夏季工况下正常服役地热能源桩承载性能原位试验研究

Field test on the bearing behaviors of geothermal energy piles in natural service under the summer condition

正确认识正常服役地热能源桩的承载性能,能有效促进其理论和设计方法的研究,为其安全性评判提供依据。为探明正常使用状态下地热能源桩的工作机制,结合现场原位试验,研究桩顶完全约束条件下静钻根植桩的承载特性。在52 m静钻根植桩桩身和桩侧埋设光纤光栅传感器(应变、温度、压力),研究地热能源桩正常运行时的桩身和土体温度、桩身轴向应力和应变、桩周径向压力等变化规律。试验结果表明,加热14 d后换热系统达到稳定状态,桩身平均温度升高15.8℃,沿深度呈两端低中部高的分布形式,温度影响范围在2~4倍桩径。桩周径向附加应力与温度变化呈直线分布,最大拟合系数(0.861倍桩长深处)为0.014 MPa/℃。轴向附加应力与温度变化量也呈线性相关,3.05 m(0.059倍桩长深)处最大线性拟合系数达0.3 MPa/℃。附加桩侧摩阻力表现为两端小中部大,最大值位于(0.649~0.861)倍桩长深度范围。试验桩桩顶受到完全约束,中性点位于桩顶,桩身没有附加负摩阻力,因此正常服役地热能源桩的承载性能与桩顶约束条件密切相关。建筑荷载+温度共同作用下最大桩顶荷载为1 954 kN,仅为桩极限承载力的42.5%,不影响桩的安全性。

Understanding the bearing behavior of geothermal energy piles in natural service will help researches in theory and design method, and provide evidence for safety assessment. A pile-loading field test for static drill rooted geothermal energy piles(SDRGEP) was undertaken to investigate the working mechanism of the piles in service under temperature cycles. In a SDRGEP of 52 m depth,the fiber bragg grating(FBG) sensors(including strain,stress and temperature) were buried to measure the temperatures of the pile and the surrounding soil,the mobilized axial and radial stresses and strains of the pile,the shaft resistance,etc. Test results show that,after heating of 14 d,the heat exchanging system reaches a stable state and the average temperature of the pile is raised by 15.8 ℃ with a distribution of low in middle and high at both ends and an influence range of 2 to 4 times of the pile radius. It is also found that the relationships of both the mobilized radial stress and the mobilized axial stress with the temperature are linear,with maximum fitting gradients of 0.014 MPa/℃ at 0.86 times of the pile length and 0.3 MPa/℃ at 3.05 m depth respectively. The mobilized shaft resistance first increases and then decreases with the depth,reaching the maximum value at 0.649–0.861 times of the pile length. Whilst the pile head is fully constrained,the null point locates at the pile top and there is no negative side shear stress along the pile,showing that the bearing capacity of the pile is closely related to the constraint of the pile head. The maximum axial force at the pile head is 1 954 kN under the mechanical and thermal loads,which is only 42.5% of the ultimate bearing capacity of the pile,indicating that the pile is safe.