Macroscopic And Mesoscopic Study On Pile Soil Interaction Of Reduced Diameter Pile Groups Under Vertical Load

With the rapid development of infrastructure construction,the scale of superstructure is expanding and the bearing capacity of the lower foundation is required to be higher,and the group pile foundation is widely used because of its advantages such as large bearing capacity and small settlement.However,due to technical and geological conditions and other factors,the foundation pile is very easy to produce necking defects in the construction and use process.At present,the research on the pile-soil interaction mechanism of the reduced group pile is still weak,which leads to the lack of reliable theoretical basis of dealing with such engineering problems.Therefore,this thesis relies on the engineering background of necking defects in group pile foundation of real engineering.By using transparent soil test technology,the vertical loading test of 1×2 necking group piles is carried out,and the corresponding discrete element numerical model of pile-soil is established to study the effect of necking on the bearing performance of group piles under vertical load and the deformation of soil around the pile by combining macro and fine view,which provides more reliable theoretical and technical basis for solving practical engineering problems.The specific research contents and main conclusions are as follows:(1)The load-settlement curves were obtained for 10 sets of working conditions by conducting transparent soil vertical loading tests of one kind of intact group piles and nine kinds of necking group piles with different necking sizes.The results show that the necking defects will lead to the change of pile force performance,and the size effect will have different degrees of influence on the group piles bearing capacity.When the axial size is certain,the smaller the radial size of the necking defect are,the larger the loss of load capacity and the larger the settlement rate of the group piles foundation;when the radial size of the necking are above 75%L_d,it has less influence on the load bearing performance,and the load-settlement curve before the ultimate load is basically coincident with the intact group piles.When the radial size are certain,the larger the axial size of the necking defect,the larger the loss of load bearing capacity of group piles foundation;when the axial length is within 2.5%L_s,the necking defect have less influence on the load bearing capacity.(2)Mat PIV was used to process the soil scatter field in the transparent soil test to obtain the two-dimensional soil displacement vectored map and displacement cloud map around the pile,and analyze the deformation law.The results show that when the necking axial size is10%L_s and 5%L_s,the necking defect causes more oblique downward concentrated deformation of the soil around the pile and produces through deformation with the soil at the upper pile body.When the reduction axial size is 2.5%L_s,the perimeter soil disturbance at the reduction is small and the overall deformation trend is very similar to that of the intact group piles.The existence of necking defects will cause a larger range of soil disturbance around the group piles,and the soil near the necking of the pile body will produce an oblique downward displacement to reduce the relative displacement of the pile and soil,resulting in the loss of lateral friction resistance.(3)Through direct shear tests,the soil particle parameters were calibrated,and a pile-soil discrete element model was established and validated.The porosity,force chain,and structure of the soil during the pile settlement process were analyzed.The research shows that the necking defect changes the stress transmission direction of the soil around the group pile and produces more contact forces in the horizontal direction.As the degree of necking defect increases,the degree of soil disturbance becomes more severe.The soil outside the pile at the necking defect will show shear failure,and the soil inside the pile will exhibit a trend of overall compression due to the shielding effect of the pile body,with a higher density than the intact group piles.The degree of soil affected is in the order of:outside the pile>inside the pile>pile end.Therefore,in practical engineering,if a necking defect occurs in a group pile,measures such as adding short piles in a certain range outside the pile can be taken to transform the soil outside the pile into the soil between the piles,strengthening the stability of the foundation structure.