Influence of Rainfall Infiltration on the Reliability of Expansive Soil Slopes

Abstract eng:
Rainfall-induced slope failure is a common occurrence in many countries, especially for expansive soil slopes. Shear strength parameters and hydraulic properties of expansive soils are essential properties that affect slope failures. Expansive soils are unsaturated, and their shear strength parameters are effective cohesion (c'), effective internal friction (φ^') and an angle defining the increase in shear strength for an increase in matrix suction (φ^b). Hydraulic properties of expansive soils are defined by soil-water characteristic curve (SWCC) and saturated coefficient of permeability (k_s), and the SWCC can be obtained by some fitting methods. Adopting the three parameter model of Fredlund-Xing for fitting the SWCC, we can calculate the fitting parameters of a, n, and m of expansive soils. Considering the highly variability of these parameters, reliability analyses are needed for analyzing the stability of expansive soil slopes under rainfall infiltration. In this paper, the stability of an expansive soil slope is studied by using the first order reliability analysis method. Because there exists a relationship between φ^b and φ^' , and the volumetric water content is insensitive to the curve fitting parameter a, the basic variables used in the reliability analysis of the expansive soil slope are c^', φ^', n, m, k_s. In order to study the influence of the rainfall intensity on the stability of a fissured expansive soil slope, three cases with different rainfall intensity with the same rainfall duration time are studied. The reliability analysis shows that the reliability index is relatively high before the rainfall infiltration. Then, the reliability index decreases with the increasing of the time during the rainfall duration. When the rain stops, the reliability index decreases to its minimum value. Then, the reliability index increases gradually with the increasing of the simulation time. On the other hand, the reliability index decreases with the increasing of rainfall intensity for the same rainfall duration, but the extent of the decreasing of the reliability index becomes small with the increasing of rainfall intensity. There exists an extreme value of rainfall intensity for the same duration of rainfall. If the intensity of the rainfall is too large, some of the rainwater cannot infiltrate into the soil slope, and the excess rainwater will flow away along the slope surface. A comparison computation shows that the effect of the rainfall on the stability of the expansive soil slope with and without fissures is different.

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