Geohazard Potential Of Rainfall Induced Slope Failure On Expansive Clay

Abstract

Each year, rain induced slope failures cause significant damages in highway infrastructures and environments, as well as tragic losses of human lives around the world. Rainfall-induced slope failure is a common problem in areas with slope constructed on high plasticity clays. These post failure costs can be significantly reduced if precautions are taken ahead of time. Development of an early hazard warning system based on weather forecast data can help identify potential slopes susceptible to failure due to rainfall. But development of such system will require a better understanding of the in situ behavior of soil due to rainwater infiltration as well as changes in shear strength characteristics of the soil. Slope stability studies in different parts of the world have indicated that infiltration of rain water into the soil has an adverse effect on the stability of earth slopes. During the infiltration process, the matric suction in the unsaturated soil slope decreases as the saturation increases with time, thereby reducing the shear strength of the soil. Therefore, it is important to identify the depth of moisture variation zone (i.e., active zone), along with field infiltration behavior, to accurately predict the response and stability of earth slopes constructed on expansive clay when exposed to a rainfall event.The objectives of this research are to determine, 1) the active zone of expansive clayey soil, 2) changes in moisture content and matric suction of soil slopes constructed on expansive clay due to infiltration of rainwater, 3) effects of rainwater infiltration on soil shear strength, 4) modeling and determining geohazard potential of soil slopes due to rainwater infiltration and finally 5) recommendation for future study. A field instrumentation program was undertaken to determine the active zone and study the infiltration behavior of embankment slope constructed on high plasticity expansive clay. An experimental program was developed to study the soil water retention characteristics and associated shear strength for different suction values. The results from laboratory testing and field instrumentation was combined with numerical modeling to study the effect of rainfall infiltration and associated geohazard potential of slope constructed on expansive clay. Results obtained from the field instrumentation, indicates that the variation in the moisture content and matric suction were different at different depth. The maximum variation often occurred near the ground surface (i.e. at 1.2 m depth) and the magnitude of variation decreased with increase in depth. Highest infiltration often occurred near the crest of the slope due to flow of water in the vertical direction which then becomes interflow within the slope. Presence of cracks at the crest also accelerated the ingress of water into the slope during rainfall events. The depth of active zone up to which moisture variation occurs was observed to be 3.6 m. Therefore, reduction in soil shear strength due to cyclic variation of weather condition is limited to a depth of 3.6 m which also matches close to the observed failure depth (3.04 m) on slopes constructed on high PI clay. Laboratory results on soil water characteristic curve (SWCC) showed that, SWCC of expansive clay yields higher air entry value and lower desaturation rate when compared with no volume change assumption during SWCC determination. Specimens compacted wet of optimum do not strongly depend on the applied stress history due to identical micro-structure formation. On the other hand, SWCC of expansive clay, compacted dry of optimum water content shows a shift to the right with high net normal stresses which indicates increase in air entry value. Results obtained from suction controlled ring shear tests indicate that both net normal stress and matric suction has significant influence on peak and residual strength of expansive clay. Both peak and residual strength increases with increase in net normal stress and matric suction. Results obtained from direct shear tests on saturated samples indicated that, shear strength of high PI expansive clay decreases when subjected to cycles of wetting and drying. The value of cohesion completely disappears due to wet dry cycles leading to shear strength condition at normally consolidated state.Based on the numerical modeling using PLAXIS, low intensity long duration rainfall was found to be most critical for expansive clay under current study which is consistent with the results previously reported for soils with low permeability. Effect of rainfall return period was found to be insignificant for the current study. Stability analyses performed for different rainfall event showed that, use of fully softened strength for active zone can reduce the factor of safety as low as twice the value as compared to the as compacted strength.