Research On The Behavior Of Grid And Soil Arch Joint Bearing Of Pile-Net Structure Based On Cyclic Loading Test

Pile-net structure is a combination of rigid pile and geogrid.Its structure can transfer the upper load from subsoil to pile top with greater strength and stiffness,so as to reduce the uneven settlement of track subgrade.The reinforcement mechanism of pile-net structure mainly comes from the joint action of soil arching effect caused by pile-soil differential settlement and tensile membrane effect of grid.At present,the influence of pile-soil differential settlement on the dynamic characteristics of pile-net structure is rarely considered.Based on the Youth Science Foundation Project "Research on the macro and micro mechanism of dynamic soil arch weakening and tension membrane coupling bearing of in-service pile-net structure",this paper designed a set of dynamic loading system which can accurately control the simulated pile-soil differential settlement.Through theoretical calculation,the theoretical formula of soil arching effect suitable for circular pile of square layout considering external load was deduced,which was confirmed with the data of test in the static load stage.Then,based on the data of dynamic load stage,the empirical formula of pile efficacy under cyclic load was deduced.The behavior law of grid and soil arch under cyclic load was summarized.The main conclusions are as follows:1.According to the literature at home and abroad,the research status of pile-net structure subgrade at home and abroad was investigated,and a set of dynamic loading system which can accurately control the simulated pile-soil differential settlement was independently designed,which can accurately control the simulated pile-soil differential settlement among 8 circular piles at the rate of 0.1mm/s.2.For circular piles of square layout considering external load,the static calculation formulas of pile-net structure by various methods were deduced.The calculated results of Terzaghi method are the closest to the test and are mutually confirmed with the test.When the pile-soil differential settlement is large,the static and dynamic difference of earth pressure is large.Based on the test results,the empirical formula of pile efficacy under cyclic load was deduced.3.Carry out dynamic model test and summarize the characteristics of soil arch and grid while settlement increases: the soil pressure at the top of pile increases sharply,while the soil pressure on the subsoil decreases sharply.The tension of grid which is mainly beared by the tension of lower grid increases sharply.When the pile-soil differential settlement reaches a certain degree,the soil arching effect reaches the strongest,and the tension of grid which is mainly beared by the tension of lower grid reaches the strongest,which generally lags behind the former.4.Summarize the characteristics of soil arch and grid in the process of cyclic loading:the cyclic load has two sides to the soil arch effect.When the soil arch effect reaches the strongest,the adverse effect of cyclic load is stronger,the soil arch effect is weakened rapidly.The greater the dynamic load,the greater cumulative loading energy,and the lower the grid strength,the weaker the final soil arch effect.The tensile force of the upper grid is generally less than that of the lower grid.After the grid reaches a certain bearing capacity,it remains unchanged or gradually decreases.The smaller the dynamic load,the smaller the cumulative loading energy and the greater the grid strength,the smaller the reduction range.5.Summarize the variation characteristics of dynamic parameters of soil arch and grid:reducing dynamic load or cumulative loading energy and increasing grid tensile strength have a good effect on the reduction of dynamic soil pressure at the plane of pile top,but increasing grid tensile strength has a poor effect on the reduction of dynamic earth pressure at the center of two piles.Reducing the dynamic load or cumulative loading energy and improving the tensile strength of the grid have significant effects on reducing the dynamic tensile force of the upper and lower grids respectively.