Failure Mechanism And Stability Analysis Method Of Soil With Vertical Free-face Between Supporting Piles

Of the many types of retaining structures, supporting piles are widely used in slope engineering and foundation pit engineering, since they have the advantages of large lateral resistance, ease for construction and limited disturbance to environment. Supporting piles is a kind of horizontal discontinuous retaining structures. The superiority of discontinuous retaining structures includes utilizing the self-supporting capacity of soil sufficiently and enlarging the embed depth of supporting piles to improve the efficiency of loads transfer to deep strata. Due to the existence of vertical free-face, how to ensure the stability of soil between supporting piles becomes a problem need to be solved by researchers and engineers.International and domestic scholars have achieved great progress in the research of discontinuous soil retaining system, but the study on local failure phenomenon of soil between piles is insufficient, the understanding of failure mechanism and influencing factors of soil between piles is not comprehensive, and the stability analysis methods of soil behind vertical free-face between piles is needed for engineering application.This thesis utilizes the theoretical analysis method, numerical simulation method and the shaking table model test method to study the failure mechanism and stability analysis procedure of soil with vertical free-face between two adjacent supporting piles, main content and achievements include:① Three kinds of failure modes of soil between supporting piles are generalized from engineering cases and model experiment phenomena. The causes, formation mechanisms, recognition features and engineering countermeasures of each failure mode are discussed. This laid a foundation for scientific studying the failure mechanism and establishing a specific stability analysis method for soil between piles.② The spatial distribution regularities of supporting stress field are obtained by mechanics analysis method. The supported soil is divided into direct restriction zone and indirect restriction zone according to the mechanism of soil retaining and the characteristic of non-uniformity supporting stress distribution. The features of indirect restriction zone are analyzed from the perspectives of stress diffusion and soil arching effects. An identification method of the boundary of potential failure zone between two adjacent piles is proposed.③The formation mechanism of three-dimensional sliding wedges between supporting piles are analyzed with the consideration of strain localization property and shear band extension mechanism. A three-dimensional sliding wedge model corresponding to the instability mode of soil between supporting piles is presented. An index system and corresponding criteria to evaluating the stability of soil with vertical free-face between piles is established. An analytic method to evaluating the stability of soil between supporting piles is proposed based on the energy balance principle and the limit analysis theorem, which can consider the earthquake impacts, as well. Meanwhile, a numerical simulation procedure to obtain the stability coefficient of soil between piles is suggested, which is based on the FLAC3 D numerical simulation platform and the strength reduction method.④The influence of vertical free-face geometry, soil strength parameters and seismic actions to the stability of soil between piles is studied with the single variable method. The theoretical results and the numerical simulation results are compared thoroughly, in order to mutual authenticate the two methods and indicate the application scope of the two methods. It is found that the stability coefficient of soil with vertical free-face between supporting piles increases with the increase of soil cohesion, increases with the increase of the internal friction angle; decreases with the increase of clear width between two adjacent piles, and the trend is gradually slow down; decreases with the increase of the height of vertical free-face, and the decrease trend gradually slow down. When the height to width aspect ratio of the vertical free-face is in the range of 4/1 to 1/1, the relationship between soil stability index and vertical free-face geometry, soil strength parameters and seismic actions obtained from the three-dimensional theoretical analysis method are generally similar with the results of numerical simulation method, and the results of three-dimensional theoretical analysis methods are slightly more secure.⑤ The pseudo-static method is employed to study the horizontal and vertical seismic effects on the stability of soil with vertical free-face between supporting piles. It is found that both the horizontal seismic action and the vertical seismic action will reduce the stability of soil with vertical free-face between piles; the stability coefficient considering the vertical seismic action and the horizontal seismic action simultaneously is small than the stability coefficient either considering the horizontal seismic action alone or considering the vertical seismic action alone. No matter the width or the height of vertical free-face of soil between supporting piles increases, the seismic yield acceleration of soil between piles is declined.⑥ Since the pseudo-static method uses inertia force acting on the soil sliding wedge to represent the earthquake process, it may overvalued or undervalued the seismic effects. A series of destructive tests are accomplished using shaking table system to explore the characteristics of dynamic failure of soil between piles. The influence of height and width of vertical free-face, the soil strength parameters and the seismic waves on the dynamic damage properties of soil between supporting piles are studied, and the experimental results are compared with the theoretical results.