Model Experimental Study On Long-term Bearing Capacity Of Pile Foundation In Frozen Soil Ground Under Different Thickness Of Ice Membrane

The long-term bearing capacity of pile foundation of railways, highways and bridges inpermafrost region is the basis to ensure the safety operation of these structures. Bearingcapacity of pile foundation in permafrost ground is affected by different frozen soil type,different frozen soil zone, different topography, different geological temperature, differentenvironment and different construction method, which will result in big difference. Researchachievement of pile foundation in permafrost ground is not as profound as in general soil,there is a lack of systematic study on the formation and development rule of freezingfrictional resistance (ice cementation force) at pile-soil interface, as well as the descriptionof creep of pile foundation in frozen soil. The problem of pile-soil interface is the key to thestudy of bearing capacity of pile foundation, and the related research is always the keytechnical problems in the field of pile foundation. Excavation of pile in permafrost foundationreveals that ice membrane exists between the pile body and frozen soil around pile,mechanism of pile-soil interaction requires in-depth study on mechanical properties ofpile-soil interface-ice membrane, as to deeply reveal the mechanical mechanism of thepile-soil system. On the basis of analyzing vertical bearing capacity of single pile foundationin permafrost ground, combined with the physical and mechanical properties of frozen soil,experimental research is conducted on the long-term bearing capacity of pile foundation.This paper adopts3kinds of ice membrane of different thickness, namely1mm,2mmand3mm, the force is respectively loaded on the pile with different ice membrane thickness，distribution rule of axial force and freezing frictional resistance of pile, as well the changingrule of settlement of the top of the pile with time are analyzed。Low temperature test chamber in Lanzhou Jiaotong University is used to conduct thetest.Test results show that the bearing capacity of pile and freezing frictional resistancechange significantly with the change of the thickness of ice membrane. Pile with the icemembrane thickness of2mm can reach the maximum freezing frictional resistance; the totalfreezing frictional resistance of pile body is also the largest, long-term bearing capacity is thehighest. Changing of the bearing capacity is: pile with2mm ice membrane thickness>1mmice membrane thickness>3mm ice membrane thickness.The axial force of pile is mainlydistributed in the upper1/3part of pile body, distribution along the pile is gradually becomingsmaller from top to bottom, and the reducing rate along the pile slows down gradually. Withthe increase of the load on the pile, distribution curve of axial force of pile changes gradually,the shape is similar to parabola.With the change of ice membrane thickness, deformation rate of the pile also hasobvious change. Changing rule of settlement rate of pile foundation is: pile with3mm ice membrane thickness>1mm ice membrane thickness>2mm ice membrane thickness, when itreaches the ultimate load, the time of the rapid development of settlement also changes, underthe same load, pile with2mm ice membrane thickness of better bearing capacity is longerthan that with1mm ice of poorer bearing capacity.At the end, the changing of long-term bearing capacity of pile foundation withtemperature is studied by numerical simulation. Through the comparison with experimentaldata, long-term cohesion of Lanzhou loess decreases to1/6of the instantaneous value underthe temperature of-2℃.Through the finite element analysis, with the decrease of temperatureof frozen soil around pile, displacement of the pile decreases under the same load, bearingcapacity of pile is greatly improved with the decrease of temperature, and it approximates aslinear growth.