Research On Negative Friction Resistance Of Long And Short Pile Foundations In Collapsible Loess Regions Under Immersion Condition

With the continuous promotion of the construction of resource-based and environment-friendly policies,long and short pile foundations are gradually introduced into the engineering practices of various regions for exploration.The coordinated working mechanism of long piles to control settlement and short piles to provide bearing capacity can effectively meet the upper bearing requirements and reduce material use and project cost.However,the research on long and short pile foundations in northwest collapsible loess is still in the preliminary stage of exploration.In order to deepen the research and reveal the difference in negative friction resistance distribution of corner pile,central pile,and side pile in collapsible loess and the change of neutral point,this paper carries out a systematic research on the negative friction resistance characteristics of long and short pile foundations based on theoretical derivation and numerical simulation method.The main work is as follows:(1)Based on the results of field test pit immersion tests in Yuzhong,Lanzhou,and Ledu,Qinghai,the water infiltration rate in collapsible loess areas is quadratically related to the depth of the soil and the distance from the edge of the test pit,and the lower limit depth of collapsible loess and the maximum radial range solution can be given by this relationship;Combined with the physical characteristics of the loess,the two parameters of the hydraulic equivalent formula are modified,the settlement calculation method is improved based on the soil-water characteristic curve and verified by combining the results of a field water immersion test.(2)Research on the calculation method of negative frictional resistance of long and short pile foundation under the action of water immersion,based on the distribution characteristics of loess in the area of large-thickness collapsible loess,the long and short pile foundation is divided into two types of load bearing: long pile and short pile are pure friction pile and long pile end-bearing friction pile and short pile friction pile,using the three-fold line soil hardening model,single pile load transfer method and improved collapsible loess settlement formula,combining with the time effect of water immersion into the elastic phase,elasto-plastic phase,full plastic phase and then deriving the average negative frictional resistance of long and short pile foundation calculation expression.The reasonableness of the pile-side negative frictional resistance calculation method is verified by combining the single pile immersion load test at a site in Guanzhong,Shaanxi Province(friction pile)and a site in Ningxia Province(end-bearing friction pile).(3)The geotechnical analysis software Plaxis 3D was used to simulate the negative frictional resistance and pile shaft force of monopiles and short and long pile foundations under submerged conditions,and to systematically study how the negative frictional resistance,neutral depth,and pile shaft force change under the five conditions of submerged pressure,pile top load,pile modulus,aspect ratio,and submerged area.Based on the results of the numerical simulation,the linear regression analysis of the average negative frictional resistance of long and short pile foundations with corner piles,central piles,side piles,and group pile foundations is carried out to obtain the linear regression equation of the average negative frictional resistance of long and short pile foundations under the submerged condition.To verify the reasonableness of the solution method of the average negative frictional resistance of long and short pile foundations based on the improved collapsible loess settlement calculation theory,and to compare with the linear regression method.