The presence of subsurface water can be a
major contributing factor to the dynamic instability of slopes and existing
landslides. Therefore, the identification and measurement of subsurface
water in areas of suspected or known slope instability should be an integral
part of the subsurface investigation. The location and extent of groundwater,
perched groundwater and potential water barriers should be defined. Subsurface
water conditions within many landslides are best considered as complex,
multiple, partially connected flow systems. McGuffey and others (1996)
have listed the following recommendations:
Surface observations are essential in determining
the effect of subsurface water on landslide instability.
Periodic or seasonal influx of surface water
to subsurface water will not be detected unless subsurface water observations
are conducted over extended time periods.
Landslide movements may open cracks and develop
depressions at the head of a landslide that increase the rate of infiltration
of surface water into the slide mass.
Ponding of surface water anywhere on the landslide
may cause increased infiltration of water into the landslide and should
Disruption of surface water channels and culverts
may also result in increased infiltration of surface water into the landslide.
Landslide movements may result in blockage
of permeable zones that were previously freely draining. Such blockage
may cause a local rise in the groundwater table and increased saturation
and instability of the landslide materials. Subsurface observations should
therefore be directed to establishing subsurface water conditions in the
undisturbed areas surrounding the landslide.
Low permeability soils, which are commonly
involved in landslides, have slow response times to changes in subsurface
water conditions and pressures. Long-term subsurface water monitoring is
required in these soils.
Accurate detection of subsurface water in
rock formations is often difficult because shale or claystone layers, intermittent
fractures, and fracture infilling may occlude subsurface water detection
by boring or excavation.
Borings should never be the only method of
subsurface water investigation; nevertheless they are a critical component
of the overall investigation.
Slope Stability Analysis
Slope stability analysis will generally be
required for cut, fill, and natural slopes whose slope gradient is steeper
than two horizontal to one vertical (2:1), and on other slopes that possess
unusual geologic conditions such as unsupported discontinuities or evidence
of prior landslide activity. Analysis generally includes deep-seated and
surficial stability evaluation under both static and dynamic (earthquake)
Evaluation of deep-seated slope stability
should be guided by the following:
Evaluation of surficial slope stability should
be guided by the following:
The potential failure surface used in the
analysis may be composed of circles, planes, wedges or other shapes considered
to yield the minimum factor of safety against sliding for the appropriate
soil or rock conditions. The potential failure surface having the lowest
factor of safety should be sought.
Forces to be considered include the gravity
loads of the potential failure mass, structural surcharge loads and supported
slopes, and loads due to anticipated earthquake forces. The potential for
hydraulic head (or significant pore-water pressure) should be evaluated
and its effects included when appropriate. Total unit weights for the appropriate
soil moisture conditions are to be used.
Calculations may be based either on analysis
procedures for stability of an infinite slope with seepage parallel to
the slope surface or on another method acceptable to the lead agency. For
the infinite slope analysis, the minimum assumed depth of soil saturation
is the smaller of either a depth of one (1) meter or depth to firm bedrock.
Soil strength characteristics used in analysis should be obtained from
representative samples of surficial soils that are tested under conditions
approximating saturation and at normal loads approximating conditions at
very shallow depth.
Appropriate mitigation procedures and surface
stabilization should be recommended, in order to provide the required level
of surficial slope stability.
Recommendations for mitigation of damage to
the proposed development caused by failure of off-site slopes should be
made unless slope-specific investigations and analyses demonstrate that
the slopes are stable. Ravines, swales, and hollows on natural slopes warrant
special attention as potential sources of fast-moving debris flows and
other types of landslides. If possible, structures should be located away
from the base or axis of these types of features.
Amplification of Earthquakes
on Steep Landslide Slopes
Ashford and Sitar (1994) presented a method
to analyze topographic amplification of site response on slopes. They specifically
addressed the expected response of very steep slopes in weakly cemented
rock. Amplification was found to increase with inclined seismic waves traveling
into the slope crest. They found that the fundamental site period dominates
the seismic response of any given slope. The relationship between wave
length and slope height controls the degree of amplification. However,
as the slopes decrease in steepness (i.e., less than 30 degrees), the slope-induced
amplification becomes less and less important, and geologic contacts between
dissimilar strata appear to exert more influence on observed failures.
Mitigation of Earthquake-Induced Landslide
For any existing or proposed slopes that are
determined to be unstable, appropriate mitigation measures should be provided
before the project is approved. The hazards these slopes present can be
mitigated in one of three ways:
The zones of required investigation for earthquake-induced
landslides do not always include landslide or lateral spread run-out areas.
Project sites that are outside of a zone of required investigation may
be affected by ground-failure runout from adjacent or nearby slopes. Any
proposed mitigation should address all recognized significant off-site
hazards. If stabilization of source areas of potential off-site failures
that could impact the project is not practical, it may be possible to achieve
an acceptable level of risk by using one or more protective structures,
as suggested below.
Avoid the Failure Hazard: Where the
potential for failure is beyond the acceptable level and not preventable
by practical means, as in mountainous terrain subject to massive planar
slides or rock and debris avalanches, the hazard should be avoided. Developments
should be built sufficiently far away from the threat that they will not
be affected even if the slope does fail. Planned development areas on the
slope or near its base should be avoided and relocated to areas where stabilization
Protect the Site from the Failure:
While it is not always possible to prevent slope failures occurring above
a project site, it is sometimes possible to protect the site from the runout
of failed slope materials. This is particularly true for sites located
at or near the base of steep slopes which can receive large amounts of
material from shallow disaggregated landslides or debris flows. Methods
include catchment and/or protective structures such as basins, embankments,
diversion or barrier walls, and fences. Diversion methods should only be
employed where the diverted landslide materials will not affect other sites.
Reduce the Hazard to an Acceptable Level:
Unstable slopes affecting a project can be rendered stable (that is, by
increasing the factor of safety to > 1.5 for static and > 1.1 for dynamic
loads) by eliminating the slope, removing the unstable soil and rock materials,
or applying one or more appropriate slope stabilization methods (such as
buttress fills, subdrains, soil nailing, crib walls, etc.). For deep-seated
slope instability, strengthening the design of the structure (e.g., reinforced
foundations) is generally not by itself an adequate mitigation measure.
The stabilization method chosen depends largely
on the type of instability which is anticipated at the project site. The
two general techniques used to stabilize slopes are: (1) to reduce the
driving force for failure, or (2) to increase the resisting force. These
consist of different mechanisms, depending on the type of failures in question.
The following list is presented to provide a range of stabilization options,
but other options may be recommended provided analyses are presented to
prove their validity.
Rock and Soil Falls
Principal failure mechanism is loss
of cohesion or tensile strength of the near-surface material on a very
Reduce driving force by reducing the
steepness of the slope through grading, or by scaling off overhanging rock,
diverting water from the slope face, etc.;
Increase resisting force by pinning
individual blocks, covering the slope with mesh or net, or installing rock
anchors or rock bolts on dense spacing; and/or,
Protect the site from the failure by
constructing catchment structures such as basins, or protective structures
such as walls and embankments.
Slides, Slumps, Block Glides
Principal failure mechanism is loss
of shear strength, resulting in sliding of a soil or rock mass along a
rupture surface within the slope.
Reduce driving force, by reducing the
weight of the potential slide mass (cutting off the head of the slide,
or totally removing the landslide), flattening the surface slope angle
(‘laying back’ the slope face) through grading, preventing water infiltration
by controlling surface drainage, or reducing the accumulation of subsurface
water by installing subdrains; and/or,
Increase resisting force, by replacing
slide debris and especially the rupture surface with compacted fill, installing
shear keys or buttresses, dewatering the slide mass, pinning shallow slide
masses with soil or rock anchors, reinforced caissons, or bolts, or constructing
retaining structures at the edge of the slide.
Flows of Debris or Soil
Principal failure mechanism is fluidization
of the soil mass, commonly by addition of water and possibly by earthquake
Reduce driving force by removing potential
debris from site using grading or excavating procedures, or diverting water
from debris so that it cannot mobilize, by means of surface drains and/or
subsurface galleries or subdrains;
Increase resisting force by providing
shear keys or buttresses, together with subsurface drainage; and/or,
Protect the site from the failure by
diverting the flow away from project using diversion barriers or channels,
or providing catchment structures to contain the landslide material.
Copyright © California
Department of Conservation,
Division of Mines and Geology,
1997. All rights reserved.