Failure mechanism of slope under several conditions by two-dimensional and three-dimensional distinct element analysis

Abstract

Landslide is a major disaster resulting in considerable loss of human lives and property damages in hilly terrain in Hong Kong, China and many other countries. The factor of safety and the critical slip surface for slope stabilization have been the main considerations for slope stability analysis in the past, while the detailed post-failure conditions of the slopes have not been considered in sufficient details. There are however increasing interest on the consequences after the failure which includes the amount of failed mass and the runoff and the affected region. To assess the development of failure of slope in more details and to consider the potential danger of slopes, the slope stability problem is analyzed by the distinct element method (DEM) in the present research. There are very few studies about slope stability using the DEM in the past due to various difficulties in DEM modeling. To investigate the progressive failure of slope, the author has managed to model the complete failure processes under several important conditions by particle flow analysis, and important new results have been obtained in the present study. In this study, Particle Flow Code (PFC) is used for the detailed investigation of the failure mechanism of slopes. Large displacement simulation of slope failure by distinct element analysis using PFC has been carried out for both two-and three-dimensional analysis to reveal the failure patterns of simple slope under the effect of self-weight, reinforced action of soil nails, buoyant force and seepage of water flow, local surcharge and also curvature effect on curvilinear slope. In two-dimensional DEM analysis, firstly, it is found that for a slope with cohesionless soil, failure firstly occurs at the crest of the slope, and the failure gradually extends to the base of the slope until finally the slope angle is equal to the friction angle of the soil and slope is completely collapsed. The analysis successfully simulates the whole flow process of simple slope failure, and the modeling includes the flow of sand from crest to toe of slope. The large scale soil movement and major change of slope geometry have revealed some important stress change and soil movement due to soil failure and flow of soil not considered in the past which is one useful new contribution in this study. Secondly, soil nails can effectively reinforce the slope stability, especially when nail head is used, and the overall stability is greatly enhanced which is demonstrated by the numerical analysis using distinct element method in this study. It is demonstrated that the soil nail can greatly stabilize the stress change along the potential failure surface even when large scale soil movement is mobilized which is another new contribution. Moreover, four influencing factors are considered in the study of the failure mode by rainfall induced water flow as followings: buoyant effect of ground water, seepage effect, strength reduction during slope failure and persistent rainfall effect. The simulation results show that the slope crest is scoured smoothly under continuous effect of rainfall induced water flow. More obvious settlements occur at the top of the slope, and extended failure zone and noticeable upheaval at the toe are developed compared with normal failure. Thus rainfall induced water flow can accelerate slope failure with an obvious decrease in the stability of the slope leading to collapse at the end. The detailed stress change and large soil movement under these cases are important and useful innovative works with no previous study. Three-dimensional DEM analysis possesses many major difficulties and is not commonly considered. Practically, most of such studies are devoted to qualitative study with very few quantitative studies at present. In the present three-dimensional DEM analysis, the author has carried out a tedious and very time-consuming analysis together with laboratory test aiming at quantitative slope failure analysis which is an innovative and new work. The comparison of the physical modeling and numerical simulation indicates that the relations between the force against displacement curves are basically similar between the physical model test and the numerical model, and the final failure loadings from the physical model test and the numerical model are also very close which is a success seldom accomplished in DEM studies. For the three-dimensional curvature influence on slope stability with or without localized loading, the study shows that the curvature defined by R0/H0 (where R0: radius of curvature, H0: height of slope) has a beneficial effect on the global stability of concave slopes, especially when the resistance of soil is relatively high and also arching effect is illustrated clearly for concave slope. As a result, only partial/localized failure is initiated in concave slope under local loading. These studies which are seldom considered in microscopic scale in the past are both difficult to be considered as well as useful in giving clearer insights about the development of failure, microcosmic failure mechanism and the post-failure mechanism of slope. The various findings from the present work are pioneer works in slope stability analysis and they are very useful and important to the better understanding of the failure and post-failure condition of a slope.