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Part 2 Geotechnical
Evaluations 5.0 PURPOSE AND SCOPE
5.1
Purpose of Geotechnical Evaluations
Part 2 of this report presents a geotechnical
engineering evaluation of the landslides that have occurred
throughout the City of Seattle (City). In Part
3, engineering evaluations will be specifically related
to three study areas: 1) West Seattle, 2) Magnolia/Queen
Anne, and 3) Madrona. In Part 4,
engineering evaluations will be related to additional study
areas: Northwest Seattle, Northeast Seattle, Capitol Hill,
and South Seattle. Based on these citywide and study area
evaluations, typical measures will be presented to improve
stability and reduce the risk of future landslides. In addition
to preventive measures, remedial schemes will also be presented
for landslides after they occur. Most of the stability improvements
presented can be both preventive and remedial.
The purpose for our
studies and recommendations regarding stability improvement
is to inform both the public and representatives of the City
of the factors that cause landslides and the steps that could
be taken to improve stability. It is important for the City
to protect utilities, drainage features, streets, and other
City facilities; however, landslides do not obey property
boundaries. Therefore, measures will be presented that could
be made by the City and/or adjacent property owners to improve
the stability of an entire landslide or unstable slope.
5.2
Scope of Geotechnical Evaluations
In order to meet the
purpose described above, the following engineering evaluations
have been made:
We studied the landslide history described previously in
this report (Part 1), with respect
to topography, geologic and groundwater conditions, slide
types, timing, City locations, and causes. This study has
provided background for making engineering evaluations.
For each type of landslide or potential landslide, we developed
stability improvement measures consisting of surface and
subsurface drainage, grading, and/or structures. Typical
measures that could be applied citywide are presented in
this report section (Part 2).
We developed unit cost estimates for the typical
measures applicable to citywide stability improvements (Part
2). These cost estimates can be extrapolated to provide
budget figures for the stability improvements recommended
in Parts 3 and 4 of this report.
We studied the general implications of City utilities and
streets as related to instability (Part 2).
For the original three selected study areas (West Seattle,
Magnolia/Queen Anne, and Madrona), we conducted detailed
studies of the types and causes of landslides, including
the effects of City utilities and streets, and we provide
recommendations and cost estimate information for stability
improvements. The results of these studies are presented
in Part 3 of this report.
For the four additional study areas (Northwest Seattle,
Northeast Seattle, Capitol Hill, and South Seattle), we
conducted studies generally similar to those accomplished
for Part 3, and we provide recommendations and cost estimate
information for stability improvements. The results are
presented in Part 4 of this report.
6.0 TYPICAL
IMPROVEMENTS RELATED TO LANDSLIDE TYPE
As presented in Part
1 of this report, most of the landslides were found to fit
into four generalized types: 1) High Bluff Peeloff, 2) Groundwater
Blowout, 3) Deep-Seated, and 4) Shallow Colluvial (includes
landslides that involve fill material). There are various
combinations of these generalized landslide types, as one
type of mechanism may lead to another. This section describes
approaches for repairing slopes with these types of landslides,
improving the stability of slopes that could be affected by
landslides, and reducing the hazard from debris flows to properties
below landslides.
6.1
Geologic Conditions that Contribute to Landsliding and Instability
Part 1 of this report
provides a detailed description of the geologic and hydrologic
conditions that contribute to landsliding in the City. In
general, the following factors affect the stability of a slope:
topography, subsurface conditions, surface and groundwater
conditions, and external loads, such as structures.
Factors that commonly
trigger landslides include:
Increased groundwater levels and surface runoff
Removing support at the toe of the slope by erosion
or by excavation
Changes in the soil strength
Loading the head of the slope with debris from another
landslide or with manmade fills
Seismic loading
Groundwater contributes
to landslides in several ways. When saturated, a potential
landslide block has more weight because of the water, which
results in a larger driving force. Groundwater moving through
the soil exerts seepage forces that further reduce stability.
Finally, the presence of groundwater reduces the strength
of the soil on a potential slide plane. Freezing weather
can be an important process in reducing slope stability because
frozen soil can impede groundwater seepage. When seepage
is impeded at the surface, groundwater levels can build to
cause an unstable condition.
Surface storm water
runoff can reduce slope stability by infiltrating into the
near-surface soils at critical locations, and by causing erosion.
Where groundwater emerges at the surface, resulting in a spring
or seep, the runoff can cause surficial erosion that can undermine
and/or oversteepen a slope. Undermining and/or oversteepening
and the consequent loss of support at the toe of the slope
can trigger a landslide.
Prior to construction
of seawalls along Puget Sound, the base of the bluffs and
slopes were subject to continual shoreline erosion and oversteepening
at the toe of the slope. Once undercut, the lower part of
the slope would slide, thereby undercutting the slope at higher
elevations. With the construction of seawalls and other shoreline
protection measures, erosion has been arrested or greatly
reduced. However, these slopes have not necessarily achieved
a stable configuration, so landsliding may continue for the
foreseeable future.
Development activities
can result in undercutting and oversteepening slopes. This
was more prevalent prior to modern building codes, such as
the Department of Design Construction and Land Use (DCLU)
Director's Rules 3-93, 3-94, and 3-97, regarding development
in geologic hazard areas. Therefore, many oversteepened and
improperly sloped or retained older cuts and excavations remain
in the City, some of which contribute to instability. In
general, modern cuts and excavations made under the guidance
of a competent geotechnical engineer have achieved a suitable
degree of stability.
Seasonal variations
in moisture and temperature, combined with plant growth and
decay, animal burrowing, and soil creep tend to reduce the
strength of soil over time. This process particularly affects
colluvial soil and glacially overridden soils that are exposed
at the surface by an excavation or by a landslide.
Loads placed on or
near the top of a marginally stable slope typically reduce
the slope stability. These loads can be caused by debris
from a landslide that occurred upslope, manmade fills or loads
from structures. Modern fills that are designed and constructed
under the direction of a geotechnical engineer generally are
suitably stable. However, many older fills were built without
proper subgrade preparation, adequate drainage or compaction.
These fills include material that was loose-dumped without
compaction in ravines and on slopes and loose sidecast road
fills. A number of recent and older landslides in Seattle
involve old uncontrolled fill material. Structures, heavy
equipment, and material stockpiles that are built or placed
on a steep slope or near the top of a steep slope can contribute
to instability.
Many of the steep
bluffs and slopes are susceptible to earthquake-triggered
landslides. Both the 1949 Olympia and 1965 Seattle-Tacoma
earthquakes caused a total of at least 41 landslides in the
Puget Sound region. Recent geological and seismological research
findings indicate that many of the large ancient landslides
identified in the bluffs along Puget Sound were triggered
by large prehistoric earthquakes.
6.2
Typical Approaches to Improve Stability
For each type of landslide,
we evaluated potential stability improvements, which could
be preventive and/or remedial. In general, the methods for
achieving suitable stability for a site or project include:
1) avoiding the slope and 2) improving stability by reducing
the forces that cause movement, increasing the forces that
resist movement, or a combination of the two. These methods
for improvement measures fall into several generalized categories,
as presented in the following table.
| APPROACH |
PROCEDURE |
EXAMPLES |
|
Avoid
the slope |
Build
structures, utilities and/or streets a safe distance
from the landslide |
Leave
property undeveloped |
|
Build
over the landslide, with supports on stable ground |
Bridge
over unstable area, build on deep piles or shafts |
|
Reduce
the
driving forces |
Remove
weight from the upper, driving portion of the landslide |
Flatten
slope, remove material from the landslide top, move
external loads away from the landslide top |
|
|
Remove
the unstable material |
Completely
or partially remove unstable materials |
|
|
Drain
surface water to reduce infiltration into the groundwater |
Grading
to promote drainage: ditches, swales, berms, storm
sewers and tightlines, low permeability covers |
|
|
Drain
groundwater to reduce the driving weight, seepage
forces and erosion |
Trench
subdrains, springhead drains, finger drains, drainage
blankets, drainage wells (horizontal, vertical and
directionally drilled), drainage tunnels and adits. |
|
|
Build
fills or replace existing soil with lightweight fills
to reduce driving weight |
Expanded
polystyrene, sawdust, cinders, bottom ash. |
|
Increase
the resisting forces |
|
|
|
Apply
external forces |
Add
weight to the resisting part of the landslide |
Buttress,
counterweights, toe berms |
|
|
Build
structural retention systems to resist part of the
driving forces |
In
situ walls (soldier pile, secant pile, tangent pile,
etc.) and gravity walls (i.e., concrete cantilever,
reinforced soil, gabion, crib, etc.) |
|
|
Install
anchors that transfer driving forces into stable
ground |
Tieback
anchors |
|
Increase
the soil strength |
Drain
the subsurface to increase the soil strength along
the failure surface |
Trench
subdrains, springhead drains, finger drains, drainage
blankets, drainage wells (horizontal, vertical and
directionally drilled), drainage tunnels and adits. |
|
|
Install
in situ reinforcement to increase the strength along
the failure surface |
Soil
nails, anchors, piles, shafts |
|
|
Replace
or modify the landslide soil to increase its strength |
Excavation
and replacement with high shear strength soil, improve
soil by compaction or lime and cement stabilization,
grouting, ground freezing |
|
|
Construct
reinforced backfill that is stable on steeper slopes
and has higher strength |
Reinforced
soil slopes and walls |
|
|
Use
biotechnical stabilization to intercept rainfall and
provide root reinforcement |
Vegetation
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
details.
6.3
High Bluff Peeloff Landslides
The main factors that
lead to high bluff peeloff landslides are nearly vertical
slopes, groundwater seepage, and surface water runoff. In
many cases, little can be done to prevent these landslides
because of their height, steepness, and inaccessibility.
The nearly vertical bluffs typically were formed by coastal
erosion. Although this erosion may have been arrested or
slowed by recent shoreline protection, the slopes have not
achieved a stable slope through erosion and landsliding.
Figure 2-1 (sheet
1 sheet2
sheet
3) shows simplified sketches of a high bluff peeloff landslide,
together with several alternatives for reducing the likelihood
of a high bluff peel off landslide, and protecting a structure,
street, and/or utility below the bluff, as shown. Unless
the bluff is low or otherwise accessible, remedial measures
to reduce the likelihood of a landslide typically are limited
to surface and groundwater improvements at the top of the
bluff, as shown. Where structures, streets, and/or utilities
are located below a bluff and in the likely landslide runout
zone, measures can be taken to reduce damage from the landslide
debris. These include building sufficiently far from the
bluff that landslide debris should not affect the structure,
building catchment, or diversion structures, and removing
trees that likely would be incorporated into the landslide
debris.
In cases where the
bluff is accessible and/or where the consequences of a landslide
are high, the slope could be retained with a wall. In general,
this type of repair is costly and may not be economical or
practical to build. Suitable wall types depend on the height
of the bluff, and the topography and geologic conditions of
the slope below the bluff. These wall types could include
soil nails with reinforced shotcrete, as shown on Figure
2-1, Sheet 2 of 3, and soldier pile walls with lagging
if the slope is low (Sheet
3 of 3). For a soil nail wall, reinforcing elements would
be required into the bluff face. For a soldier pile wall
with lagging, the soldier piles could be tied back or cantilevered.
Installation of these soil nails or anchors would be expensive
and access to the bluff would pose safety concerns for the
workers.
We recommend removing
hazard trees, other large vegetation, or structures that likely
would be incorporated into a high bluff peeloff landslide.
Such objects incorporated into landslide debris have damaged
structures located below bluffs. Trees that are isolated
or subjected to high winds that accelerate over bluffs are
more likely to be uprooted. An uprooted tree can initiate
a high bluff peeloff landslide.
6.4
Groundwater Blowout Landslides
Groundwater blowout
landslides occur where a relatively permeable soil overlies
a less permeable soil, resulting in perched groundwater and
seepage towards the slope face. The high groundwater levels
and seepage towards the slope face result in destabilizing
seepage pressure and reduced soil strength. Seeps and springs
that form where groundwater exits the slope face often cause
erosion that can undermine and oversteepen a slope further
reducing the stability.
Figure 2-2 (sheet
1 sheet
2) shows four simplified sketches of a groundwater blowout
landslide, together with several alternatives for reducing
the likelihood of a landslide. Because the primary driving
force is groundwater seepage, suitable remedial measures usually
include drainage to lower the groundwater level and to control
seepage at the slope face. Drainage measures usually are
most effective when they intercept groundwater at the contact
between the relatively permeable soil and the underlying less
permeable soil.
Sketch A on Figure
2-2 (Sheet 1) shows the application of an interceptor
trench subdrain and a springhead drain. Both improve stability
by lowering the groundwater level in a landslide or potentially
unstable slope, thereby reducing the driving forces and increasing
the soil strength. The springhead drain is used to collect
water that emerges from the slope in a concentrated area,
thereby reducing erosion potential and improving stability.
Trench subdrains generally are applicable to slopes where
the contact with the underlying low permeability material
is relatively shallow. An interceptor trench subdrain is
installed across the slope to intercept the groundwater before
it reaches the slope face. Sketch
B shows another type of trench subdrain, called a finger
drain. It is similar in construction to an interceptor trench
subdrain, except that it is installed along the slope fall
line (perpendicular to slope contours).
Sketch C on Figure
2-2 (Sheet 2) shows two alternatives for drilled drains:
horizontal drains and directionally drilled drains. Drilled
drains are typically used to improve stability of slopes and
landslides where the groundwater cannot be intercepted with
trench subdrains, or where it is not practical to excavate
trench subdrains. Drilled drains are commonly used to improve
the stability of large deep-seated landslides. Horizontal
drains are drilled from the slope face, which limits their
application to sites that have suitable access near the toe
of the landslide mass. Directionally drilled drains usually
are installed from the top of the slope and can be aimed to
intercept a specific zone where the drainage is needed. Vertical
wells (not shown) can be used in special cases; however, their
suitable application is limited. Vertical wells require continual
pumping to maintain lower groundwater levels. As such, they
incur the cost of electricity and are subject to power outages
during critical rainy periods.
A replacement earth
buttress is sometimes used to improve a marginally stable
slope and more commonly to repair a landslide that has already
occurred. As shown by Sketch
D, the landslide mass or potentially unstable soil is
removed and replaced with a well drained fill material. In
some cases, the excavated soil can be recompacted to form
the earth buttress, while in others a suitable imported backfill
is compacted to form the earth buttress. In either case,
an effective drainage layer and subdrain should be constructed
under the earth buttress.
6.5
Deep-Seated Landslides
As described in Section
4.1.3, deep-seated landslides can occur in a variety of
geologic settings. Most deep-seated landslides consist of
a relatively large block of soil that may remain partially
intact as it slides downhill on an arc- or wedge-shaped failure
surface. The size of a deep-seated landslide can vary from
a single backyard to a city block or more. Groundwater usually
contributes to deep-seated landslides, although the source
of the groundwater may not be clearly related to a contact
between relatively high and low permeability soils. Because
of the varied geologic and hydrologic conditions that contribute
to deep-seated landslides, the alternatives for repairs are
equally varied.
Figure 2-3 (sheet
1 sheet
2 sheet
3) shows five simplified sketches of a deep-seated landslide,
together with several alternatives for improving the stability.
Depending on the soil types and groundwater levels, the various
schemes for dewatering that are shown on Figure
2-2 for the groundwater blowout landslide may be applicable.
If the landslide failure surface is too deep to drain with
trench subdrains, horizontal or directionally drilled drains
could be used.
Deep-seated landslides
often can be repaired by adding weight to the toe of the landslide
and/or removing material from the upper, driving part of the
landslide. Sketches A through F show various schemes for
improving the stability of a deep-seated landslide or potential
landslide.
Sketch A on Figure
2-3 (Sheet
1) shows an earth buttress constructed at the toe of an
existing or potential landslide to add weight to the lower,
resisting part of the landslide. In general, earth buttress,
counterweights and toe berms should include a drainage layer
beneath the main fill to reduce the potential for destabilizing
groundwater conditions. Sketch
B shows a similar situation, except that the toe of the
fill is retained to accommodate a street, property line, or
other construction-access limitations. The wall at the toe
of the retained earth buttress fill could be a reinforced
soil wall, as shown on the Sketch, or another type of wall.
Both in situ and gravity wall types, as defined in the table
with Section 6.2, could be suitable, with
the choice of wall type depending on the geometry of the earth
buttress fill.
Sketch C on Figure
2-3 (Sheet
2) shows an example where soil is removed from the upper,
driving part of the landslide and removed from the site.
The slope behind the landslide or marginally stable slope
should be graded so it will be stable. Alternatively, a retaining
wall could be used to reduce the loss of level ground above
the landslide. In this instance, a soldier pile or other
in situ wall probably would be most effective, although other
types could be used if a suitable foundation can be provided.
Sketch
D shows an example where the stability is improved by
a combination of removing soil from the upper driving part
of the landslide and adding an earth buttress fill to the
lower resisting part of the landslide. For the example shown,
the excavation and fill quantities are approximately equal,
so that most material would be derived and disposed of on
site. Retaining walls could be used at the top or at the
toe of the landslide to keep within rights-of-way or natural
grade limitations.
Another alternative
that reduces the driving weight is shown on Sketch
C. After removing soil from the upper driving portion
of a deep-seated landslide, lightweight fill material could
be placed to restore grades. Wood chips, cinders, and expanded
polystyrene (geofoam) are frequently used in this type of
application.
Sketch A on Figure
2-3 (Sheet
1) also shows the use of an in situ wall to retain soil
in the upper driving part of a marginally stable slope or
landslide that has had incipient failure or small displacements.
The piles also provide reinforcement across the landslide
failure surface. In situ walls include soldier pile walls
with lagging, secant piles and tangent piles (details given
in Section 7.0). This type of repair is
appropriate when other less expensive alternatives, such as
drainage and regrading are not practical because of site limitations.
Sketch E on Figure 2-3 (Sheet
3) shows an example where an in situ wall is used to retain
the slope above the scarp after a landslide occurs. In this
example, the wall retains the ground above the landslide and
prevents progressive upslope failure. However, the ground
below the scarp is not restored nor is the stability improved.
Sketch
F shows an example where a drained earth buttress is built
at the bottom of the slope to improve stability and a slope
fill is built to restore the grades. A reinforced soil wall
or other type of gravity wall could be used to steepen the
toe of the drained earth buttress if needed for property or
site access limitations.
6.6
Shallow Colluvial Landslides
As described previously,
colluvium is present on most slopes in Seattle. Because of
the typically shallow depth, colluvium is particularly susceptible
to rapid saturation from infiltration of surface runoff, direct
infiltration of precipitation, groundwater seepage, discharge
from pipes, or a combination of these sources of water. Shallow
colluvial landslides are the most common type of landslide
in Seattle.
Figure 2-4 (sheet
1 sheet
2) shows simplified sketches of a shallow colluvial landslide,
together with several alternatives for reducing the likelihood
of a landslide. They also show alternatives for protecting
a structure, street, and/or utility below the slope, if it
is not practical to stabilize the slope.
Sketch A Figure 2-4
(Sheet
1) shows a combination of surface drainage, subsurface
drainage, removing hazards from the site, and a catchment
wall. Depending on the circumstances, these measures could
be used individually or in combination. As discussed previously,
colluvium is particularly sensitive to rapid saturation from
both surface water runoff and groundwater seepage. Therefore,
all storm water runoff from roof drains, paved surfaces, foundation
drains, etc., should be collected into a tightline and discharged
to a suitable location. Ideally, storm drainage collected
from the top of the slope should discharge to a storm sewer
located at the top and back from the edge of the slope. If
this is not possible, a tightline should convey this storm
water runoff to the bottom of the slope or to a storm sewer
located downslope. It is usually advisable that the tightline
not be buried, to prevent breakage from soil creep or landslide
movement.
Trench subdrains are
often effective for providing subsurface drainage in colluvium.
The depth to the underlying glacial soil is usually within
the reach of a backhoe or even hand-excavated trenches. Both
interceptor trench subdrains, as shown on Sketch A, and finger
drains, as shown on Sketch B of Figure 2-2 (Sheet
1), can be effective. Drilled drains usually are not
practical, except where site access precludes construction
other than directionally drilled drains.
In some cases, it
is not practical or possible to improve the stability of the
slope. A catchment wall, as shown on Sketch A of Figure 2-4
(Sheet
1), can protect structures, streets, and/or utilities
below the landslide. As stated previously, we recommend removing
trees, other large vegetation, or structures that likely would
be incorporated in a landslide. Trees and other large debris
incorporated in a debris flow can cause as much or more damage
than the moving soil. It is also possible to use the catchment
wall as an in situ wall designed to retain potential sliding
soil.
Sketch
B shows a retaining wall to provide support for a marginally
stable slope, or to allow an excavation at the toe of a slope.
The type of wall will depend on the site conditions and access
limitations. An in situ wall type, as shown on the sketch,
often is needed to construct the wall within property lines.
In addition, such walls can be built before making the excavation,
so the slope is continually supported. Other improvements
that could be made include an earth buttress and subsurface
drainage.
Sketches C and D,
on Figure 2-4 (Sheet
2), show two schemes for repairing a shallow colluvial
landslide. Sketch C shows a case where site access limitations
prevent extensive work on the slope above a bench where a
structure, street, and/or utility is located. If springs
or seeps are present on the slope, springhead drains could
be installed to promote good drainage. Finger drains could
be installed in the remaining colluvium at the base of the
slope, as shown on Sketch B of Figure 2-2 (Sheet
1).
Sketch
D shows a repair where the slope is restored by removing
the landslide debris and unstable colluvium, and placing a
well-drained structural fill or reinforced soil slope. Depending
on the required slope, desired use, and property limitations,
a gravity wall or other type of wall could be constructed
at the toe of the new fill.
Landslides involving
fill material typically are similar to shallow colluvial landslides.
Therefore, the repairs are also similar. Figure
2-5 shows two sketches of stability improvements for landslides
involving fill material that was placed at the top of a slope,
or of a sidecast road fill. Both sketches show one or more
walls to restore a level area damaged by a landslide. The
sketches show that a gravity wall (e.g., reinforced soil,
concrete cantilever, crib, etc.) or an in situ wall (e.g.,
soldier pile) could be effective. The number, type, and location(s)
of the walls would depend on the slope geometry, the final
grades desired, site access, and other site limitations.
The fill material and underlying loose colluvium could also
be replaced with a drained earth buttress, as shown on Sketch
D, Figure 2-2 (Sheet
2), or a reinforced soil slope.
7.0
DETAILS REGARDING IMPROVEMENTS
Many different engineered
systems are currently used to mitigate landslides in the Seattle
area. Sometimes a single system is enough to provide the
necessary level of slope improvement or property protection.
Often a combination of several mitigation systems is required
to adequately increase the stability of a landslide or a marginally
stable slope. Site accessibility and the mitigation scheme
required to improve stability are the primary factors that
govern the total cost of a slope stability improvement project.
Sections 6.1 and 6.2
describe the geologic conditions that contribute to instability
and typical approaches to improving stability. Sections 6.3
through 6.6 show typical applications of
these typical approaches to improving stability for each of
the four landslide types. The following subsections discuss
details regarding mitigation measures that are commonly used
in the Seattle area. These include typical details for the
following types of improvement measures:
- Surface Water
- Groundwater
- Retaining Structures
- Soil Reinforcement
- Grading
Note that final design
details are not provided in this report. Final details and
the selection of the appropriate improvement measure or measures
should be developed by a geotechnical engineer experienced
in landslide repairs and based on site-specific explorations
and engineering evaluations.
7.1
Surface Water Improvements
As described in Section
6.1, surface water runoff can contribute to landsliding
by causing surficial erosion and/or rapid saturation of the
ground. Surface water improvements generally are the least
costly measures that can be implemented to reduce landslide
potential or mitigate existing instability. These improvements
can be effective where storm water runoff, including water
from streets, other paved areas, and roofs flows onto or near
steep slopes and potential landslide areas. In most cases,
surface water improvements consist of capturing storm water
runoff and redirecting it away from sensitive slope areas.
Storm water can be captured in appropriately located ditches,
swales, roof drains, curbs, and catch basins. Once collected,
the runoff should be conveyed in a tightline (an unperforated
pipe) to a suitable discharge location. A suitable discharge
location includes a storm sewer with adequate capacity or
the bottom of a slope. Where storm water runoff is discharged
to the ground surface at the bottom of a slope, appropriate
erosion control measures should be placed at the discharge
point.
Runoff from roofs
should be collected by gutters and conveyed with downspouts
to a catch basin or other structure that permits periodic
cleaning. From the catch basin, the runoff should be conveyed
in a tightline to a suitable discharge location. The downspouts
at some homes discharge into footing subdrain pipes. We strongly
recommend against this practice. It introduces surface water
rapidly into the ground, which can trigger landslides and
can cause foundation settlement. In addition, poorly drained
foundations are often the cause of wet basements.
Where surface water
runoff occurs toward a potential landslide slope, we recommend
constructing a paved or lined swale near the slope top to
intercept runoff. The adjacent ground should be graded to
drain into the paved swale. The swale should be sufficiently
large to convey the design storm with some blockage from leaves,
ice, and other debris. Water in the swale should be collected
at a catch basin and then conveyed to a suitable discharge
location.
Surface water management
on roads and other City-owned pavement surfaces located on
or near the top of the slope is discussed in detail in Section
9. In general, the concepts and need for controlling
surface water runoff are the same for any property; however,
City property does have some special implications because
of streets that create large areas of low permeability surface
that can generate considerable storm water runoff.
7.1.1
Tightlines
Tightline
systems are an integral part of a surface water system.
As such, it is essential that all tightline pipe systems
are properly designed and durable. The primary functional
design requirements include the inlet, pipe capacity (pipe
size and slope), and outlet. An important design factor
in landslide areas is that the tightline might be subjected
to landslide ground motion. The location and type of inlet
will depend on the storm water collection system. However,
it should include some provision for preventing debris from
entering the tightline and for periodic cleaning. For example,
a catch basin allows large debris to settle before the water
enters the tightline and provides for periodic cleaning.
The
pipe size is a function of the anticipated runoff discharge
and the pipe slope. All pipes should be continuously graded
to prevent settlement accumulation in the pipe that could
eventually block the pipe or reduce its capacity.
Tightlines
can consist of a variety of materials including different
types of plastic and metal pipe. Each one should be selected
based on the particular project requirements. Tightlines
that extend across steep terrain and unstable slopes should
consist of durable plastic pipe, such as high-density polyethylene
(HDPE). The joints should be durable and able to carry
axial loads and accommodate flexural deformation of the
pipe. Welded or through-bolted, flanged joints are examples
of suitable joints. The pipe should be installed on the
surface with an anchor system to prevent the pipe from being
pulled apart by soil creep or by a landslide and to allow
regular inspection. Figure 2-6 (sheet
1 sheet
2) shows examples of tightline anchoring systems.
Tightlines
may consist of less expensive, jointed, flexible, corrugated
plastic pipe in less critical stability areas, and depending
on other project requirements. In addition, tightlines
may be buried in stable areas.
As
described previously, all tightlines should discharge to
a suitable location. They should not be allowed to discharge
at the top of a slope, directly into or onto a slope, or
onto a mid-slope bench.
7.1.2
Surface Water Systems - Maintenance
All
surface water systems should be regularly checked and maintained.
Ideally, the City and residents on or near slopes should
collectively implement and maintain the drainage features
described above. We recommend designing maintenance programs
in landslide prone-areas that use a partnership between
the City and residents. The City could set up a regular
maintenance program to:
Clean catch basins and storm water runoff systems
on a regular basis. Initially, the City could establish
a frequent maintenance schedule in landslide hazard areas
based on previous experience and current schedules. However,
a record-keeping system should be implemented to identify
an appropriate schedule for specific locations, and typical
storm events that could block catch basins.
Inspect streets in landslide-prone areas for significant
cracking and surface wear. Repair significant cracks,
as appropriate, where found to be needed to reduce storm
water infiltration.
Provide residents in landslide prone areas with information
on measures they should implement to reduce surface water
runoff and infiltration. These measures should include
the issuance of publications such as the Washington Department
of Ecology "Surface Water and Groundwater on Coastal
Bluffs" (1995), and free inspection programs that
might be similar in organization and implementation to
the energy audits provided by Seattle City Light.
In many cases, residents on steep properties cannot
discharge storm water runoff into a storm sewer, either
because there is no nearby storm sewer or because the
grades are inappropriate. Depending on the property ownership,
installing a tightline to the bottom of the slope may
be an alternative. The City could work with these residents
and their neighbors to identify storm water disposal alternatives.
These might include installing tightlines that cross more
than one property (including City property) to convey
runoff to the bottom of the slope or to another suitable
discharge point below the property.
Form "Landslide Block Watch Groups," in
which groups of residents would regularly inspect City
storm water catch basins for debris. This group could
perform some surficial cleaning when necessary and could
alert the City when additional work is needed. The groups
could provide annual or more frequent reminders to their
neighborhood when regular maintenance, such as cleaning
gutters and drains, should be performed.
Landslide Block
Watch Groups could also assist the City with disseminating
information on reducing residential surface water runoff
and infiltration. This could include providing a liaison
with City personnel, meeting with new homeowners, and identifying
and helping to resolve neighborhood storm water runoff problems.
7.2
Groundwater Improvements
Intercepting groundwater
upslope and from within the slope can reduce landslide potential
and improve stability of existing landslides. Groundwater
improvements can be effective on many slopes, and when used
appropriately, they are often the most cost-effective approach.
The primary goal is to remove groundwater in areas where groundwater
reduces stability by adding weight to potentially unstable
soils, causes seepage forces, and reduces the soil strength.
However, capturing water flowing within the ground requires
some different methods compared to surface water improvements.
Common groundwater improvement methods include:
Interceptor trench subdrains and finger drains
Springhead drains
Drainage blankets
Drilled drains
All groundwater improvement
schemes should be designed based on the site-specific subsurface
conditions. To perform effectively, the system must lower
the groundwater level near the landslide failure surface,
which requires an understanding of the soil and groundwater
conditions that cause instability at the location. The following
sections provide a description of common groundwater improvements
with some typical design details and requirements.
7.2.1
Interceptor Trench Subdrains and Finger Drains
Trench subdrains
are relatively narrow trenches that contain a drainage pipe
and permeable backfill. Figure 2-7 (sheet
1, sheet
2) shows typical trench subdrain cross-sections. Groundwater
preferentially flows into the permeable trench backfill
and then into the drainage pipe at the bottom of the trench.
From there, it is conveyed into a tightline that discharges
the groundwater to a suitable discharge location. Trench
subdrains are most effective when they penetrate at least
1foot below the contact between the layer being drained
and an underlying clay, silt, or less permeable layer.
This contact is commonly also at or close to the slide plane.
Two basic types of trench subdrains include interceptor
trench subdrains and finger drains. An interceptor trench
subdrain is usually oriented across the slope (parallel
to the contours) to intercept groundwater as it flows downslope.
Finger drains are frequently used to lower the water level
within an active landslide mass by extending a trench subdrain
from the toe of the slope up into the landslide debris.
Other than their orientation (perpendicular to the contours),
finger drains are constructed in the same manner as trench
subdrains.
Trench Excavation
Most
trench subdrains are excavated using a backhoe or a track-mounted
excavator. Therefore, the practical depth for most trench
subdrains is about 15 feet or less. Track-mounted excavators
are available that can excavate 20 feet deep or more. However,
deep trenches are often difficult and expensive to excavate
because of the shoring required to maintain stable trench
sideslopes. Where groundwater is shallow and site access
is limited, hand dug trench subdrains may be practical.
The
depth of the trench is generally determined by the maximum
practicable depth of the excavating equipment, site conditions,
shoring requirements, and other project limitations. As
mentioned previously, trench subdrains are most effective
when they penetrate though the layer being drained and at
least 1 foot into an underlying less permeable soil.
Excavating
open trenches in marginally stable soil is often difficult
because of groundwater infiltration and the tendency for
the trench sidewalls to collapse. Where practical, we recommend
beginning the excavation at the outfall and proceeding upslope
to allow water to drain away from the advancing trench excavation.
It may be necessary to periodically stop work for a day
or more to let the site drain before advancing the trench.
Drainage Pipe
The
drainage pipe in an interceptor trench subdrain typically
consists of a 6-inch (minimum) diameter slotted or perforated
plastic pipe. The slots or perforations in the drainage
pipe allow water to enter the pipe. However, the drainage
pipe only conveys water when the groundwater level rises
higher than the pipe invert. When lower water levels are
present in portions of a trench subdrain system, the water
flows through the surrounding permeable trench backfill.
Therefore, the pipe may not need to be placed at precise
grades. The pipe should be graded to drain continuously
with no sags or depressions where water could infiltrate
into the subgrade.
The
drainage pipe at the bottom of the trench may consist of
rigid or flexible and perforated or slotted pipe. Each
type has its advantages and disad4vantages. The rigid pipe
is more durable and less susceptible to crushing during
installation. However, a worker must be present in the
trench to fit the pieces of pipe together and to prepare
the bedding gravel. Having workers in a trench generally
requires shoring, which results in a higher cost and a longer
time for construction. The other alternative, flexible
plastic pipe, is easily crushed if workers are not careful
or if it is not properly bedded. The primary advantage
of the flexible pipe is that it can be lowered down into
a deep, unshored trench excavation without workers being
in the trench.
The
size of the perforations or slots should be compatible with
the drainage backfill material around the pipe and the anticipated
groundwater flow rates into the pipe. The following paragraph
describes filter requirements for perforations and slots
in more detail.
Trench Backfill
The
trench backfill material depends on the anticipated groundwater
inflow and the grain size of the surrounding soil. The
backfill should be sufficiently permeable so that water
easily flows from the surrounding soil into the trench subdrain.
However, the backfill should also act as a filter to prevent
migration of the surrounding soil into the subdrain trench.
This migration process, known as piping, can eventually
plug the subdrain pipe, the drainage backfill or both.
In some cases, extensive piping can cause extensive settlement
around the trench subdrain from the ground lost by piping.
The backfill must also be compatible with the perforated
or slotted drainage pipe. If the openings are too small,
water cannot enter the pipe fast enough. However, if the
openings are too large, the backfill will enter and plug
the pipe. To meet the requirements of adequate permeability
and filter characteristics, the backfill material should
be designed for each specific situation. The following
paragraphs describe examples of typical backfill materials
that have been successfully used in Seattle soils.
In
the example shown on Figure 2-7, Sheet
1 of 2, the perforated or slotted pipe at the bottom
of the trench is bedded in washed pea gravel to provide
a highly permeable material directly around the pipe. The
pea gravel should be underlain by a geotextile where it
rests on silt or clay soil. The remaining backfill can
be less permeable, because the groundwater is flowing across
a larger area. Therefore, a clean drainage sand or sand
and gravel often is appropriate, as shown on the figure.
Modified City Type 26 aggregate (Seattle Standard Specifications,
1989, Section 9-03.16) provides adequate permeability for
many applications. It is also an adequate filter material
for many Seattle soils. The example shown on Figure 2-7,
Sheet
2 of 2, shows a slotted pipe with drainage sand and
gravel used for both bedding and backfill.
Backfill
is placed in the trench in layers and either tamped with
a backhoe or systematically compacted. Generally, if the
trench subdrain is located in a landscape area or an unused
portion of property, the backfill can be moderately tamped
in place with the backhoe bucket to reduce subsequent settlement.
However, backfill in trench subdrains located where subsequent
settlement of the backfill is not appropriate should be
placed and compacted as structural fill material.
Tightline Connections
At
the end of the trench subdrain and/or prior to daylighting
the drainage pipe on the slope, the slotted or perforated
pipe should connect to a tightline pipe. At this transition,
the subdrain trench should be filled with concrete or clay
to force water from the permeable subdrain trench backfill
into the slotted or perforated pipe. Figure
2-8 shows an example of a drainage dam constructed with
concrete or compacted clay. From the concrete or clay dam,
the tightline should extend to a suitable discharge location.
Trench Cover
The
upper 12 to 18 inches of the trench subdrain should be backfilled
with a relatively low permeability material to prevent direct
infiltration of surface water. Often the soil excavated
from the trench is adequate because it should have a similar
or lower permeability than the surrounding soil when recompacted
in the trench. However, if the trench backfill must be
compacted as structural fill, the trench excavation spoils
may be too wet to achieve sufficient compaction without
some drying and aeration.
In
non-structural areas, where compaction is moderate, the
backfill should be mounded slightly over the trench to prevent
low areas from forming when the trench backfill settles.
The surface should be graded to prevent water from ponding
near the trench subdrain.
A
paved or lined swale installed in conjunction with an interceptor
trench subdrain can be used to limit infiltration into the
trench subdrain. This remediation tactic is commonly used
to control both surface and groundwater near the crest of
a slope or close to the edge of a bluff, as shown on Figure
2-1, Sheets 1
and 2
of 3.
Geosynthetic
Applications
Geosynthetic
materials have been used in several trench subdrain applications.
These include geotextiles that provide a filter for trench
backfill materials and composite drainage materials that
form both the drainage material and filter material.
Geotextiles
can be used to separate the permeable trench backfill from
the surrounding soil and prevent migration of fines into
the trench subdrain. For this alternative, the drainage
backfill could be a coarse-grained permeable soil that is
a poor filter for the surrounding soil, such as uniformly
graded gravel. The geotextile would be selected based on
its ability to pass water and its filter characteristics
to prevent migration of fines from the surrounding soil.
The geotechnical engineer should specify this application
on a case-by-case basis after careful consideration of the
soil conditions. Note that geotextiles are made for many
purposes. Therefore, not all geotextiles are appropriate
for this application. In deep trenches where shoring boxes
are required, the use of a filter geotextile can increase
the time and labor costs of the project. Refer to Figure
2-7, Sheet
1 of 2, for the use of a geotextile to separate pea
gravel from on-site soil.
One
common misuse of geotextiles is wrapping the fabric directly
around the perforated subdrain pipe. Because of the small
area of the fabric around the pipe, it can quickly clog
with fines, effectively blocking groundwater flow into the
pipe.
7.2.2
Springhead Drains
Springhead
drains are installed to intercept point-source springs,
seeps, and shallow water-bearing zones in slopes or on existing
landslides. They reduce the possibility for surficial erosion
that can reduce stability by undercutting and oversteepening
a slope. In addition, they reduce the amount of groundwater
that can seep into the surficial colluvial and fill soils,
which are often particularly susceptible to landsliding
when saturated.
Springhead
drains are placed at the point where springs and seeps emanate
from the slope, to direct water through pipes to the base
of the slope. Springhead drains have filter soils placed
at the beginning of the drainpipe to reduce the potential
of piping (migration) of soils into the springhead system.
Figure
2-9 shows an example of a typical springhead drain installation.
The
installation generally begins with an excavation to expose
the seepage zone. The size of the excavation depends on
the lateral extent of the seep or spring and on the practical
size of a springhead drain. Difficult access on steep,
wet slopes may require making excavations using hand tools.
The excavation should extend at least 1 foot deeper than
the seepage level to form a collection pool. A perforated
or slotted 4-inch (minimum) diameter pipe is placed in the
excavation perpendicular to the direction of the slope with
the pipe ends capped. The pipe is connected to a tightline
pipe and a dam of sandbags, concrete, or clay is placed
around the connection to seal the leaks and force water
into the tightline pipe. The installation must be completed
in such a way that the entire seepage zone is backfilled
with a free-draining aggregate that is sufficiently permeable
to accommodate the anticipated seepage. Often the perforated
or slotted pipe is backfilled with pea gravel or other more
permeable clean granular aggregate to accommodate the increased
flow rates as the collected seepage is concentrated near
the collector pipe. Drainage sand and gravel, such as Seattle
Type 26 Aggregate, may be suitable for the remainder of
the backfill in the seepage zone. The selection of pipe
diameter, perforation or slot size, and backfill materials
depends on the amount of seepage and the grain size of the
surrounding soil. As described in Section
7.2.1, the backfill material(s) must be adequate filters
for the surrounding soil to prevent piping.
7.2.3
Drainage Blankets
When
fills are constructed on a slope, a drainage blanket should
be placed between the fill and the prepared subgrade surface
to intercept seepage from the underlying soil and to improve
drainage of water that infiltrates from the surface. Fills
where a drainage blanket should be considered include toe
buttresses, embankment fills, and slope fills placed to
restore grades. A drainage blanket consists of a permeable
layer of soil that is placed over the prepared subgrade
before a fill is placed. Because it is designed to transmit
groundwater, a drainage blanket should be designed as a
filter for the subgrade and fill soils. Otherwise, piping
of fines could plug the filter blanket and/or cause loss
of ground.
The
drainage blanket should be designed so it is capable of
conveying the maximum anticipated seepage and infiltration
water without saturating its full thickness. Figure
2-10 shows an example of a drainage blanket placed beneath
an earth buttress fill. The design elements that need to
be evaluated for each site include:
The
anticipated groundwater seepage and surface water infiltration
rates.
Permeability
of the drainage blanket material and its thickness.
The
maximum distance to an interceptor trench subdrain or
outlet.
Seals to prevent direct surface water infiltration.
If
build on steep slopes, the drainage blanket should be
built in benches or steps that penetrate into the natural
slope. The drainage blanket should be continuous across
the benches and should be graded to drain continuously.
7.2.4
Drilled Drains
Drilled
drains consist of generally small-diameter drainpipes installed
in drilled holes to a water-bearing soil layer. They are
used to lower the groundwater level in a landslide or marginally
stable slope where the depth to groundwater is too deep
for dewatering using trench subdrains. The main advantage
of drilled drains is that they can be installed at virtually
any depth. Limitations include relatively high cost and
the ability to intercept a sufficient amount of the permeable
water-bearing zones to effectively lower the groundwater
level. A thorough understanding of the subsurface soil
and groundwater conditions is essential in planning a dewatering
system using drilled drains. A geotechnical engineer and
a hydrogeologist should explore the subsurface conditions,
evaluate groundwater flow, and perform slope stability studies
to develop an optimum drain configuration. The hydrogeologist
should design the most appropriate drain spacing, well diameter,
and well screen size. Pumping tests or other aquifer tests
are commonly required to evaluate the effectiveness of proposed
drilled drains. If drilled drains are selected as an element
in improving the stability of a slope, groundwater monitoring
wells should also be installed and monitored before and
after drain construction to verify that the drains are achieving
the degree of lowering in the groundwater levels desired.
These groundwater monitoring wells can also be used to monitor
the effectiveness of the system over time.
The
three general categories of drilled drains include nearly
horizontal drains (commonly called horizontal drains), directionally
drilled drains, and vertical drains or wells. Horizontal
and directionally drilled drains capture groundwater and
drain it away from a sensitive slope area with gravity flow.
Vertically drilled drains typically require pumping to remove
groundwater, although gravity drainage is possible in certain
circumstances, as subsequently described. Figure 2-11
shows a schematic of the various types of drains. If drilled
drains are suitable, the site access limitations, the subsurface
conditions, and construction costs typically dictate which
system is feasible for a particular site.
Horizontal Drains
Horizontal
drains are installed by drilling a nearly horizontal boring
from a point at the bottom of a slope. Therefore, access
to the bottom of the slope for a large, track-mounted vehicle
must be possible for this option. Typically, two or more
horizontal drains are radially drilled from one or more
points to intercept the water-bearing stratum. The drilled
holes extend as far into the hillside as necessary to intercept
and lower the groundwater level. The length of drilled
drains can be 200 feet or more. They are drilled straight
at a constant upward inclination of 2 to 10 degrees from
the horizontal, depending on the site access and elevation
of the water-bearing zone. The installation technique generally
consists of drilling a subhorizontal boring and concurrently
placing a steel casing into the hillside. A slotted or
screened plastic pipe is then placed inside the casing,
which is then withdrawn leaving the plastic pipe in-place.
A tightline pipe is attached to the end of the plastic pipe
and a low permeability plug installed to force the water
into the tightline for conveying the discharge water to
a suitable location. Each pipe is generally fitted with
individual valves for shutting-off and cleaning-out.
Directional Drains
Directional
drains are similar to horizontal drains except that they
are typically drilled from the top of the slope using a
remotely guided drill to intercept a water-bearing soil
layer at a predetermined location. Once the drilled hole
reaches the target water-bearing layer, the drill bit continues
until it exits the slope at the desired collection point.
From there, the water is conveyed in a tightline to a suitable
discharge location. The advantage to directionally drilled
drains is that access to the bottom of the slope for heavy
equipment is not needed. For many landslides or marginally
stable slopes, access is not otherwise practical.
The
drill rig is typically set up some distance away from the
top of the landslide, with an initial drilling inclination
on the order of 20 degrees from the horizontal. The position
of the drill bit is monitored using an electronic tracking
device. The drilling assembly can be steered using a specially
tooled drill bit to direct it to the desired dewatering
zone and exit point. The allowable radius of curvature
of the drill steel limits the amount of steering. Once
the hole is completed, it can be reamed if a larger diameter
is needed. A slotted or screened plastic pipe, usually
2- or 4-inch-diameter polyvinyl chloride (PVC), is pulled
through the drill hole from bottom to top to complete the
drain. The discharge is captured at the lower end in a
tightline pipe system and conveyed to a suitable discharge
location. The upper end of the pipe is capped and encased
in a monument at the surface to allow access for maintenance
and cleaning.
Vertical Drains
Vertical
drains consist of vertically drilled bore holes that extend
into or through a water-bearing soil layer and remove the
water either by constant pumping or in certain circumstances
by gravity flow. Pumped vertical drains are essentially
water wells and, as such, are designed and built in the
same manner as water wells. Typically, the boring for a
well is drilled through the permeable unit where dewatering
is planned and into the underlying low permeability soil
layer. The well consists of a screened section of well
casing that extends through the permeable saturated soil
and solid casing extending to the surface. A sand pack
is placed between the screen and the native soil to increase
the effective diameter of the well and to form a filter
between the surrounding soil and the screened well casing.
The filter prevents the well from piping fines from the
surrounding soil that could cause loss of ground and impair
the capacity of the well. Water is removed from the well
using a submersible pump that is controlled with a switch
activated by rising water level or hydrostatic pressure.
Several different pumping system configurations are available.
Dewatering wells can intercept water-bearing units that
otherwise are not accessible by other types of subsurface
drainage. However, they require continual pumping and maintenance,
which can be costly. In addition, a reliable power source
is essential because of the likelihood of power outages
during wet stormy periods. Backup power systems require
frequent maintenance and testing to ensure that they will
function when the normal power system is interrupted.
A
vertical gravity drain is installed in a similar manner
as the pumping well, but instead of using a pump to remove
water, the well drains groundwater to an underlying layer
of permeable soil. This type of system requires specific
subsurface conditions to be practical. These are:
-
The water-bearing
layer that is reducing the slope stability must overlie
a lower permeability layer (aquitard).
-
The aquitard
must be underlain by a zone of permeable soil (e.g.,
sand or gravel).
-
The lower
permeable zone must be below the level of slope instability.
-
The lower
permeable zone must be able to drain the upper water-bearing
layer, i.e., it must have sufficient permeability,
thickness, and there must be a sufficiently large
hydraulic gradient.
The design of this
type of system requires detailed knowledge of the hydraulic
characteristics of the entire system. A hydrogeologist
is typically required to evaluate the soil parameters and
design the well system. Another concern associated with
vertical gravity drainage is the potential for cross contamination
between upper and lower aquifers. If the groundwater in
the upper soil layer is contaminated, vertical drainage
into an underlying aquifer would be prohibited by environmental
regulations. Even if the upper aquifer is not contaminated,
the Washington State Department of Ecology or other local
environmental regulatory agencies may require work to demonstrate
that the underlying aquifer would not be degraded.
7.2.5
Other Subsurface Drainage Systems
Numerous
other subsurface drainage systems have been used to lower
groundwater levels in landslides and in marginally stable
slopes. These other systems typically are appropriate for
specific subsurface geologic and groundwater conditions
and are not widely applicable. Some systems have largely
been replaced because of technological advances. For example,
during the Depression, several U.S. Works Progress Administration
(WPA) projects were undertaken to install drainage in landslide
areas. In many cases, the drainage consisted of hand-excavated
tunnels that were subsequently backfilled with drain pipe
and sand and gravel. Today, many of these drains would
be installed by horizontal or directional drilling. Still
drainage tunnels have specific, if limited, use for subsurface
drainage. Other subsurface drainage systems include: electro-osmosis,
vacuum dewatering, and siphoning. Because of the limited
and site specific applications, these methods are not discussed
in this report.
7.2.6
Monitoring and Maintaining Subsurface Drainage Systems
Subsurface
drainage systems are only effective if they lower the groundwater
level at least to the level assumed for design and if they
maintain the lowered groundwater level. Therefore, we recommend
performing regular maintenance and installing a monitoring
system so that the effectiveness of a subsurface drainage
system can be monitored. The type of monitoring depends
on the site conditions, the type or types of subsurface
drainage system(s) used, and the degree of reliability required.
Maintenance includes clearing vegetation from outlet pipes
and tightlines, inspecting and repairing damage to surface
installations, removing accumulated sediment from catch
basins, and jetting pipes to remove sediment and encrustation.
Subsurface
drainage systems are usually monitored by measuring the
groundwater level in one or more monitoring wells and measuring
the discharge rates from drain outlets. The continuity
of a drain line can also be evaluated by adding water at
an uphill cleanout location and observing the flow at a
downhill discharge location. However, this type of test
should only be performed during the dry summer season.
The groundwater level in monitoring wells can show that
the drainage system is lowering the groundwater to the levels
assumed in design. Monitoring wells should be installed
before the subsurface drainage system is installed to establish
pre-construction groundwater level(s). Often the monitoring
wells that were installed during the initial site explorations
can be used for long-term monitoring. After the subsurface
groundwater drainage system is installed, the groundwater
levels should be monitored on a regular basis to evaluate
the performance of the drainage system, including its response
to seasonal rainfall events. The measurement and data recording
interval should be determined for each site. Depending
on the complexity and criticality of the subsurface drainage
system, an automated data recording system may be justified.
Once the groundwater response to seasonal and rainfall events
is established, groundwater level monitoring should be conducted
at least once on an annual basis thereafter. Unanticipated
changes in groundwater levels typically show the need for
cleaning or other maintenance. The discharge rates from
subsurface drains should also be measured and recorded.
Declines in the discharge rate may indicate buildups of
encrustation or sediment that reduce the effectiveness of
the system.
Subsurface
drainage systems require regular maintenance to perform
as designed. Maintenance should start by designing surface
installations that are protected from damage. For wells,
this could be accomplished by installing guard posts and
steel monuments to prevent vandalism and accidental damage.
All surface installations, such as wells, drains, and tightlines,
should be placed in locations where they can be easily found.
Vegetation around these installations should be regularly
trimmed to allow inspection for deterioration, breaks, leaks,
and other damage. If groundwater monitoring and/or discharge
rates indicate a decline in the performance of the subsurface
drainage system, it should be cleaned by flushing, jetting,
or other appropriate means.
7.3
Retaining Structures
Structures can be
built to increase the stability of marginally stable slopes
or existing landslides by:
Retaining
fills that add weight to the resisting part of the landslide
Retain
part of the driving forces
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