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Subsurface Water
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: 
  1. Surface observations are essential in determining the effect of subsurface water on landslide instability.
  2. Periodic or seasonal influx of surface water to subsurface water will not be detected unless subsurface water observations are conducted over extended time periods.
  3. 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.
  4. Ponding of surface water anywhere on the landslide may cause increased infiltration of water into the landslide and should be investigated.
  5. Disruption of surface water channels and culverts may also result in increased infiltration of surface water into the landslide.
  6. 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.
  7. 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.
  8. 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.
  9. Borings should never be the only method of subsurface water investigation; nevertheless they are a critical component of the overall investigation.

Slope Stability Analysis

General Considerations

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) loading conditions. 

Evaluation of deep-seated slope stability should be guided by the following: 

  1. 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.
  2. 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.
Evaluation of surficial slope stability should be guided by the following: 
  1. 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.
  2. Appropriate mitigation procedures and surface stabilization should be recommended, in order to provide the required level of surficial slope stability.
  3. 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 Hazards

Basic Considerations

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: 
  1. 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 is feasible.
  2. 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.
  3. 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 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. 

Stabilization Options

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 steep slope. 
Mitigation Strategies
  1. 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.;
  2. 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,
  3. 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. 
Mitigation Strategies
  1. 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,
  2. 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 shaking. 
Mitigation Strategies
  1. 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;
  2. Increase resisting force by providing shear keys or buttresses, together with subsurface drainage; and/or,
  3. 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.