Experimental Study On Bearing Behavior And Failure Mode Of The Wrapped Sand Pile Composite Foundation

The composite foundation of sand piles is a kind of composite foundation with loose material piles.It is suitable for the treatment of loose sand,cohesive soil,soft soil,and miscellaneous filling foundations.Soft soil foundation is widely distributed around coastal areas and inland rivers and lakes in China,because of its good compaction and drainage,sand pile can not only improve the bearing capacity of foundation,reduce the uneven settlement of foundation,but also improve the overall stability of foundation.In recent years,sand piles are increasingly being used in engineering.Sand pile method was first proposed by French engineers in 1835.After continuous improvement and optimization,the construction method and technology have gradually matured.Many experts and scholars at home and abroad also studied sand piles,and drew some conclusions.However,the research on sand pile is mainly based on the numerical simulation and the theory of single pile's bearing capacity.The research on group pile effect,pile failure mode,failure location,the change of lateral pressure on pile bottom and the bearing capacity of composite foundation are very rare.On the basis of discrete material pile composite foundation pile soil interaction mechanism,with the diameter of 75 cm,high 100 cm metal drums filled with sand made of composite foundation model,and buried sand pile in it.70 g standard A1 paper and white geotextile are used as wrapping materials to make model sand pile.On the basis of group pile effect,the static load model test of single pile is carried out on the composite foundation of wrapped sand pile,carry out the following research:1.The self-made loading device of the composite foundation of the wrapped sand pile isused as the packing material of the sand pile with A1 paper,and then indoor static load test,by changing the pile length,pile diameter,pile spacing and other factors,the failure mode and the damage position of the loaded pile under the static load test are investigated;the influence and failure mode of the piles surrounding the loaded pile under the load;bearing behavior and failure mode of the wrapped sand pile composite foundation.2.Using geotextile as a sand pile wrapped material,separate inquiry the influence of the change of pile length,pile diameter,pile spacing and length to diameter ratio on the bearing capacity and settlement of composite foundation.The test results show that,increasing the wrapped sand pile in foundation soil can effectively restrict the lateral deformation of the foundation soil,and exert the vertical bearing capacity of the wrapped sand pile,so that the bearing capacity of the composite foundation is obviously improved,and the settlement is significantly reduced;the influence of pile diameter,pile spacing and long diameter ratio on the bearing capacity and settlement of the composite foundation of wrapped sand pile is more significant,and the effect of pile length is not obvious.3.Excavation observation of geotextile wrapped sand pile composite foundation after static load test,the observation results show that,the change of pile spacing has a significant effect on the damage of the pile surrounding the pile group,through analysis,for the wrapped sand pile composite foundation static load on the surrounding pile influence range is not greater than 4D(D is the pile diameter,the same below).4.The earth pressure box is embedded at the end of the model pile,the 1/3L side of the pile side(L is the length of the pile,the same below)and the 2/3L side of the pile side to explore the change of additional earth pressure at the end of the pile and the pile side during loading,test results show that,the soil pressure of pile side decreases with depth,and the proportion of load distribution on pile side and pile end has little relation with pile length.5.In order to further explore the distribution of soil pressure on the side of the pile under the static load test,tests are carried out at the end of the pile,the bottom of the pile side,the 1/2L of the pile side,the 1/3 L at the pile side,the 1/6L at the pile side,the top of the side of the loading side,and the soil pressure box between the piles and the pile.Test results showthat,along the length of the pile,the soil pressure on the side of the pile increases gradually from the end of the pile to the top of the pile,the earth pressure at the 1/6L position on the top of the pile reaches the maximum soil pressure at the whole pile side,and the soil pressure at the top of the pile side is smaller.6.The theoretical calculations of the ultimate bearing capacity of a single pile wrapped in a sand pile composite foundation are compared with the average value of the ultimate bearing capacity obtained from the model test,and the error is 17.03%,the two results are close to each other,which further validates the rationality and reliability of the lab model test results,therefore,it can be considered that this formula is suitable for the calculation of the ultimate bearing capacity of a single pile with a wrapped sand pile composite foundation.