冻土区输电线路桩基础抗拔承载特性数值模拟研究

Numerical analyses of uplift behavior of pile foundation for transmission line structure in frozen soil regions

输电线路工程现已成为我国冻土工程的重要组成部分,而桩基础是冻土区输电线路杆塔较为通用的基础型式。输电铁塔是典型的高耸结构,抗拔与抗倾覆稳定性是铁塔基础设计的主要控制条件。通过回顾国内外相关文献,发现冻土区桩基础抗拔承载性能研究相对较少,尤其是上拔与水平荷载共同作用时,对其承载机理、荷载传递规律等认知模糊不清,给冻土区桩基础设计带来不便。为此,采用数值计算方法,分析了季节冻土区与多年冻土区粉质黏土、砾砂地基中桩基础抗拔承载性能。结果表明:冻土区桩基础破坏以上拔为主;上拔荷载-位移曲线呈缓变型;同种地基土质条件下,相较融化期,冻结期桩基础抗拔承载力提高20%;相较粉质黏土,砾砂地基承载力提高20%;随着水平荷载增加,桩顶竖向位移增大,导致桩基抗拔承载力下降。

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 FLAC3D 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.