Characteristic Of Unsaturated Soil And Slope Stability Analysis Considering Antecedent Rainfall

The landslide caused by slope instability, together with earthquake and volcano, are the three most frequent geological disasters. Landslides are triggered by many factors, in which the rainfall-induced landslides occupy 90% of all the landslides records. Rainfall-induced landslides severely restrict the development of economic, and throw a direct threat to the lives and properties of the people. Though numerous investigations have been conducted on rainfall-induced landslides, the mechanism of rainfall-induced landslides is far from attaining complete comprehension at present. The factors controlling the disaster are not fully discovered, especially, the research on effects of antecedent rainfall on slope stability is insufficient. This thesis, focus on the strength and deformation properties of unsaturated soil, first studies the estimation of suction in the unsaturated soil using filter paper technique. Then, the compressibility of unsaturated soil is modeled. Finally, the influence of antecedent rainfall on slope stability is explored. The details and conclusions of the work are summarized as follows.(1) The background and the significance of the work are presented first. A literature review of researches on rainfall-induced landslides, including aspects of the mechanism of rainfall-induced landslides, the method of numerical simulation, and the effect of antecedent rainfall on slope stability, is then conducted. The researches on the characteristic of unsaturated soil, including the constitutive model, the influence of matric suction on unsaturated soil and the estimation of the matric suction, are summarized finally.(2) Beginning from the stress state variables, the independent stress approach and the effective stress approach are investigated. Some relative equations are derived. The pros and cons of the two approaches in describing the characteristic of unsaturated soil are compared. The most frequently used soil water characteristic curves, the permeability functions and the shear strength theory of unsaturated soil are summarized.(3) The test data of three types of homemade "Double Circle" filter papers from the literature are categorized. By analyzing the data, new calibration equations which are more applicable are raised. Results show that the bilinear equations can fit well the relationship between suction and the water content of "Double Circle" filter papers. Same equation can be used to calibrate both matric suction and total suction for same kind of filter paper. The test data of No.201 and No.203 filter papers are sufficient and consistent enough, thus the calibration curves are reliable.(4) The problem of volume change of unsaturated soils is studied to quantify how the compression index ?, which is the slope of the NCL, varies with the change of saturation. The empirical model of NCL for unsaturated soil proposed in the literature, in which the ? is dependent on saturation, is studied in detail. Parameters in the empirical model are estimated by fitting the test data of five different soils taken from literature. Emperical equations are built to relate the parameters in the empirical model to the average particle size parameter Dso. Comparison between values of ? calculated from test data and estimated using empirical equations suggests that the regression equations perform well.(5) Based on the daily rainfall data in the Three Gorge Reservoir area, GeoStudio is used to investigate the effect of different initial conditions on slope stability. The method to get the initial condition, which can reflect the different response of initial pore water pressure distribution of different soils to the climate environment, is suggested. Then, the sandy and clayey slopes are used as examples to investigate the effect of antecedent rainfall on slope stability. Longer than 15-day Antecedent rainfall should be considered in the slope stability analysis.(6) A homogeneous slope model is taken as an example to investigate the effects of antecedent rainfall patterns and soil-water characteristic curves (SWCCs) on pore water distribution and slope stability. The effect of antecedent rainfall pattern on slope stability is studied using four different types of antecedent rainfall. The effect of the shape of SWCC on slope stability is investigated by keep the saturated permeability constant while the other parameters of SWCC varies in the Fredlund-Xing model.