Upper Bound Stability Analysis Of Slurry Trench With Centrifuge Model Test

Recent years have witnessed the rapid growth of urbanization in our country,the exploration of underground space has also gained increasing attention from the public.During the exploration of underground space,slurry-supported trench is a popular supporting structure for foundation engineering due to its high stiffness,high strength and low environmental impact.In the past,trench stability is usually assessed by previous engineering experience and simplified equations.As the depth of slurry trenches is increasing,the existing methods are no longer applicable.The focus of the present study is placed on the stability analysis of slurry-supported trenches by using upper bound stability analysis and centrifuge model test.The key points of this study is summarized as follows:(1)Centrifuge model tests on trench stability in undrained clay and sandy soil with confined water pressure are conducted,the failure pattern and underlying mechanism is studied.Soil undrained shear strength profile is obtained by T-bar tests.The effect of slurry density and confined water pressure on trench stability are investigated.The evolving pattern of ground settlement and water pressure are analyzed and the soil movement pattern is acquired by PIV technique.(2)The streamline-based upper bound stability analysis method is established,which solves the problem for construction of kinematically admissible velocity field in undrained clay.Based on the previous work,the general form of the streamline velocity field in rectangular coordinate,polar coordinate and three-dimensional curved coordinate system is proposed,and the exact form of the velocity field can be determined based on the specific problem.The usage of streamline velocity field is illustrated by examples of trench stability analysis and tunnel stability analysis.The applicability and problems of the present study is clarified by comparison with existing studies.(3)Upper bound trench stability analysis by spatial discretization technique in non-uniform undrained clay is established.The proposed method is verified by comparison with centrifuge test results and solutions from the finite element limit analysis.The discretization scheme discretizes the mechanism into several points,and velocity boundary condition is satisfied on these points.In this manner,mechanism geometry no longer needs to follow standard geometries such as horn or torus,and can better adapt to trench boundary conditions,which significantly improves the solutions of analytical torus mechanism.Spatial discretization technique is utilized to improve the accuracy of simplified translation mechanism for shallow tunnels and the possibility of construction combined mechanism based rotational movement is discussed.(4)The discretization based upper-bound analysis method is further extended to layered soils,trench stability in horizontally layered soils is discussed from both twodimensional and three-dimensional angle.The result of the present study is verified by comparison with shear strength reduction finite element method.The effect of slurry density,slurry depth,along with site conditions on trench stability and the most critical failure pattern are discussed in detail.The influence by the position of the weak layer on trench stability is also investigated.The applicability of the simplified methods by using weighted average parameters for trench stability evaluation in layered soils is also discussed in detail by comparison with the method of using original soil parameters.(5)By discussion on the analytical model of the horizontal slice method and comparison with the discretization-based rotational mechanism,the problems of the horizontal slice method for trench stability analysis are revealed and a wedge model based on an equivalent trench length in undrained clay is proposed.The connection between three-dimensional horizontal slice method and simplified methods based on sliding wedge method is revealed through analytical derivation.Horizontal slice method is adopted for case analysis on an engineering project in Shanghai and the most critical failure pattern is obtained,which can serve as a reference for the soil zone to be strengthened.