The Basics of Slope Stability Analysis for Geosynthetics

Slope stability analysis is vital to preventing slope failures in a variety of engineering applications including landfill design, roads, dams and embankments, to name a few. Proper analysis anticipates both natural and manmade slopes and failure of foundations and retaining walls. This blog covers the necessity of slope stability analysis, how to conduct the analysis and the role of geosynthetics in slope stability.

Slope Stability Meets Four Key Objectives

The geometry of the slope, incorporated materials, soil, environmental (soil gasses) and geological forces (seismic and groundwater) are only a few of the factors to consider in determining slope stability. A careful analysis can assess the likelihood of sliding or collapse of the slope, its impacts and proper reinforcement techniques.

The correct geosynthetic reinforcement must be determined to meet both long-term strength and soil interaction requirements. Failures commonly occur at interfaces, especially where geotextile and soil interact. Therefore, a stability analysis can evaluate the compatibility of the slope with a geosynthetic material as well as ensure the safe design of a slope.

Slope stability analysis accomplishes four key objectives: it determines the long-term survivability of existing and excavated slopes, evaluates the effectiveness of proposed reinforcements, calculates shear strength and designs a successful slope.

Conducting Slope Stability Analysis

In general, there are two types of slope failures; global and veneer. Global slope failure has been commonly observed to follow the arc of a circle. In some circumstances, failure will occur in a predominantly horizontal direction. Most techniques for stability analysis operate on the assumption of a circular failure arc. There are many different ways to conduct slope stability analysis, and they all come with their own benefits and limitations. In the past, engineers used calculators and graphs, but now software is widely accepted as the standard for slope calculation. Most software is based on the limit equilibrium method of analysis.

Limit equilibrium analysis is the most common technique and includes several different variations. It requires the crucial surface to be chosen and is typically assumed to resemble an arc. It judges the equilibrium of the soil on the slope by assessing failure mechanisms such as internal stresses, strains, and lateral displacements.

Slope stability analysis determines stability based on a “factor of safety.” The factor of safety is basically a ratio of material shear strength properties to applied shear stress. It can also be thought of as a ratio of the maximum load a slope can sustain to the actual load that is applied. The factor of safety can be determined using cohesive and friction interface strengths. Cohesive forces are typically ignored unless a strength-based analysis is used instead of sole reliance on friction interface angles.

Global stability and geosynthetics interact in base-liner situations where a failure plane resembling an arc can develop. This can be a very complicated calculation involving slope geometry, internal shear strength of the overlying and underlying mass, and veneer shear strength of the geosynthetics involved. For more complex slope analyses, methods other than limit equilibrium such as finite element analysis will be needed.

The Important Role of Geosynthetics

Limit equilibrium principles still apply to veneer stability situations. Veneer stability can essentially be thought of as a “sandwich” of geosynthetic and soil materials. Selection of the correct geosynthetics will provide increased stability and endurance to a constructed slope for environmental protection projects. The design objective in geosynthetic design is to drive the failure plane into the subgrade soils. This allows for steeper slopes (as allowed by the native soil properties) that can sustain longer durations of wear and tear without sliding or collapse.

Steepened slopes lined with a properly designed geosynthetic system can increase land usage for environmental projects, hence reducing land capital and development costs. Agru America’s MicroSpike® Liner provides high veneer shear against cohesive and non-cohesive soils as well as geotextile and geocomposite materials. In addition to MicroSpike, where stronger interface forces are required, Agru’s Super Gripnet® and Grip Liner® products provide exceptional shear strength properties, especially against sandy type subgrade soils which are the most common an engineer will encounter versus a pure clay soil.

Agru’s liner products have consistent thickness due to the flat die extrusion process that gives them consistent properties. It has the best thickness control of any geomembrane in the industry, and the consistent surface structuring gives Agru’s geomembranes reproducible friction angle values with narrow confidence intervals.

For capping applications, Agru also offers the ClosureTurf® geosynthetic final cover system as an alternate to the standard closure capping systems. ClosureTurf avoids nearly all the slope stability issues associated with final cover stability. Call an Agru Technical Services representative at 1 (800) 373-2478 for more information.