Slope Stability Analysis, Design, and Repair

Alpha Adroit Engineering Ltd provides geotechnical engineering expertise and materials testing services for slope stability analysis, slope design, slope failure investigation, and slope and landslide repair. Alpha Adroit also provides consulting services for preventive and remedial measures for small scale and large scale landslides throughout Alberta, British Columbia, Saskatchewan, and Northwest Territories. Please contact us hereshould you have any enquiries.

This article provides a brief description on the required knowledge and analysis techniques for slope stability analysis, slope failure repair, and other geotech aspects related to manmade and natural slopes. This article is not intended to be comprehensive or all-inclusive.

Knowledge Required for Slope Stability Analysis

For slope stability analyses to be useful, they must represent the correct problem, correctly formulated (modelled). This requires:

mastery of the principles of soil and rock mechanics;
accurate knowledge of geology and site conditions (including also topography, stratigraphy, groundwater conditions, etc); 
accurate knowledge of the properties and mechanical behavior of the soils and rocks at the site; and,
 expertise in conducting correct and accurate numerical and computational analysis for Slope Stability Analysis purposes. 

Analysis techniques

The difference between undrained and drained conditions in slope stability analysis is time.

Undrained signifies a condition where changes in loads occur more rapidly than water can flow in or out of the soil. The pore pressures increase or decrease in response to the changes in loads.

Drained signifies a condition where changes in load are slow enough, or remain in place long enough, so that water is able to flow in or out of the soil, permitting the soil to reach a state of equilibrium with regard to water flow. The pore pressures in the drained condition are controlled by the hydraulic boundary conditions, and are unaffected by the changes in loads.

Analyses of drained conditions

Drained conditions are those where changes in load are slow enough, or where they have been in place long enough, so that all of the soils reach a state of equilibrium and no excess pore pressures are caused by the loads.

In drained conditions pore pressures are controlled by hydraulic boundary conditions. The water within the soil may be static, or it may be seeping steadily, with no change in the seepage over time and no increase or decrease in the amount of water within the soil. If these conditions prevail in all the soils at a site, or if the conditions at a site can reasonably be approximated by these conditions, a drained analysis is appropriate.

A drained slope stability analysis is performed using:

Total unit weights
Effective stress shear strength parameters
Pore pressures determined from hydrostatic water levels or steady seepage analyses

Analyses of undrained conditions

Undrained conditions are those where changes in loads occur more rapidly than water can flow in or out of the soil. The pore pressures are controlled by the behavior of the soil in response to changes in external loads. If these conditions prevail in the soils at a site, or if the conditions at a site can reasonably be approximated by these conditions, an undrained analysis is appropriate.

An undrained slope stability analysis is performed using:

Total unit weights
Total stress shear strength parameters

Progressive Failure

An "implicit assumption" of limit equilibrium slope stability analyses is that the soils exhibit ductile stress-strain behavior (some may consider it as an intrinsic deficiency of this method).

Progressive failure is a strong possibility in the case of excavated slopes in overconsolidated clays and shales, particularly stiff-fissured clays and shales. These materials have brittle stress-strain characteristics, and they contain high horizontal stresses, often higher than the vertical stress.

When an excavation is made in stiff fissured clay or shale, the excavated slope rebounds horizontally; shear stresses are very high at the toe of the slope, and there is a tendency for failure to begin at the toe and progress back beneath the crest. With time, the slope would continue to rebound into the cut, due to a delayed response to the unloading from the excavation, and possibly also due to swelling of the clay as its water content increases following the reduction in stress.

Progressively, through this process, failure would spread around the slip surface, without ever mobilizing the peak shear strength simultaneously at all points along the slip surface.

Because progressive failure can occur for soils with brittle stress-strain characteristics, peak strengths shall not be used for these soils in limit equilibrium analyses; using peak strengths for brittle soils in slope stability analysis can lead to inaccurate and unconservative assessment of stability.

Experience with slopes in overconsolidated clays, particularly fissured clays, has shown that "fully softened strengths" are appropriate for these materials in cases where slickensides have not developed, and "residual strengths" are appropriate in cases where slickensides have developed.

Loading Conditions Frequently Considered for Earth Slopes

End-of-construction stability

The end-of-construction stability analysis is analyzed using drained or undrained strengths, depending on the permeability of the soil.

Long-term stability analysis

The analysis reflects conditions after swelling and consolidation are complete and is analyzed using drained strengths and pore water pressures corresponding to steady-state seepage conditions.

Rapid drawdown

This condition removes the stabilizing effect of external water pressures and subjects the slope to increased shear stress. Either drained or undrained strengths are used, depending on the permeability of the soil.

Earthquake

Earthquakes subject slopes to cyclically varying stresses and may cause reduction in the shear strength of the soil as a result of cyclic loading. Shear strengths measured in cyclic loading tests are appropriate for analyses of stability during earthquakes.

Staged construction

Slope stability analysis for staged construction of embankments requires consolidation analyses to estimate the increase in effective stresses that results from partial consolidation of the foundation.

Surcharge loading

Depending on whether the load is temporary or permanent, and whether the soil drains quickly or slowly, undrained or drained strengths may be appropriate. If the surcharge loading occurs shortly after construction, the undrained strengths would be the same as those used for end-of-construction stability.

However, if the load is imposed some time after construction, and the soil has had time to drain (consolidate or expand), the undrained strengths may be different and would be estimated using the same procedures as those used to estimate undrained strengths for rapid drawdown.

Partial submergence and intermediate water levels

For the upstream slopes of dams and other slopes where the level of an adjacent body of water has an influence on stability, the lowest water level usually produces the most adverse conditions.

In the case of slopes that contain zones of materials with different strength characteristics, the factor of safety of the upstream slope may be lower with a water level at some elevation between the top and the toe of the slope. The most critical water level for these conditions must be determined by repeated trials.

 

Back-analysis

Back-analysis (or back-calculation) is the process of determining the conditions and establishing a suitable model of the slope from a slope failure case.

Useful information on the conditions in the slope at the time of the failure can be understood when a slope fails due to sliding: at the time of failure the factor of safety against sliding is unity (1.0). This knowledge and an appropriate method of analysis can be used to develop a model of the slope at the time it failed. The model will consist of the unit weights and shear strength properties of the soil(s), groundwater and pore- water pressure conditions, and the method of analysis (including failure mechanisms). Such a model can be used for:

understanding and evaluating the cause(s) of failure (forensic engineering)
back-calculating material properties at the time of failure
analysis, design, and evaluation of the effectiveness of the remedial measures

One should note that:

  • Only one strength parameter (c, c' or Φ, Φ') can be calculated by back-analysis if the location of the slip surface is not known

  • Back-calculation of an average shear strength expressed as a cohesion, c (Φ = 0) can produce misleading results when a slope has failed under long-term drained conditions

  • Each combination of cohesion and friction angle that produces a factor of safety of 1 produces a unique location for the critical slip surface. Accordingly, the location of the slip surface can be used to calculate unique values for both cohesion (c, c') and friction angle (Φ, Φ')

  • Use of the location of the slip surface to back-calculate both cohesion and friction has had mixed success and does not seem to work when there is significant progressive failure or distinct layering and inhomogeneities in the slope

  • The degree of uncertainty in the back-calculated shear strength parameter will be no less than the degree of uncertainty in all of the other variables that affect the stability analysis. In fact, the back-analysis should actually be conceived as a back-analysis to determine all of the variables that are applicable to the failure, rather than only shear strength parameters. To reduce the uncertainty in this determination it is important to utilize all the information that is known or can be estimated by other means prior to performing the back-analysis, such as:

knowledge of the shear strength of soils and how the shear strength varied with depth

knowledge of drained and/or undrained conditions

knowledge of slip surface(s)

knowledge of historical slope and groundwater events

knowledge of historical human and seismic activities at the site

knowledge of groundwater and pore-water pressures in soils

knowledge of natural slope conditions and the movements that had already taken place (residual strength?)

 

Slope Stabilization and Repair

The causes and the nature of a slope failure should be understood before embarking on repair action. When investigating what caused a slope failure, one shall remember that there may be more than a single cause.

Thorough geological study and detailed exploration are the first steps to investigate slope failures. Topographic surveys and measurements on surface markers help to define the area affected and the magnitudes of vertical and horizontal movements. The location of the shear zone can often be determined using test borings, accessible borings, trenches, or slope indicators. Piezometers and observation wells can be used to determine groundwater levels within the slope.

When soil strengths and other conditions have been assessed through back-analysis, it "might be" justified to use lower-than-conventional factors of safety for the stabilized slope.

Factors governing selection of method of slope stabilization and repair

In choosing among the methods that are technically feasible, the following factors need to be considered:

  What is the purpose of stabilizing the slope? to prevent further large movements? or to restore the capacity of the moving ground?

  How much time is available?

— How accessible is the site, and what types of construction equipment can be mobilized there?

  What would be the cost of the repair?

 

Commonly used methods of stabilization include:

  Drainage is by far the most frequently used means of stabilizing slopes. It can be used alone or in combination with other methods and often provides effective stabilization at relatively low cost.  Drainage improves slope stability in two ways:

(1) it reduces pore pressures within the soil, thereby increasing effective stress and shear strength; and

(2) it reduces the driving forces of water pressures in cracks.

 Flattening a slope reduces the shear stresses along potential sliding surfaces, increasing the factor of safety.

Prestressed anchors and anchored walls do not require slope movement before they impose restraining

Conventional gravity retaining walls, mechanically stabilized earth (MSE) walls, and soil nailed walls, which are not prestressed, must move before they can develop resistance to stabilize a landslide.

Piles or drilled shafts that extend through a sliding mass, into more stable soil beneath, can be used to improve slope stability. A combination of limit equilibrium slope stability analyses and p-y analyses can be used to design piles or drilled shafts to achieve the desired increase in factor of safety of the slope.

Slopes have been stabilized using lime piles, grouting with cement, vegetation, thermal treatment, and construction of a reinforced concrete bridge on the ground surface, followed by excavation of soil from beneath the bridge to unload the head of a landslide.

When a sliding mass has been disturbed significantly as the result of slope movement, and the slide area must support structures or pavements, it may be necessary to excavate and replace the entire sliding mass. Excavation and replacement, with good compaction and drains beneath the fill, provide a very reliable means of restoring full usefulness to a slide area. The cost of the method is large when the surface of sliding is deep beneath the ground.

Alpha Adroit Engineering Ltd provides:

Stability analysis of natural and man-made slopes including excavations, embankments, dams, dykes, dumps, and fills

Design of shallow and deep open excavations, open pits, waste dumps, and high embankments

 Analysis of structural stability of rock masses and global stability of soil, rock, ore, and waste masses

Advanced coupled stress-deformation, seepage, seismic, and limit-equilibrium analysis

Design of alternative instability prevention and landslide remediation measures (45 methods available)

Instrumentation, monitoring, and Observational Method based analysis and design

Forensic geotechnical engineering (failure investigation and expert witness) services

Some of Alpha Adroit's high profile slope stability analysis and remediation projects include several slope stability studies, analysis, and remediation design projects for Riverbend slope failures in Edmonton, Alberta, re-development of old sites abutting North Saskatchewan River, new land developmends abutting creeks, and forensic investigation of landslides (failure investigation and expert witness).  

We provide our slope stability analysis, design, and repair services throughout Alberta, British Columbia, Saskatchewan, and Northwest Territories. We also provide services in international locations directed through our main office in Edmonton, Alberta. Major cities served include: Edmonton, Calgary, Red Deer, Fort McMurray (ABAlberta), Vancouver (BCBritish Columbia), and Saskatoon (SKSaskatchewan).

 

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