetation.  For example, grasses tend to provide a relatively small amount of protection.  Generally, native vegetation is desirable because it can be maintained without irrigation during the dry season.  Ideally, the vegetation should require no more moisture than what typically occurs in the region to reduce the need for watering on the slope.  Publications such as "Slope Stabilization and Erosion Control Using Vegetation" (Washington State Department of Ecology Publication 93-30) provide guidance in selecting plant species.  As described previously, yard debris, or any other debris or fill, should not be placed on the slope as the additional loading adversely affects slope stability and inhibits plant growth.

8.0  COST ESTIMATE

Representative unit costs of improvement measures are presented in Table 2-1, Typical Improvement Unit Costs.  This table can be used for calculating a preliminary budget of the remedial work contemplated.  For a specific project, a more definite cost should be based on a more accurate cost breakdown that includes labor, materials, equipment, and engineering and administrative project costs. 

The unit costs presented are based mostly on recent contractor bids on work which Shannon & Wilson, Inc., was involved within the Seattle area.  Some prices were obtained from telephone interviews with specialty wall manufacturers and contractors.  In some cases, unit prices from bids in other States and Means Cost Data were used.  Reference projects were both small and large and included residential and commercial work as well as public and private projects.  The prices are representative of the prevailing cost for the 1997-1998 period.  Budget estimates based on this table should be adjusted for inflation in the following years.

The unit prices are shown as a range and as an average.  In some cases, we did not have sufficient information to provide a range of cost.  In these cases, we provided only the average unit price.  Typically, the greater unit price should be used in smaller residential projects and the lesser unit price should be applied for larger projects.  Other costs that should be added include:

        Incidentals and contingencies.
       A mobilization fee of approximately 10 to 15 percent of construction cost should be added to the unit cost items.
       Engineering costs for design that typically range from 10 to 15 percent, depending on the size and complexity of the job.
       Contract administration and construction observation costs.
       City administrative costs.
        State sales tax.

A number of other factors should be considered in making a preliminary cost estimate.  These include:

        Anticipated weather conditions
        Access difficulties
       Availability of staging areas
       Environmental constraints such as wetlands and erosion control
       Availability of qualified contractors
        Union or non-union labor wages
        Required traffic control
        Noise constraints imposed by neighbors
        Available work hours

The effect of these and other factors specific to the site need to be included in the final budget estimate.  Total project costs can increase by a factor of approximately two or three when all the above factors and cost additions are included.

The cost items considered in Table 2-1 follow in general the outline of improvements discussed in Section 7.

9.0  UTILITIES AND STREETS

Buried utilities and streets can affect the stability of the slopes they are built on, into, or adjacent to.  The presence of a buried utility or a street can act either to enhance or reduce stability.  For example, a utility trench could be designed and built to act as a trench subdrain that would remove groundwater from a slope, thereby improving the stability.  The same utility trench, if not properly graded, covered, or drained, could provide a conduit to rapidly convey surface and/or groundwater to a critical portion of a slope, and then infiltrate the water at that location.  For another example, low permeability street pavements typically inhibit infiltration of surface water into the groundwater, thereby improving stability.  However, if the storm water system is inadequate or not present, then uncontrolled, concentrated surface water runoff can discharge onto a slope and reduce its stability.  This section presents recommendations and typical design concepts for using buried utilities and streets to improve stability.  For instances where stability improvements are not practical, this section makes recommendations for reducing possible destabilizing effects.

9.1  Streets

Streets can be used in a number of ways to improve the stability of the slopes they are built on or adjacent to, which include the following:

        Reduce infiltration
        Storm water runoff control
        Subsurface drainage
        Structures and grading improvements

We recommend that the City consider these types of improvements when building a new street, performing maintenance, or when rebuilding an existing street.  The following sections provide typical details and design recommendations regarding these improvements.

9.1.1   Reduce Infiltration

Most streets are paved with low permeability asphalt concrete or Portland cement concrete.  As such, streets tend to reduce infiltration into the subsurface.  Where reducing infiltration could improve stability, we recommend adopting the following measures:

Use low permeability pavements.  Do not use pavement materials that are designed to allow rapid infiltration of surface water, such as Class F asphalt concrete.

Design the pavement section for a high degree of reliability and long service life to reduce deterioration and cracking that would increase the permeability of the pavement surface.

The performance of a pavement depends largely on the condition of the subgrade.  Therefore, subgrade improvements should be made where practical, such as with new streets or major renovations and repairs.  Subgrade improvements include overexcavating soft, loose, and compressible soil until undisturbed, firm, and unyielding native soil is exposed.  Any backfill or embankment fill materials should be placed and compacted in accordance with the Seattle Standard Specifications, except that all fill material should be compacted to at least 95 percent of the maximum dry density (American Society for Testing and Materials [ASTM] D 698).

Perform regular inspections and maintenance to detect and seal significant cracks, if necessary.  Evaluate the subgrade in areas of chronic cracking.  Correct soft or loose subgrade conditions that lead to poor drainage and/or cracking, if found to be needed.

Provide adequate storm water drainage system.  Grade pavement surfaces as may be found needed to promote rapid runoff and to prevent ponding.

9.1.2   Storm Water Runoff Control

Storm water runoff from streets and other low permeability surfaces, including areas of low-permeability soil, is sometimes a contributing factor to landslides.  Developers and private property owners must assess existing conditions and take steps to protect their property and comply with existing drainage codes.  In general, storm water drainage is beyond the scope of this study.  We understand that another study has been commissioned by the City to study storm water drainage citywide, which will address issues such as adequacy of storm drainage collection in landslide-prone areas and storm sewer capacity.

The following measures can be used to reduce the flow of storm water from streets onto or adjacent to slopes:

Provide curbs and/or lined storm water ditches to prevent runoff onto or adjacent to slopes.  Curbs or ditches should be designed to contain and convey all runoff to a storm sewer or other appropriate facility.  In some cases, curbs that are higher than normally built could be effective in controlling runoff in landslide-prone areas.  The capacity of the ditch or curb should take into account the design storm, and reasonable allowances for reductions in capacity from debris and/or melting snow or ice.

We do not recommend constructing unlined ditches to convey storm water runoff in landslide-prone areas.

Grade streets to drain into storm sewer catch basins.  Provide curbs and berms as needed to ensure proper runoff into the catch basins.

Regularly inspect and maintain curbs, ditches, and storm drains.

Educate and enlist the assistance of neighborhood organizations or individual residents regarding storm drainage facilities.  Residents could perform simple surface cleaning of debris and/or could notify the City when maintenance is needed.  Communication lines to the City need to be open, accessible, and made known to the public.

9.1.3   Subsurface Drainage

Subsurface drainage can be incorporated during construction and/or renovation of streets and adjacent storm water ditches.  In general, subsurface drainage associated with streets would fall into two general categories:  trench subdrains built under or adjacent to a street and a drained pavement base course. 

A drained pavement base course can intercept shallow groundwater and surface water that infiltrates through the pavement surface.  It can improve stability of slopes below the road to the extent that groundwater is intercepted and infiltration is reduced.  Usually, this type of shallow drainage will have the greatest benefit in improving the stability of roadway embankment fills.  However, in areas of shallow groundwater, drainage in the base course can effectively drain natural slopes.  While not related to slope stability, well-drained pavements generally perform better and have a longer service life.  A drained pavement base course is constructed in the same manner as a normal base course, with the following exceptions:

Grade the pavement subgrade to drain into a perforated or slotted collector pipe.  The collector pipe should be constructed in the same manner as a trench subdrain, described in Section 7. 

The collector pipe should be graded to drain to a suitable discharge point, such as a storm sewer.  It should not be allowed to discharge directly onto the surface.

Cleanout points should be provided for the collector pipe, and a regular cleaning and maintenance program adopted.

The base course aggregate should meet the requirements listed in Section 4-04.2 of the Seattle Standard Specifications, with the following additional requirements. 

The aggregate should have not more than 3 percent passing the No. 200 mesh sieve, based on the minus 3/4-inch fraction in a wet sieve analysis (ASTM D 422).

The aggregate should also meet filter criteria with respect to the underlying subgrade soil.  A non-woven filter fabric could be placed between the subgrade and the drainage base course layer in lieu of using a base course aggregate.

Trench subdrains associated with streets would not be substantially different in their application and construction from those described in Sections 6.0 and 7.0.  Streets in landslide-prone areas are generally parallel to the slope.  As such, they are well suited for constructing a groundwater cutoff trench subdrain, either in or adjacent to the street.  A groundwater cutoff trench subdrain could be particularly effective in improving the stability of an embankment fill that was placed over a soil with relatively low permeability, such as Lawton Clay or a fine-grained colluvium.  It could also effectively dewater relatively permeable colluvium that overlies Lawton Clay or other low permeable soil.

A trench subdrain could be built either in the roadway, and then paved over, or on the upslope side of the roadway.  If a storm water drainage ditch is being excavated next to the roadway, a trench subdrain could be incorporated.  The trench would be excavated to the depth needed for the subdrain.  The trench subdrain materials would be placed and then covered with a low permeability liner material for the storm water ditch.

As with all subsurface drainage, the collector pipes should discharge to a suitable location, such as a storm sewer.  The system should include provisions for periodic inspection and cleaning.  The City should adopt a regularly scheduled program for inspecting, cleaning and maintaining subsurface drainage.

9.1.4   Structures and Grading Improvements

New street construction or a major street renovation provides the opportunity to incorporate structures and grading improvements that would improve stability.  In general, these types of improvements are described in Sections 6.0 and 7.0.  Specific applications of structures and grading improvements that could be applied when building or renovating a street include the following:

All proposed structures, embankment fills, and excavations, whether retained or not, should be evaluated to determine the effects, if any, they will make on the stability of the slope.  Both the slopes above and below the proposed improvements should be evaluated.

Streets constructed near the top of a slope could make use of retained or sloped excavations to remove load from the upper portions of a marginally stable or potentially unstable slope.

Where fills are required near the top of a slope or midslope, lightweight fill materials can be used to reduce new loads imposed on the slope, or to reduce the loads currently imposed on the slope.  A common lightweight fill material used in roadway construction is expanded polystyrene (geofoam).  Other types of lightweight fill material are discussed in Section 7.5.2.

Retaining structures can be used to retain cuts and fills to improve the overall stability of the slope.  Walls designed to retain excavations on the upslope side of a street could also be designed as catchment walls.  This "double duty" would be relatively inexpensive, yet it could provide considerable protection of streets in areas where debris flows are likely.

Streets constructed near the toe of a slope could be built on an embankment fill that also serves as a toe buttress to improve the stability of the slope above.

9.2  Buried Utilities

Buried utilities, such as water, sewer, and storm drainage pipes and electrical and communication lines, could be used to improve the stability of a slope by providing subsurface drainage.  In some cases, grading changes could be made when a buried utility is installed that could also improve stability.  In some cases, buried utilities have triggered landslides.  These cases include pipe leaks and breaks, and possibly when groundwater is conveyed to a marginally stable slope in the trench backfill.  The following sections describe methods to improve slope stability associated with buried utilities, including:

        Subsurface drainage
        Groundwater control methods
        Old buried utilities
        Grading improvements

9.2.1   Subsurface Drainage

A buried utility trench could also be used as a trench subdrain.  In general, the use and design of a trench subdrain that is associated with a buried utility is the same as presented in Sections 6.0 and 7.0.  The utility location limits where drainage can be installed.  Therefore, the potential effectiveness of such drainage as well as the possibility for conveying groundwater into an inappropriate location should be carefully evaluated.  In addition to the recommendations presented in Sections 6.0 and 7.0, we recommend that trench subdrains constructed in conjunction with a planned buried utility include the following elements:

The trench subdrain should extend all the way through saturated or potentially saturated soil.  Portions of the utility trench that are excavated in permeable, unsaturated soil should be backfilled with clay or another low permeability material.  The collector pipe should be connected to a tightline through such permeable trench sections.  If a perforated pipe was placed in unsaturated permeable soil, water could flow from the perforated pipe into the soil it was intended to drain.  Under these circumstances, the trench subdrain could actually reduce the stability of a slope.

A slotted or perforated collector pipe should be included as part of the trench subdrain system.  It should be designed with sufficient capacity to convey the anticipated groundwater inflow.

Concrete or clay dams should be built wherever perforated pipes are connected to tightlines.  The concrete or clay dams will force the water into the tightline and prevent water from moving along the outside of the tightline.  Section 7.2.1 provides additional information on tightline connections.

The trench and the collector pipe should be continuously graded to drain so there are no low spots where water would tend to pond.

The trench backfill, collector pipe and native soil should be compatible with respect to filter criteria to prevent piping of fines that could cause loss of ground or clogging of the collector pipe.

Provide cleanouts and provisions for maintaining the collector pipe.

Install groundwater monitoring wells along the utility trench to verify that the trench subdrain is functioning as intended.  The monitoring wells should be used to determine when maintenance is required.

9.2.2   Groundwater Controls

Some landslides have been attributed to buried utilities.  Usually, these instances involve a pipe leak or break in a water, sewer, or storm drainage line.  The following section provides recommendations for constructing pipes in landslide prone areas.  The utility bedding and/or trench backfill can also provide a path for groundwater to migrate to a landslide area.  In such cases, bedding and trench backfill for utilities should be made in a manner that either does not change the drainage characteristics of the soil, or in a way that inhibits groundwater migration to the slope.  This section provides recommendations for constructing buried utilities to prevent groundwater migration to potential landslide zones.

Buried utilities can provide an adverse path for groundwater migration under the following circumstances:

The backfilled trench passes through saturated soil, i.e., a groundwater source, and then into an area of unsaturated permeable soil that is marginally stable.

The pipe bedding and/or trench backfill is more permeable than the native soil, but is not sufficiently well-drained to maintain groundwater levels in the backfill that are below the groundwater level in the adjacent native ground.

The trench is not continuously graded, so there are low areas where water can infiltrate from the pipe bedding or trench backfill into the native ground.

The trench backfill is not covered with a low permeability soil at the surface, thereby allowing surface water to infiltrate into the permeable backfill.

Water can migrate along a buried utility either in a permeable backfill material, or along small voids between the pipe and the backfill material.  The latter process, piping, can also result in ground loss by erosion of the backfill material around the pipe.  The following measures can reduce the potential for undesirable groundwater migration along a buried utility.

When practical, backfill the trench with compacted native soil.  This should result in a trench backfill that is hydraulically similar to the undisturbed ground.

Backfill the upper 1 to 2 feet of a utility trench with low permeability soil to reduce surface infiltration.  In landscaped areas, mound the backfill soil over the trench and grade the surrounding area to promote runoff away from the trench and to reduce the possibility of ponding.

Install concrete or clay dams at intervals along the pipe to prevent groundwater flow in the pipe bedding and/or backfill.  Concrete or clay dams can also reduce the potential for piping.

Pervious granular bedding material is often required for certain types of pipes.  In these cases, consider alternate pipe materials or install a sufficient number of concrete or clay dams to prevent groundwater migration into sensitive areas.  If possible, collect water from behind the concrete or clay dams with a tightline.

Install utilities above ground.

Provide subsurface drainage at key points.  For example, a trench subdrain could be installed where saturated soil is encountered in the utility excavation.  A concrete or clay dam should be installed at the end of the trench subdrain section to force the groundwater into the collector pipe and to prevent further groundwater migration along the buried utility.

9.2.3   Old Buried Utilities

Numerous existing pipelines, and especially sewers, were constructed by bedding the pipe in pea gravel.  This pea gravel bedding material has a relatively high permeability that provides the capacity to convey potentially large volumes of water.  If water is conveyed out of a potentially unstable slope, the stability of the slope is improved.  However, as noted in Section 9.2.2, the opposite can also occur.  That is, pea gravel pipe bedding can act as a conduit to rapidly convey water into an unstable slope, thereby reducing the stability of the slope.  We recommend establishing a program to evaluate buried utilities that are in or adjacent to landslide-prone areas.  Those that may have pervious bedding and/or backfill material, and especially pea gravel pipe bedding, should be further evaluated to determine if they have the potential to adversely affect slope stability.  For buried utilities that could adversely affect slope stability, we recommend the following:

If the utility is old and close to its design life, consider early replacement.  The replacement utility should be designed to improve subsurface drainage, as described in Section 9.2.1, or to prevent adverse groundwater migration, as described in Section 9.2.2.  If this alternative is selected, the old buried utility should be excavated to remove pervious bedding and/or backfill materials.

Install concrete or clay dams at key locations to prevent groundwater migration along the pervious bedding and/or backfill material.  If possible, drainage should be installed at each concrete or clay dam location to collect and convey the groundwater to a suitable discharge location.

Install adjacent drainage to intercept water that the buried utility may convey into a marginally stable slope.  Such drainage could include trench subdrains that are located downgradient from the buried utility.

Grout the pipe bedding and/or backfill to reduce the permeability.  While this alternative may be technically feasible, it is also relatively expensive.  Therefore, we anticipate that it would be used only for relatively short sections where other alternatives are not practical.

9.2.4   Grading Improvements

In most cases, buried utilities are placed in trenches that are subsequently backfilled to restore the original grades.  For these cases, slope stability improvements are mostly limited to subsurface drainage as described in the previous section.  However, in certain circumstances, grading improvements could be made in conjunction with placing utilities.  In most cases, grading improvements would be made when desirable for maintaining stability of the proposed utility installation.  For example, excavations could be made at the top of a slope to reduce the driving forces of a marginally stable slope in conjunction with installing a pipeline.  Lightweight fill materials can be effective in improving stability or reducing adverse effects when a fill is needed midslope or at the top of a slope.  As mentioned previously, the stability of any fills or excavations should be evaluated to demonstrate that the stability both above and below the proposed grading is not adversely affected.

Large utility excavations that extend below a landslide failure surface or potential failure surface could be backfilled with compacted angular aggregate to form a shear key.

9.2.5   Other Considerations

Landslide prone areas pose a breaking or rupture hazard to buried utilities.  Water lines, sewers, and storm drains that are damaged by ground movement can cause leaks that further exacerbate the unstable conditions.  Therefore, before installing new buried utilities, the utility route should avoid areas where ground movement is likely.  Where these areas cannot be avoided, several alternatives could be considered as may be appropriate to reduce the likelihood of damage.  These include:

Install the utilities above ground.  Above ground installations generally are less susceptible to damage from relatively small ground movements.  Also, they can be readily inspected for damage.  Storm drainage and communication lines are particularly well suited to above ground installations.

Use materials that are more tolerant to ground motion.  For example, bell and spigot concrete pipe is sensitive to relatively small movement as compared to HDPE pipe that has fused joints.

Install flexible connections and joints that also allow for some extension or compression.

Use pumped sewer lines in landslide areas instead of gravity drainage.  A pumped sewer line does not need granular bedding material to set the pipe at the grades required for drainage.  Also, if small movements cause grade changes, a pumped line would not be affected, whereas a gravity line may not function as designed.