Deformation Coordination Mechanism And Performance Evaluation Of Load Transfer Platform Of Composite Foundation

The core problem of the composite foundation is the cushion deformation and the stress coordination.In order to explore the working mechanism of composite foundation,this paper adopts the multi trapdoor model test box device,which is made by the research group,to carry out 6 sets of model tests with two sands of different particle sizes.DEM discrete element method was used to carry out numerical simulation of the test,and the results of the model test were verified.Four factors?the upper load P,cushion thickness H,net distance between piles?s-a?,pile width a?are selected to design numerical simulation orthogonal experiment with four different levels,then the evolution patterns of the composite foundation cushion and the change of the pile-soil stress distribution ratio are explored,and to record the reaction force of the cushion at the bottom of the upper foundation.Two indexes of the deformation coordination ability are proposed to evaluate the cushion.Through model test and DEM numerical simulation test analysis,the results are as follows:?1?By using the PIV technology,the particles of the composite foundation are monitored,and the displacement contour map of each moment of the cushion particles is revealed when the trapdoor is sinking.There are three kinds of modes in the displacement of cushion particles revealed by the test displacement contour map.When the cushion thickness is small,the particle displacement is small at pile top.With the increase of the thickness of cushion,a constant upward diffusion displacement of the triangular area is formed with the sinking of the trapdoor,and inverted wedge region appears at pile top.When the thickness of the cushion layer is high,the triangular slip surface is not extended and the top of the sliding surface appears.?2?By the pile-soil stress distribution ratio curve of the model test,when the trapdoor is not moved,uniform force is between the pile and trapdoor.Once the trapdoor is down,the stress ratio of the pile is up to the peak value quickly;?3?The model test is simulated by 2D DEM method.Through the comparison with the model test,it is found that the DEM method and the model test results have a large degree of similarity.?4?The composite foundation cushion is simulated by orthogonal numerical test of 4 factors with 4 levels.Deformation evolution of 3 kinds of cushion is similar to the model test,and there are triangular expanding pattern,equal settlement plane pattern and overhead pattern.A complete force chain bridge is formed between the pile top in the triangular expanding pattern,and force chain network is both formed between the pile top and between the pile top with soil in the equal settlement plane pattern.The overhead mode load is transmitted directly through the cushion particles of the pile top,and the base plate is without foundations.?5?Two evaluation indexes are proposed on the coordination ability of the cushion deformation,those are the change rate of the total volume of the cushion V and the particle volume change rate of the cushion V₁.The slope K of curve of the cushion volume change rate with settlement of soil between piles is as the value of the cushion deformation coordination ability.For the plane strain condition,when the K is in the[0.105,0.214]interval,it can be considered that the deformation coordination ability of the composite foundation is good.?6?Four factors of the orthogonal test is analyzed by range analysis and variance analysis,and the analysis results show that the influence degree of the cushion deformation coordination is cushion thickness H>net distance between piles?s-a?>pile width a>the upper load P,and the test result is significantly influenced by the cushion thickness H and the net distance between piles?s-a?,but the effect of the pile width a and the upper load P is not significant.In this experiment,the small size model test box is carried out,which is inevitably limited by the model boundary conditions.There is no reinforced test type of geotechnical materials,so there are some differences in the use of the cushion with the actual composite foundation.In order to be more ready to explore the need for further research,we can be using 3D large scale model test or field test.