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| Field test on the bearing behaviors of geothermal energy piles in natural service under the summer condition |
| FANG Pengfei1，2，GAO Xiang3，LOU Yang3，ZHANG Rihong4，XIE Xinyu3，WANG Zhongjin1，2，ZHU Dayong1，2 |
(1. NingboTech University，Ningbo，Zhejiang 315100，China；2. Ningbo Research Institute，Zhejiang University，Ningbo，
Zhejiang 315100，China；3. Research Center of Coastal and Urban Geotechnical Engineering，Zhejiang University，Hangzhou，Zhejiang 310058，China；4. ZCONE High-tech Pile Industry Co.，Ltd.，Ningbo，Zhejiang 315145，China) |
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Abstract 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.
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2021, Vol. 40 
