Research On Setback Distance Of Strip-foundation Under Surface Rupture In Strong Earthquakes

The occurrence of structural strong earthquakes is often accompanied by active faults and subsequent surface rupture,which usually cause serious damage to engineering structures under appropriate conditions.Therefore,how to consider the influence of active faults on its impact and avoidance in engineering construction has important significance.In this paper,through the large-scale model test method of surface rupture and numerical simulation of large rupture deformation,the deformation characteristics of the overlying clay and sand layers under normal fault action and the expansion of the rupture zone are studied,giving advice on the avoidance of strip foundations.The main research contents include:Together with this research group,we have developed a large platform fault displacement model box that can meet the thickness of the cover layer up to 200cm,the length*width is 4.6*1.85m,the fault dip angle is 45°to 70°,and can realize the normal and reverse fault displacement body device.Six free field tests of normal fault displacement simulation were carried out to discuss the effects of dislocation amount,fault dip angle and overlying soil layer thickness on surface fracture,the main conclusions are:(1)the surface uplift or uneven settlement will gradually increase as the amount of bedrock dislocation increases,and finally a V-shaped shear zone formed in the soil body,and the damage is concentrated in the hanging wall area;(2)In the clay site,the thickness of the overlying soil layer increases,and the surface uplift height and the influence range of the rupture zone decrease;in the sand site,the fault dip angle increases,the surface uplift height and the influence range of the rupture zone increase,and there is a tendency to spread to the hanging wall;And giving advice on setback.Six field tests were conducted under the strip foundation of normal fault displacement simulation.The effects of dislocation amount,fault dip angle and overlying soil thickness on the foundation were analyzed,and reasonable avoidance distances were given.The main conclusions are:(1)With the gradual increase in loading,the slope difference between the two ends of the strip foundation increases,and a V-shaped shear zone eventually formed in the soil to cause damage to the surface and foundation;(2)In the clay strip foundation site,the thickness of the overlying soil layer increases,the influence range of the rupture zone decreases,and the height of the foundation surface uplift decreases;(3)In the clay and sand strip foundation site,the fault dip angle increases,the impact zone of the rupture zone increases,and the foundation uplift height increases;(4)After considering the thickness of the overburden and the inclination of the fault,suggestions for the avoidance of the strip foundation are given.The maximum reasonable avoidance distance is 45m for the upper plate and 25m for the lower plate.The maximum reasonable setback distance is 45m for the hanging wall and 25m for the footwall.Using particle flow PFC~2DD software for model construction,a particle flow large deformation analysis program based on sand-strip foundation site was prelimin-arily developed,when verifying the previous typical test conditions,it was found that the basic trends of the two were consistent,the main conclusions are:(1)Through the numerical analysis of the thickness of the cover layer and the amount of dislocation under multiple conditions that simulate the free field of sand,increasing the thickness of the cover layer or reducing the amount of dislocation can alleviate the surface damage to a certain extent;(2)By simulating the numerical analysis of the foundation burial depth and size of the sand-strip foundation site under multiple conditions,the foundation is located near the main rupture zone,increasing the foundation burial depth or size can effectively reduce the damage effect.Leaving the upper and lower ranges outside this area,changing the base burial depth or size did not significantly improve the reduction of strong earthquake damage.