Three dimensional slope stability analysis and failure mechanism

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Three dimensional slope stability analysis and failure mechanism

 

Author: Wei, Wenbing
Title: Three dimensional slope stability analysis and failure mechanism
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Soil stabilization.
Slopes (Soil mechanics)
Department: Dept. of Civil and Structural Engineering
Pages: xiv, 221 p. : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2239573
URI: http://theses.lib.polyu.edu.hk/handle/200/1662
Abstract: For slope stability problem, two-dimensional analysis is commonly used for simplicity, though all slope failures are three-dimensional (3D) in nature. There are only limited applications of 3D analysis due to the various limitations of three-dimensional slope stability methods. Recently, there are various important progresses for 3D analysis. 3D limit equilibrium method (LEM) for general asymmetrical problem together with innovative optimization method for locating the general critical 3D failure surface has recently been developed. In addition, 3D strength reduction method (SRM) can now be conducted within a tolerable duration. Until now, there is still a lack of detailed investigation of 3D slope stability failure mechanism and the application of 3D LEM and SRM under different 3D conditions. This study aims to conduct an extensive three-dimensional slope stability analysis and to investigate the failure mechanism under different situations. The 3D effect considered in this study includes the important factors such as slope geometry, boundary conditions, water, soil nail and pile. Both the LEM and SRM are conducted in this study, and some interesting differences between these two methods are discovered. It is concluded that both methods have their own merits and limitations, and a good understanding of these methods are required before a good solution can be obtained. The suitability of these two methods under different conditions is investigated and precautions when these methods are applied are suggested. In this study, some results which appear to be different from common understanding are obtained. Some of the results are also different from published studies, and careful investigations have revealed that the present detailed studies have provided a better and more reasonable understanding about the failure and stabilization mechanism. For example, for a simple slope extending to infinity, the critical slip surface is still basically two-dimensional until the external loading is large to induce a three-dimensional failure. The failure mechanisms due to the self weight of soil and external loads are actually different. The discretization domain required for SRM analysis is found to decrease with the increase of external loads which is also out of expectation. The distribution of tension force in soil nail is found to be influenced by the state of the slope (service state, limit state) and the failure modes (external failure, internal failure). In general, the line of the maximum tension may not correspond to the critical slip surface as commonly believed, except when the failure mode is an internal tensile failure. For slope supported with one row of piles, the slip surface is divided into two parts when the pile spacing is very small, and these two parts gradually become connected to form a clear single slip surface mechanism with the increase of pile spacing. The point of maximum shear force in the pile which is commonly used to determine the location of the slip surface in traditional design is found to be not a valid assumption, and this is important for design of slopes reinforced with piles. Some engineers have questioned the disturbance of the seepage pattern due to the presence of soil nails and piles. Such blocking effect is however found to be negligible. With the detailed study on 3D failure and stabilization mechanism, a clearer and better understanding of three-dimensional slope failure has been achieved in the present study.

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