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