Model Test Study On The Mechanism Of Foundation Pit Anti-heave Piping Considering Soil Nailing Effect Of Engineering Piles

In recent years,the heave piping of deep foundation pit with confined water has become a prominent engineering problem gradually.For the deep foundation pit with confined water,the excavation will reduce the thickness of confining layer overlying the confined aquifer.When it is reduced to a certain extent,the confined water will crack the bottom and cause heave piping,eventually lead to instability of foundation pit.Therefore,how to judge the stability of foundation pit is a difficult problem in the design and construction of foundation pit.In the areas with high groundwater level,the number of engineering piles is more larger,and the influence of engineering piles on heave piping is seldom studied.In this paper,the mechanism of heave piping considering the influence of engineering piles is studied.The failure modes and deformation of soils and stress of piles are analyzed by model test on heave piping of foundation pit.The influence of different pile length,pile spacing,soil strength and other factors on the capacity of anti-heave piping is analyzed by using ABAQUS.Finally,the method of anti-heave piping analysis is developed.The main work and achievements of this paper are as follows:(1)The existing methods of anti-heave piping for foundation pit are summarized,and their shortcomings are analyzed.This paper puts forward a method to analyze the anti-heave piping of foundation pit,which is based on the factors such as the size of foundation pit,the uplift bearing capacity of engineering piles and the strength of soil.Different methods should be used when the overlying soil has different properties.(2)In this paper,the stress of piles and deformation of soil are analyzed by laboratory model test.The results show that the magnitude of axial force of piles are as follows: the biggest is the center pile and the side pile between the center pile and the corner pile.In the range of the confining layer,the pile skin friction is positive,and develop with the increase of water pressure.The pile skin friction below the soil interface is negative.The vertical displacement curve of pile and soil distribute asparabola,and the deformation at the center is the largest.The existence of engineering piles has a significant inhibitory effect on the deformation of the soil.(3)Numerical results show that with the increase of the pile length,the uplift bearing capacity of engineering piles increase gradually.Increase the length of pile can improve the ability of resisting heave piping.The heave piping occurs also related to the pile spacing,the thickness of confining layer and the strength of soil.Under the condition of a certain number of piles,the smaller pile spacing or the greater thickness and strength of confining layer,the higher water head when heave piping occurs and the smaller deformation of the soil with a same confined water pressure.The plastic zone of soil is generated at the interface of pile and soil,which is mainly distributed at the bottom of the confined layer and developed with the increase of water pressure.(4)Two methods of analyzing the safety factor of anti-heave piping are put forward by using the single pile and the group piles foundation model.Considering the comprehensive correction coefficient of pile ultimate skin friction,the formula for calculating the critical thickness of confining layer is proposed.