Lateral Bearing Behavior And Seismic Resistance Of Strength Composite Piles In Soft Soils

Due to the lateral load and seismic liquefaction,excessive lateral displacement or failure of pile foundations causes engineering accidents frequently.It is an urgent challenge to effectively improve the lateral bearing capacity and seismic performance of pile foundation in weak stratum,which is also a hot and difficult issues in pile foundation engineering.The strength composite pile is a novel type of pile technology which combines the high-strength precast concrete pile and deep cement-mixing column(or high-pressure jet grouting column).The engineering practices show that the cement-treated soil around the precast concrete pipe pile can greatly improve its vertical bearing capacity.However,the studies on the lateral response and seismic behavior of the composite piles are still in the preliminary stage,which is difficult to guide the engineering practice.This paper systematically studied the lateral bearing behavior of strength composite piles in soft soils and their seismic performance in liquefiable soils by means of field test,model test,numerical simulation,and theoretical analysis.The research results provide theoretical basis for the analysis of the lateral capacity and seismic behavior of strength composite piles in weak strata.The main research contents and results of this paper are as follows:(1)Through the field lateral load tests of piles in three different sites,the reinforcement effect of soil-cement column on the lateral bearing capacity of the precast concrete pile was evaluated.The test results from six strength composite piles and three PHC pipe piles show that the presence of soil-cement column can effectively enhance the lateral critical load and ultimate bearing capacity of the PHC pipe pile as well as evidently reduce the lateral deflection and bending moment in the pile.For the strength composite piles with the diameter ratios of soil-cement column to concrete-cored pile of 1.5～2.5,their lateral critical loads are33%～50% higher than that of unreinforced PHC pipe piles,and the lateral displacements at the pile-head are 40%～70% smaller than that of PHC pipe pile under the same load.Moreover,increasing the diameters of the soil-cement column and concrete-cored pile can improve the lateral bearing capacity of the strength composite pile.(2)Increasing soil resistance and reducing or delaying the development of tensioninduced damage in the pile are reinforcement mechanisms of soil-cement column to laterally loaded single precast concrete pipe piles.For laterally loaded single composite piles,the concrete-cored pile,cement-improved soil and soft soil around the pile can work together to resist the lateral load.The presence of soil-cement column can not only greatly increase the lateral soil resistance of the pile to limit the development of pile deformation,but also obviously reduce or delay the development of tensile damage in the concrete pile,thus improving the lateral performance of the composite pile.Additionally,the effect of design parameters of the soil-cement column on the lateral behavior of the composite pile was revealed.Increasing the diameter of soil-cement column and enhancing the strength of cement-improved soil can increase the initial stiffness and ultimate soil resistance of the composite pile,but there is a critical strength of cement-improved soil.When the length of the soil-cement column is smaller than 10 times the diameter of concrete-cored pile,the presence of soil-cement column is effective in improving the lateral bearing capacity of the pile.(3)Considering the proportion of lateral resistance provided by the cement-improved soil and the soft clay surrounding the pile,as well as the nonlinear behavior of the concrete-cored pile,a p-y curve method for analyzing the lateral bearing characteristics of single strength composite piles in soft soil was proposed.Firstly,the cement-improved soil was assumed to be the stiff clay.Then,the proportion of lateral resistance provided by the cement-improved soil,as well as the soft clay surrounding the pile was deduced based on the typical p-y curves for both soft and stiff clay.Subsequently,a p-y curve model was established to predict the lateral response of composite piles in soft clay.In this model,the nonlinear behavior of the concrete-cored pile was considered by introducing the moment-curvature relation.The rationality of the proposed analytical approach was verified by the field measurement results of engineering cases.Based on the proposes p-y curve model,the influence of the diameter,length and strength of the soil-cement column,elastic modulus of precast concrete pile as well as the constraint condition of pile head on the lateral behavior of laterally loaded composite pile was analyzed.(4)The cement-soil column reinforcement can effectively improve the seismic performance of precast concrete pipe piles in liquefaction soils.Increasing the restraint effect on the pile and reducing the cyclic shear strain of soil around pile are improvement mechanisms.Under the action of earthquake,the presence of the cement-soil column can obviously restrain the development of excess pore pressure in soil around the pile group foundation,and thus limit the stiffness degradation of soil and increase in the fundamental period of the site caused by seismic liquefaction.In the case of composite pile,the lateral displacement of superstructure and settlement of raft are greatly reduced due to the reinforcement of cement-soil column.Moreover,the larger the reinforcement depth of cement-soil column,the more significant the decrease in lateral displacement and settlement.The cement-soil column can effectively restrain the increase of bending moment of the pile in the range of reinforcement depth,which may lead to a 70% reduction in the maximum pile moment.However,the location of the maximum moment in the pile is different under various reinforcement depths of cement-soil column.(5)The effects of the diameter,length and modulus of soil-cement columns,relative density of sand,and seismic intensity on the seismic response of sand-composite pile-superstructure were clarified.In addition,the anti-liquefaction performance of liquefiable ground and the bending failure of composite piles were evaluated.Based on the evaluation results,the key points of seismic design for composite piles in liquefiable soil were proposed.Increasing the diameter of soil-cement column can greatly improve the seismic performance of composite pile.When the ratio of shear modulus of cement-improved soil to sand is less than 45,increasing shear modulus of cement-improved soil can effectively improve the seismic performance of composite pile.When the thickness of the liquefiable soil layer is small,the length of soil-cement column should pass through the liquefiable layer,while in deep-thick liquefiable soil sites,the length of soil-cement column should not be smaller than10 m.A large moment response may occured in the pile at the interface of cement-improved soil and liquefiable sand,so necessary seismic structural measures should be considered at the part.(6)The influence law and mechanisms of connection type of pile-raft on the seismic response of strength composite piles in liquefiable soils was revealed.Compared to the connected piled raft(CPR)case,the use of the disconnected piled raft(DPR)foundation can mitigate the degree of soil liquefaction,and thus limit the stiffness degradation of soil due to seismic liquefaction.Due to the seismic-isolation effect of cushion,the acceleration responses of the soils and superstructure in the DPR case are clearly lower than in the CPR case.The integrity and rigidity of the DPR foundation are relatively poor,which results in an increase of more than 50% in the lateral displacement of superstructure and the settlement of raft compared with those of CPR.In the CPR case,the peak bending moment mostly occurs at the top of the composite pile.However,the maximum moment locates nearby the depth of1/3～1/2 pile length beneath the pile head in the DPR case.Moreover,the maximum pile moment in the DPR case is approximately 50% lower than that in the CPR case.