Frost Jacking And Bearing Capacity Of Pile Foundations In Cold Regions

Pile foundation is a type of foundation that meets the specific requirements of structures,and it has been widely adopted in special soil regions,such as frozen ground regions.The instability of frozen soil would bring a series of issues to engineering practice,as well as new challenges to the in-service performance of the pile foundations and superstructures.Research on the frost jacking and bearing capacity of piles would serve as a reference for the design,construction,and maintenance of foundations,as well as other engineering works in cold regions.In this thesis,the frost jacking stability of screw piles and cast-in-place piles are investigated by laboratory tests,theoretical derivation and numerical methods;based on the direct shear tests of frozen soil-concrete interface and previous works,the shearing characteristics of pile-frozen soil interface are analyzed,while the calculation formulas and variation law of a pile’s bearing capacity are obtained from the perspectives of direct shear tests and pile loading tests.Main contents are summarized as follows:(1)Based on the working scenario,a model test is designed and carried out for the screw piles under a unidirectional freezing condition,and the responses of various pile types are compared and analyzed,as well as temperature profile and soil pressure distribution.The vertical displacements of different piles with no restrictions applied to the pile head are obtained,indicating that increasing order of vertical displacement is:large-diameter multi-helix pile<small-diameter multi-helix pile<double-helix pile<straight-shafted pile<full-length-deployed helical pile.The axial forces are collected with the pile head vertically constrained.The variations of forces and displacements are observed for the thawing process after test ceased,and the results demonstrate that small-diameter multi-helix pile recovers the most.The bearing capacity of piles subjected to frost jacking is derived from a conventional calculation method,and it agrees well with the test results.(2)A numerical model is established to simulate the frost jacking response of screw piles under the same test condition,and its validity has been proved by comparing to test results.Optimal design is conducted to obtain geometrical parameters of a screw pile that can mitigate the frost jacking the most.(3)In the context of cast-in-place pile in permafrost regions,a pile-soil interaction system is established under two-dimensional axisymmetric condition.Within a framework of hydro-thermo-mechanical sequentially coupled theory,a volumetric strain model and a pile-frozen soil interface model are proposed based on test results.The frost jacking of a single pile under natural permafrost condition is predicted using this model,sequentially,a couple of countermeasures to mitigate frost jacking are discussed.(4)The shearing properties of a frozen soil-concrete interface are investigated using orthogonal design.Temperature,normal pressure,moisture content and strain rate,each having three levels,are considered in the test.A visual analysis is undertaken for evaluating the impacts of factors on the mechanical properties of the interface.After each test group,the sample was remounted and stabilized under the same test conditon for 8 h.Then,a shear test was conducted again to obtain the bond healing value.The results indicate that recovered bond strength can exceed the residual shear strength of the first test under favorable conditions.(5)The current progress:in the direct shear test of pile-frozen soil interface and pile loading test are systematically organized and is then connected to pile design code in cold regions.In general,two indicators can be used for pile design,namely adfreeze bond strength and pile settlement rate.