Research On Bearing Characteristics Of Flexible-Pile Reinforced-composite Foundation Under Progressive Building Loads

The flexible-pile composite foundation has the advantages of low cost,environmental protection,short construction period,quick construction,and easycontrolled quality.In particular,the characteristics of environmental protection and easy access of pile materials(soil,lime soil,sand,gravel,or construction waste etc.)cater to targets of countries("13th Five-Year Plan" energy-saving emission reduction).Therefore,the rational use of flexible pile materials,environmental protection,easy to take advantage of the advantages of pile materials,building a "green" foundation,for the country advocated "energy-saving emission reduction" policy to make its due contribution.However,the bearing capacity of flexible-pile composite foundation is smaller than that of rigid piles,and the post-construction settlement is large.Therefore,its application range has limitations.Increasing the bearing capacity of flexible-pile composite foundation has always been a problem for exploration by geotechnical workers.Many scholars put forward some measures that can improve the bearing capacity and post-construction settlement,such as reinforced cushions,piles wrapped,etc.,but still need to further refine their research to make it more rational and effective be used.This article relied on the foundation treatment project of the “C1 Middle School Dormitory Building in Taiyuan Civil Political Park Shelter District Renovation and Resettlement Project Area.” During the construction process,the pressure on the top of the pile,the soil between the piles,and the foundation settlement were regularly monitored.The changes of the curve of the earth pressure between piles/pile,the deformation curve of the lattice tension and the curve of the settlement curve were analyzed in depth.The load-bearing mechanism of the flexible-pile reinforced composite foundation was comprehensively analyzed based on various monitoring data.The reduction model was established using FLAC3 D with the field test physical parameters as the background.The effect of the reinforced cushion's working mechanism,the optimal reinforced-position of the cushion and the number of reinforced-layers was further studied.analysis.The results of comprehensive field tests and numerical simulations are as follows:1> Analysis of grid tension deformation analysis: About 60% of the grid's tensile deformation occurs in the initial deformation under initial base pressure,and the deformation in the latter stage is small;The grid is at 90° The tensile strain in the direction is always greater than 0° and 60°(the nodes that are vertically connected through the grid bars).In addition to the 90° grid strain,the tensile curve shows an abrupt increase in the initial period.Others The stretching and deformation steps in all three directions are basically the same,and the "tri-line" deformation characteristics similar to the earth pressure at the pile top(segment)also appear.2> Analysis of "Tear effect" of grid bars and joints: The grid deformation in the directions of 0° and 60° is harmonized,and the "tear effect" 60i' between 60° and 90° increases with pressure.The gradual increase,and the greater the base pressure,the more obvious the incompatibility of deformation between 60° and 90°;the macroscopic phenomenon of the “tear effect” is represented by the change of direction of the hinge force between the tendon and the node,and the long-term tearing.Under the action,the articulation force between the tendons and the joints will yield,and stress relaxation will occur,affecting the force mechanism of the grid or tearing the tendons at the hinges,thereby affecting the stress distribution and bearing capacity of the composite foundation.3> The correlation between the total settlement and the hypothesis impact factors: In addition to the direct impact of the larger base pressure on the total settlement,the earth pressure between the piles and the 90° direction grid tension deformation of the grid also have an indirect effect;The positive correlation between the tension deformation of the grate and the total settlement is due to the lateral deformation of the soil-soil interface and the effect of the tensile membrane effect on the compression deformation of the soil between piles;the basic type(rigid and flexible)of the soil between piles.The correlation between pressure and total settlement has a certain influence.In the same composite foundation,the degree of correlation between rigid foundation and total settlement is slightly higher than that of flexible foundation and total settlement.4> Comprehensive analysis of staged load-bearing mechanism: With the increasing of progressive pressure,the curve of each index of flexible pile-net composite foundation shows a phase change.The phased characteristics show that each physical state of the foundation(including pore/voidage rate,pore water pressure,compressive modulus,etc.)has its corresponding "stage bearing domain".The initial segment and the tail segment showed different characteristics at each stage.Specifically,the slope of the tangential line in the initial segment changed more,and the stress redistribution phenomenon was more obvious;the tail segment was relatively stable,and the longer the path was,indicating that the physical state was The range of the bearing domain value in the stage is greater;the turning point between the two stages indicates that the stress and strain relationship of the foundation under the physical state of the previous stage is in the yield state,and begins to enter the initial stress and strain state of the physical state in the next stage.5> The laid grid has little influence on the ultimate bearing capacity of the foundation,but it can reduce the settlement of the foundation to a certain extent.The grid reduces the settlement of the foundation by limiting the lateral displacement through the friction between the soil and the soil.It is concluded that when the grid limits the lateral movement of the nearby soil,it has a certain range of influence.The soil outside this range is not affected by the grid;the number of grid layers is increased in the cushion,and the grid can be increased to limit the amount of soil.The influence of the lateral shift in the vertical direction will further reduce the settlement of the foundation;the grid will not affect the foundation settlement when it is laid out in different positions,but it will gradually deviate from the top of the pile with the placement position,or the placement position will be closer to the application.The location of the load,the smaller the settlement of the foundation.