Temperature effects on the in-situ mechanical response of clayey soils around an energy pile evaluated by CPTU

Abstract

Thermomechanical behaviour of clay has been widely investigated by well-controlled laboratory experiments, while in-situ test of thermal effects on soil is rare. The piezocone penetration test (CPTU) is first used in this study to directly evaluate the temperature effects under in-situ stress condition. A full-scale precast high strength concrete (PHC) pile was utilized as the heat source and a series of CPTU were conducted in adjacent layered clays after monotonic heating and thermal cycle. The results show that unprecedented temperature elevation leads to an apparent reduction of soil strength in all soil layers; this degradation was alleviated after the thermal cycle. After exposure to a higher temperature, the normally consolidated soil was improved even when tested at a lower elevated temperature; whereas, the overconsolidated soil could nearly recovered to the pre-heated value. The measured pore pressure (u2) also decreased after heating indicating a more overconsolidated or dilatant behaviour during shearing. By comprehensively considering the in-situ responses, the in-situ soils were believed to be unloaded by the thermally induced pore fluid pressurization and densified after cyclic thermal loading. Finally, a hypothesis is proposed to qualitatively explain the thermomechanical responses of in-situ soil in the framework of critical state soil mechanics theory.