The failure mechanism and influence area of granular soil slopes under dry and rainfall conditions
Date Issued
2012
Date
2012
Author(s)
Kuo, Kwo-Jane
Abstract
The purpose of this study was to understand the failure mechanism of granular soil slopes and the influence area resulting from the slope failures. Two kinds of model tests were carried out, i.e., sliding of dry granular soil slope and the slope failure caused by rainfall infiltration. Combined with the test results, numerical analysis was also employed and the software used was PFC (Particle Flow Code), which employs discrete element method to simulate the interaction of soil particles.
For the sliding of dry granular soil slope, tests were conducted under various combinations of slope angle, slope height, and the relative density of soil. The displacement of the soil was measured by a particle image velocimetry (PIV). These series of images clearly displayed the movement and development of granular flows. The final profile of the soil and the shape of deposition area were then measured. Based on the measured displacement of flow, the front velocity of the flow was obtained. Among various parameters, slope angle and the internal friction angle of soil are found to be the most significant factors influencing the front velocity. Furthermore, a simplified equation based on Newton’s law of motion and energy conservation is developed to predict the run-out of flow. Comparison is then made between the measured and predicted run-out distances, and they are generally in good agreement. Finally, the profile of slope after failure can be simplified and represented by the friction angle of the soil.
For slope failure caused by rainfall infiltration, the model test was conducted on slopes with convex and concave profiles, and the slopes were non-homogeneous and had a dip stratum underneath. In addition, different fines contents (0-12%) and rainfall intensities (78 and 287 mm/hr) were considered as variables. During the experiment, variations in pore water pressure and volumetric water content in the soil were measured. The characteristics of the failure mechanism and the responses of pore pressure and water content in the model slopes are compared and discussed as functions of variables. For samples with fines content of 12%, failure was more likely initiated by surface erosion. In contrast, for samples with fines content less than 10%, slope failures were initiated near the toe. The failure length and the time when piping was observed also varied with fines content and rainfall intensity. The reason could be due to the samples with more fines content having lower friction angle and higher unit weight.
The profiles of total head above the impervious stratum were generally nonlinear and the pressure head near the toe of the slope could be even higher than the equivalent head of overburden pressure. Moreover, the initial failure in concave slopes occurred sooner than that in convex slopes; it was attributed to the thin soil layer near the toe of the slope, so that a high hydraulic gradient was induced to cause piping to be initiated. It is also note worthy that these ratios for failure length and run-out distance under high and low rainfall intensities approximate the ratio of rainfall intensity. This result also implies that the rainfall would have caused as much soil to fail as it could if there was enough soil.
In the sensitivity analysis on micro-parameters, the coefficient of friction of the soil was found to be the most critical. Furthermore, with the proposed values of the micro-parameters from this study, the error in predicting run-out distance is less than 10%. The simulation of a real slope failure was also in good agreement. Hence, this numerical model is suitable to be applied in real cases to estimate the velocities of a moving mass, its impact force on structures, and the travel distance and time of the soil mass.
The stability of a slope under seepage is mainly influenced by the permeability of the soil, which in turn is proportional to the square of the effective size of the soil particle. Therefore, geometrical similarly must be considered in the numerical analysis on the seepage velocity and the permeability because the size of soil particle is a dominant factor. The result from the numerical analysis shows that the strength of unsaturated soils has great influence on the failure mechanism of the slope, in which the matric suction can be simulated by the micro-parameter of parallel bonds.
Keyword: slope, granular soil, model test, failure mechanism, PIV analysis, PFC numerical analysis.
Subjects
slope
granular soil
model test
failure mechanism
PIV analysis
PFC numerical analysis
SDGs
Type
thesis
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