Research On Deformation And Bearing Capacity Of Single Pile Under Vertical Load In Permafrost Region

As the scale and complexity of the infrastructure construction is growing, pilefoundation is widely used in projects in permafrost region because its high bearingcapacity and less thermal disturbance on the surrounding permafrost.Although people have accumulated very profound relevant construction practice ofpile foundation in unfrozen soil areas, the complex nature in the surrounding of pile inpermafrost and the imperfect design theory inevitably lead to a lot of new scientificquestions of the study of pile in permafrost. With the background of the newconstruction of Tahe-Mohe transmission lines, the load-carrying properties, the criterionof the static loading test and the failure mode of the pile foundation in permafrost wereresearched systematically.Firstly, the most reliable method to study on the bearing capacity of the pilefoundation is the static loading test. Therefore, the filling pile in situ static loading testwas conducted. While the static loading test of the pile was carried on, the temperatureof the permafrost around the pile and the original temperature of the site were comparedto judge whether the permafrost around the pile refrozen. The result of the two batchesprototype tests (before the permafrost around the pile refreezing and after the permafrostaround the pile being refrozen) shows that the tops of the pile’s load-displacementcurves slowly changed, and that there are no plunged section in the curves of thesettlement of the pile and the logarithmic of time. For instance, when the number5pilewas suffering the load on the top of the pile for2000kN, the settlement of the top of thepile is only2.3mm.Secondly, the nuclear magnetic resonance (NMR) analyzer imported from Englandwas used to fulfill the indoor experiment to determine the unfrozen water content withthe undisturbed soils (four layers in total) from the permafrost in Mohe region. Usingthe nitrogen pressure in the experiment to simulate the in-site gravity pressure onundisturbed soils, the relationship between temperature and unfrozen water content canbe obtained. Then, the relationship between the modulus of frozen soil and unfrozenwater content can be established. The indoor experiment results give service to thefollow-up numerical parameters. Thirdly, based on the software ABAQUS, the three-dimensional finite elementcomputational model of the site in the prototype number5test pile is established. Thenumerical results and prototype test were compared and analyzed. Then by changing thelength of the pile, the diameter of the pile, the modulus of the pile, the pile Poisson’sratio, the internal friction angle of the pile-tip bearing soil, the cohesion of the pile-tipbearing soil, the unfrozen water content of the pile-tip bearing soil in differentconditions (a total of29kinds of conditions), the curves of the pile’s load-displacementin different conditions and the main factors which influence the bearing capacity of pilefoundation in permafrost regions can be achieved.Finally, the anchor pile reinforcement is pulled off and the static loading test is notdestructed. Therefore, the loading test of the number5pile continued until it reached todamage. According to the static loading test in current norms which determine theultimate bearing capacity of pile foundation with the settlement deformation of40mmon the top of the pile exist problems. The settlement of the top of the pile used todetermine the ultimate bearing capacity in permafrost, according to the uniaxialcompressive strength test value and the numerical calculation, should be matched withthe temperature of the permafrost, and be certainly less than40mm. And then thefailure mode of the pile foundation were analyzed when the temperature of the pile-tipis2℃and-6℃. The results of the study show that the failure mode of the pilefoundation in the permafrost is triaxial pressure-shear failure.