Study On Bearing Characteristics Of Concrete Pipe Pile Composite Foundation In Island Permafrost Area Based On Pile-soil Interaction Mechanism

Highway construction inevitably changes the soil hydrothermal conditions in the island permafrost area.It intensifies the degradation of permafrost and the downward of permafrost table,resulting in the significant reduction in the bearing capacity of foundation.This leads to uneven settlement,longitudinal cracks,waves deformation and other damages in the roadbed.For this reason,this paper proposes the application of concrete pipe pile composite foundations to improve the foundation bearing capacity in the island permafrost area.The bearing mechanism and load transfer characteristics of composite foundation are investigated by combing the indoor tests,theoretical equation derivation and numerical simulation metod,and the main research contents and results are as follows:(1)Based on the strain-controlled direct shear tests,the contact surface characteristics,stress-strain characteristics,shear strength parameters and residual shear strength of the clayconcrete interface during thawing process were studied.Also,the effects of temperature,moisture content,and normal stress on the shear characteristics were analysed.The results show that the shear strength of the thawing clay-concrete interface decreases with increasing shear temperature and approaches the thawed state;the shear strength at-0.5°C is lower than that at positive temperature under the condition of high moisture content and high normal stress.(2)The shear stress-displacement curves of the thawing soil-pile interface at different melting states were fitted to obtain the models of the pile-soil interface considering different degrees of melting.A Gompertz model with hydrothermal coupling effect was established at-3°C to-1°C;A three-stage model with Gompertz model-linear model-ideal plasticity model was established at-0.5°C;And an exponential function model was established at positive temperature.The results show that the models of pile-soil interface action proposed in this paper have good applicability.On this basis,a calculation method for load bearing deformation and load transfer of monopiles in permafrost foundations under vertical load was proposed.(3)In order to analyse the bearing mechanism of composite foundations during the degradation of permafrost,a test device for testing the bearing performance of composite foundations in permafrost areas was developed and the bearing tests of composite foundations under different thawing depths were carried out.The results show that the bearing capacity of the composite foundation decreases during the degradation of permafrost,and the pile-soil stress ratiodecreases gradually.Also,the extra load is transferred to the soil between piles gradually.The distribution patterns of pile shaft resistance in composite foundations with various melting depth are different.When the melting depth is less than 0.7 times of pile length,the pile shaft frictional resistance can be divided into the plastic deformation zone with negative friction,the coordinated deformation zone and the elastic zone with positive friction.Conversely,the pile shaft frictional resistance can be divided into the plastic deformation zone with negative friction,the coordinated deformation zone and the plastic zone with positive friction.(4)Based on the distribution patterns of pile shaft resistance under different thawing depth and the slip characteristics between the pile and soil,the analytical expression for the settlement of the rigid pile composite foundation in frozen soil area is derived.The differential equation for load transfer between the pile and soil was apllied in the calculation process.Also,the changes in mechanical properties during the freezing and thawing of the soil and the pile-soil interface were considered.Finally,the applicability of this settlement calculation method was verified.(5)A numerical model of concrete pipe pile composite foundation in the island permafrost area was established,and the characteristics of temperature and deformation during the construction and operation periods were analysed.The results show that the reinforcement zone is in gradual thawing state due to the driving of the piles into foundation.The heat brought by the piles causes the continuously and long-term degradation of the island permafrast.The difference in temperature field at the pile-soil interface between the warm and cold seasons results in a different pile load transfer mechanism.Meantime,the road is in a more stable state during the cold season.The concrete pipe pile composite foundations have good applicability for treating the foundation in island permafrost area.