摩擦型能源桩荷载–温度现场联合测试与承载性状分析

In-situ tests and thermo-mechanical bearing characteristics of friction geothermal energy piles

能源桩集地源热泵技术与建筑桩基于一体,桩基承载性状受荷载–温度耦合作用而不同于常规工程桩。开展了昆山某摩擦型能源桩工程的荷载–温度现场联合测试,测试了多级荷载水平与不同换热工况下桩身的温度、应力分布及桩顶位移变化,整理得到桩身的附加温度荷载、桩身轴力及桩侧摩阻力分布曲线,分析了摩擦型能源桩荷载–温度作用下的承载性状与荷载传递特征。测试结果表明:能源桩的温度变化将引起附加温度荷载,桩身轴力和附加温度荷载分布特征和桩顶荷载作用、桩端约束有关,承载性状不同于单一荷载作用下的工程桩。加热工况引起桩身上、中部多处出现负摩阻力,但荷载的增加有利于减小升温引起的负摩阻力效应;制冷工况下,桩端附近产生负摩阻力,能源桩荷载传递特征受荷载–温度耦合作用而改变。设计荷载作用下,能源桩顶加热时隆起而制冷时下沉,加热工况引起的桩顶位移在停止加热回温后可基本恢复,但制冷工况引起的桩顶位移在回温后会导致桩顶产生附加沉降,荷载–温度耦合作用也引起了能源桩沉降性状的变化。

Geothermal energy pile is the new pile technology that combines the ground source heat pump technology and building pile foundation. Due to the pile as the heat transfer carrier of ground source heat pumps, the bearing characteristics of energy piles are different from those of the conventional engineering piles because of thermo-mechanical coupling. In-situ thermo-mechanical tests on drilled friction geothermal energy piles of a Kunshan project are performed. The temperature, stress distribution of pile shaft and displacement of pile tip are investigated under multi-level loadings and heat exchange cases. The distribution curves of temperature-induced loading, axial force and friction resistance of pile shaft are obtained, and the thermo-mechanical bearing properties and load transfer characteristics of drilled friction energy piles are analyzed. The results indicate that the additional temperature loading is induced in energy pile shaft by temperature change. The characteristics of axial force and the temperature-induced loading of energy piles are affected by pile loading and constraint of pile toe. The bearing properties of the energy piles is not the same with that of the engineering piles by loading only. Under heating condition, the negative friction resistance of pile shaft is produced at the upper and middle parts of pile, but the effect of negative friction resistance decreases by the increasing loading. While under cooling condition, the negative friction resistance of pile shaft is produced near the pile toes. So the load transfer characteristics of energy piles are changed by the thermo-mechanical coupling effect. Under the design working loading, the head of energy piles is uplift under heating case and dropped under cooling case. The displacement of energy pile head at heating process is almost restored after temperature is recovered, but the temperature-induced settlement of energy piles at cooling process remains after cooled. So the settlement characteristics of energy piles are changed by the thermo- mechanical coupling effect.