Slope Stability

Slope Stability

Oklahoma Transportation Center

PI: Jerry Miller (OU)

co-PI: Amy Cerato (OU) Rifat Bulat (OSU)


Introduction

Shallow slope failures in roadway cuts and on embankments are frequent problems along Oklahoma highways, and most other States for that matter; and they and represent a significant burden on maintenance budgets. Often these failures are associated with clayey soils having relatively high plasticity. Generally, during construction these soils have relatively high shear strength, a stiff consistency, and produce stable slopes. However, over time the soils experience cyclic wetting and drying resulting in a net increase in soil moisture content and corresponding decrease in shear strength. Eventually, the reduction in shear strength results in a slope failure usually triggered by a rainfall event. Desiccation cracks exacerbate the problem that develops during extreme drying periods allowing water to penetrate the soil deeper and faster.

The loss of shear strength in clayey soils due to wetting is associated with two inter-related phenomena. First, as the moisture content is increased the matric suction is reduced, which reduces the intergranular or effective stress in the soil. Decreasing effective stress equates to reduced shear strength. Second, as the moisture content increases the diffuse double layers surrounding clay particles expand and take on water with a corresponding increase in soil void ratio. Reduced dry density and increased water content results in softening and substantially lower shear strength. An additional consequence of wetting is that the driving mass of the soil increases with increased moisture content. Thus, while shearing resistance (or strength) is decreasing, imposed gravitational shearing stresses are increasing, further reducing the slope stability.

Research is needed so engineers can better understand the problem, better predict shallow slope stability, and implement preventive measures if necessary. Proposed research will examine the mechanics of the soil in shallow slopes as related to matric suction changes, soil type, and expected degree of wetting. Research will involve studying at least two field cases where shallow slope instability has been a problem; at least one case will involve a cut slope section and one case will involve an embankment slope. Successful completion of this research will provide engineers with tools for improved analysis of shallow slope stability and recommendations for preventing landslides.

 

Goals of Proposed Research

There are three primary goals of the proposed research:

1) To provide geotechnical engineers with a method for predicting stability of cut slopes and embankment slopes composed of unsaturated soil, incorporating soil moisture condition and suction into the analysis. The focus is on high plasticity clays for which these problems are most prevalent.

2) To provide geotechnical engineers with methods for predicting changes in soil moisture conditions and suction in said slopes as a function of climate changes so that a proper “design moisture condition” can be selected. This will also all for predicting the slope stability over time based on predicted moisture content changes.

3) To provide recommendations to minimize the climate impacts on slope stability including, as necessary, reducing adverse impacts of desiccation cracking in clayey materials.

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