Study On Failure Mechanism And Avoidance Measures Of Friction Pile Group Under The Influence Of Normal Fault

The activities of faulting have caused varying degrees of damage to the pile foundations of near-fault buildings.At present,pile failure mechanism and technique of partition wall are not yet investigated sufficiently.Centrifuge and numerical modeling established by FLAC^3D were conducted in this paper to investigate the pile failure mechanism subjected to normal faulting,and determine setback distance of pile group foundations.A soil bentonite wall to maitigate the influence of normal faulting on pile group has been proposed.Design parameters relevant to the effect of soil bentonite wall are analyzed in detail.The soil anti-pressure under pile cap has a significant effect on the fault rupture path.When the sand under the pile cap bears part of the upper load,its stress increases and the shear strength improved.Significant stress redistribution in the sand under the pile cap is induced by normal faulting.Part of the pile cap is even separated from the sand,that results in stress release and the shear strength reduction.When the propagating fault rupture approaches the cap,it shifts to the position where the soil stress is lower.According to the response characteristics of pile cap displacement and tilt at different pile locations,it can be divided into three characteristic areas:safety area,avoidance area and translation area.When the pile group is located in the safe area,it is not affected by faulting and remains relatively static.When the pile group is located in the avoidance area,its overall inclination exceeds the allowable inclination value of pile foundation of multi-storey buildings by 0.004（JGJ94-2008）.And the load redistribution on pile top is significant,which causes the pile group to break down under pressure or under tension.Therefore,it is necessary to strengthen fortification.When the pile group is located in the translation area,it makes rigid translational motion along the direction of faulting,and does not tilt.The surface rupture diverge from pile cap.Along the direction of surface rupture,The influence range of 1×3 pile group,2×3 pile group and 3×3 pile group is 4.5S_d,5.0Sd and 6.0Sd.When the bedrock is moved by normal fault with a small fault dip angle,the angle between the fault rupture and the horizontal plane decreases.Correspondingly,the avoidance area increases and leans to the footwall side.When the fault rupture passes through a thicker overlying soil to emerge from the surface,a larger amount of fault displacement is required,and the induced pile settlement and tilt are reduced.In the range of S≤-0.2H in the avoidance zone,the increase of the upper load makes the lateral constraint of soil on the pile cap stronger.In the range of S>-0.2H in the avoidance zone,the increase of the upper load will aggravate the upward side inclination of the pile group.Due to its low shear strength and high compression,the soil bentonite wall between the bedrock fault and pile group foundation absorbs the soil shear deformation caused by normal faulting,so that the fault rupture propagates upward along the soil bentonite wall,which can effectively mitigate the impact of normal faulting on pile group.The mitigation performance of the soil bentonite wall can be improved by increasing the thickness and reducing the stiffness of the wall.