Part 2 Geotechnical
5.0 PURPOSE AND SCOPE
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.
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.
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
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
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
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.
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
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.
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.
structures, utilities and/or streets a safe distance
from the landslide
over the landslide, with supports on stable ground
over unstable area, build on deep piles or shafts
weight from the upper, driving portion of the landslide
slope, remove material from the landslide top, move
external loads away from the landslide top
the unstable material
or partially remove unstable materials
surface water to reduce infiltration into the groundwater
to promote drainage: ditches, swales, berms, storm
sewers and tightlines, low permeability covers
groundwater to reduce the driving weight, seepage
forces and erosion
subdrains, springhead drains, finger drains, drainage
blankets, drainage wells (horizontal, vertical and
directionally drilled), drainage tunnels and adits.
fills or replace existing soil with lightweight fills
to reduce driving weight
polystyrene, sawdust, cinders, bottom ash.
the resisting forces
weight to the resisting part of the landslide
counterweights, toe berms
structural retention systems to resist part of the
situ walls (soldier pile, secant pile, tangent pile,
etc.) and gravity walls (i.e., concrete cantilever,
reinforced soil, gabion, crib, etc.)
anchors that transfer driving forces into stable
the soil strength
the subsurface to increase the soil strength along
the failure surface
subdrains, springhead drains, finger drains, drainage
blankets, drainage wells (horizontal, vertical and
directionally drilled), drainage tunnels and adits.
in situ reinforcement to increase the strength along
the failure surface
nails, anchors, piles, shafts
or modify the landslide soil to increase its strength
and replacement with high shear strength soil, improve
soil by compaction or lime and cement stabilization,
grouting, ground freezing
reinforced backfill that is stable on steeper slopes
and has higher strength
soil slopes and walls
biotechnical stabilization to intercept rainfall and
provide root reinforcement
and vegetation combined with structural slope stabilization
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
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
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
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
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.
Groundwater Blowout Landslides
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
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
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.
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
Figure 2-3 (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.
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
Sketch A on Figure
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
Sketch C on Figure
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.
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
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
Sketch A on Figure
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.
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.
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
Figure 2-4 (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
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
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
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
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
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
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.
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.
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:
- Surface Water
- Retaining Structures
- Soil Reinforcement
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.
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
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
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.
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.
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.
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
2) shows examples of tightline anchoring systems.
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.
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.
Surface Water Systems - Maintenance
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
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.
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.
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
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.
Interceptor Trench Subdrains and Finger Drains
are relatively narrow trenches that contain a drainage pipe
and permeable backfill. Figure 2-7 (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
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.
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.
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 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.
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.
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 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.
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,
2 of 2, shows a slotted pipe with drainage sand and
gravel used for both bedding and 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.
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.
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.
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.
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
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.
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
1 of 2, for the use of a geotextile to separate pea
gravel from on-site soil.
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
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
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.
2-9 shows an example of a typical springhead drain installation.
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.
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
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:
anticipated groundwater seepage and surface water infiltration
of the drainage blanket material and its thickness.
maximum distance to an interceptor trench subdrain or
Seals to prevent direct surface water infiltration.
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.
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.
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.
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.
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.
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
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.
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:
layer that is reducing the slope stability must overlie
a lower permeability layer (aquitard).
must be underlain by a zone of permeable soil (e.g.,
sand or gravel).
permeable zone must be below the level of slope instability.
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
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.
Other Subsurface Drainage Systems
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.
Monitoring and Maintaining Subsurface Drainage Systems
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.
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
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.
Structures can be
built to increase the stability of marginally stable slopes
or existing landslides by:
fills that add weight to the resisting part of the landslide
part of the driving forces
driving forces into stable ground
the resisting forces of the soil along the failure surface
oversteepened scarp areas to prevent progressive upslope
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:
to the site
of the slope
of slide forces
and cost of materials
risk to life and safety
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.
In Situ Walls
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.
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
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).
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.
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.
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.
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
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.
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
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.
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.
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.
Reinforced Soil Walls
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
2-16 presents an illustration of a typical geogrid-reinforced
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.
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
reinforced soil structures must have adequate drainage.
Soil Nail Walls
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
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.
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
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.
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
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
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
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.
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.
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
Decrease Driving Weight
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.
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.
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.
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.
Increase Resisting Weight
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.
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.
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.
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.
Increase Soil Strength
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.
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.
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.
Remove Unstable Soil
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.
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.
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.
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.
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.
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.
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.
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
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
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
Availability of staging areas
Environmental constraints such as wetlands and erosion
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
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
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:
Storm water runoff control
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.
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
permeability pavements. Do not use pavement materials
that are designed to allow rapid infiltration of surface
water, such as Class F asphalt concrete.
pavement section for a high degree of reliability
and long service life to reduce deterioration and
cracking that would increase the permeability of the
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).
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.
adequate storm water drainage system. Grade pavement
surfaces as may be found needed to promote rapid runoff
and to prevent ponding.
Storm Water Runoff Control
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
following measures can be used to reduce the flow of storm
water from streets onto or adjacent to slopes:
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.
to drain into storm sewer catch basins. Provide curbs
and berms as needed to ensure proper runoff into the
inspect and maintain curbs, ditches, and storm drains.
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.
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
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:
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.
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.
points should be provided for the collector pipe,
and a regular cleaning and maintenance program adopted.
course aggregate should meet the requirements listed
in Section 4-04.2 of the Seattle Standard Specifications,
with the following additional requirements.
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).
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.
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.
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.
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.
Structures and Grading Improvements
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:
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.
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
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
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.
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.
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:
Groundwater control methods
Old buried utilities
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:
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.
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
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.
and the collector pipe should be continuously graded
to drain so there are no low spots where water would
tend to pond.
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.
cleanouts and provisions for maintaining the collector
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.
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.
utilities can provide an adverse path for groundwater migration
under the following circumstances:
trench passes through saturated soil, i.e., a groundwater
source, and then into an area of unsaturated permeable
soil that is marginally stable.
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
is not continuously graded, so there are low areas
where water can infiltrate from the pipe bedding or
trench backfill into the native ground.
backfill is not covered with a low permeability soil
at the surface, thereby allowing surface water to
infiltrate into the permeable backfill.
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.
backfill the trench with compacted native soil. This
should result in a trench backfill that is hydraulically
similar to the undisturbed ground.
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
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.
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.
utilities above ground.
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.
Old Buried Utilities
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
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
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.
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.
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.
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.
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.
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
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
flexible connections and joints that also allow for
some extension or compression.
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.