关键词: 冻土, 桩基础, 抗拔, 承载力, 破坏模式

Abstract:

Pile foundation is one of the most commonly used and suitable foundations to support transmission line structure， especially in seasonally frozen soil regions and permafrost regions. Axial compression is the controlling condition in the design of foundations for such structures as bridges and buildings， while uplift and overturning will control the design of transmission line structure foundations. This paper presents an extensive overview of previous studies including experimental （e.g.， laboratory model test and full-scale field load test）， analytical/theoretical （e.g.， limit equilibrium and limit analysis based on plasticity） and numerical （e.g.， finite difference and finite element methods）. The review indicates that study on the uplift behavior of pile foundation in frozen soil is relatively limited， particularly in the case of combined effect of axial uplift and lateral loading. Interaction between pile and frozen soil and mechanism of load transfer along the pile shaft and around the pile tip still remain unclear. Therefore， this paper implements finite difference analysis within FLAC^3D to investigate the behavior of pile foundation in frozen silty clay and gravelly sand under axial uplift behavior and the effect of ground condition and lateral loading on the uplift behavior. Because of the axisymmetric condition of the problem studied， only half of the model is simulated. The chosen domain of the medium is discretized into a set of quadrilateral elements and the pile is discretized by the cylinder element. The interaction between the soil and pile is considered according to interface elements. Mohr-Coulomb criterion is adopted to model the soil behavior （perfectly elastic-plastic）， while the pile is simply considered as a rigid body. The soil parameters such as Young’s modulus， cohesion and internal friction angle used for numerical analyses are determined by laboratory tests and estimated according to the empirical correlations with in-situ tests. The present numerical modeling is verified with the results from field loading tests on pile foundations in Qinghai-Tibet ±550 kV transmission line project. On this basis， parametric studies are carried out to uncover the behavior of pile in frozen soil. It is observed that pullout is the dominant failure mechanism of pile and the uplift load-displacement curve clearly exhibits an asymptote， consisting of initially linear elastic， nonlinear transition， and finally linear regions. These results are consistent with the observations in a few previous studies. In addition， larger uplift capacity of pile foundation in freezing period and gravelly sand is gained （about 20%）. Lateral loading increases the deflection and therefore， decreases the uplift capacity of pile foundation. For the convenience of using the results obtained in practice， the values of uplift factor for pile foundation in silty clay and gravelly sand are provided. Finally， it should be noted that the method used， and the results obtained in the current work could be useful for engineers and designers， at least providing them some qualitative evidence for pile design in seasonally frozen soil regions and permafrost regions. This is important and necessary to ensure the safety of construction in such regions. Meanwhile， numerical analyses in the current work can be a benchmark example for subsequent research studies.

Key words: frozen soil, pile foundation, uplift, capacity, failure mechanism