Comparison And Selection Of Treatment Methods And Properties Of Large Thickness Collapsible Loess Foundation Based On Engineering Practice

With the development of society,more and more projects are built on the site of large thickness collapsible loess.The problem of effective foundation treatment methods for large thickness collapsible loess site has attracted the attention of geotechnical scholars.On the basis of previous studies,this paper relies on a large thickness collapsible loess foundation treatment project in Shanxi,and compares and selects the foundation treatment schemes according to the engineering geological conditions,relevant engineering cases and site conditions,The deep dynamic compaction(DDC)method in the hole and the dynamic compaction method on the surface of pile foundation are preliminarily selected.The filler of DDC pile body is cement soil(belonging to semi-rigid pile),and the dynamic compaction pile body of surface pile foundation is cast-in-place pile.Through relevant field tests(load test,indoor geotechnical test,standard penetration test,etc.),the bearing capacity and collapsibility elimination effect of the two foundations are tested;The finite element simulation software ABAQUS is used to establish the DDC single pile composite foundation model and the cast-in-place pile single pile model,and the settlement change law,bearing characteristics and load transfer law of the two kinds of piles under load are deeply studied;Pile group models of two foundation treatment methods under the same working condition are established,and their treatment effects are compared and analyzed from five aspects: overall structure,pile and plate,surface settlement,differential settlement and economy,so as to provide practical experience and theoretical support for similar large thickness collapsible loess projects in the future.The main results of this paper are as follows:(1)DDC scheme and pile foundation surface dynamic compaction scheme can effectively deal with large thickness collapsible loess foundation;According to the soil collapsibility coefficient curve and compaction coefficient curve between DDC piles,under the same pile forming process,the discrete type of soil between surface piles is significantly larger than that between deep piles,and the difference boundary is about 10 m.This depth can be used as the difference boundary of DDC pile construction on site.When the upper pile is formed,the tamping times or tamping energy can be appropriately increased to achieve the same overall treatment effect of foundation.(2)Under the action of loads at all levels,the axial force of DDC pile increases first and then decreases along the buried depth of the pile.The maximum value is 0.13 times of the pile body.The pile side friction decreases gradually along the buried depth of the pile,but the size remains basically unchanged after the depth reaches 7M;The bearing capacity of foundation is dominated by side friction,supplemented by end resistance.The inner resistance of pile body is synchronous,and the end resistance and side resistance are asynchronous;The maximum surface settlement of single pile composite foundation under load is located at the second node near the pile.(3)Under the action of various loads,the axial force of cast-in-place pile decreases nonlinearly along the buried depth of pile,and the pile side friction increases gradually along the buried depth of pile.The inner resistance of the pile body is asynchronous,and the end resistance and side resistance are also asynchronous;After the deep soil is collapsible,the foundation settlement becomes larger under the same load,the bearing capacity of pile-soil at the collapsible position is completely lost,the axial force increases first and then decreases with the depth,the side friction turns from positive to negative,and the lower pile-soil plays the role of bearing capacity earlier;With the increase of the upper load,the axial force of the pile body and the positive friction of the pile side increase,but the negative friction of the pile side decreases and the neutral point moves upward.(4)DDC pile composite foundation has the advantages of complete elimination of soil collapsibility within the treatment depth,high safety,stronger integrity and small surface differential settlement value;The dynamic compaction foundation on the surface of pile foundation has the advantages of stronger bearing capacity and less surface settlement deformation,but there are certain potential safety hazards in deep collapsible loess,which should be paid great attention in engineering practice.