Non-isothermal Shear Test And Model Study Of Saturated Clay And Pile-soil Interface

Shallow geothermal energy is a kind of clean and renewable energy with abundant reserves,which can be applied to indoor heating and cooling.This will greatly reduce the dependence of buildings on traditional energy sources and achieve energy saving and emission reduction.Energy pile technology is an innovative architectural form for the utilization of shallow geothermal energy resources,which has been preliminarily applied and promoted.The operation of energy piles is accompanied by periodic temperature changes,and the mechanical properties of the pile-soil interface and the soil around the pile are changed under the action of temperature cycling.A large number of field test results indicate that the temperature cycling caused by the operation of the energy pile may cause changes in bearing characteristics and lateral damage.Therefore,it is of great economic and social significance to clarify the changes in the frictional properties of the pile-soil interface and saturated clay under the complex temperature-stress path,and to explore the stable properties under the thermo-mechanical coupling condition,so as to ensure the safety of the energy pile during the operation cycle and realize the further promotion of this technology.In this paper,the mechanical properties of the soil around the pile and the pile-soil interface under the temperature effect are studied experimentally and theoretically.In terms of experiments,this paper conducts shear tests on concrete-saturated clay interface and saturated clay samples with different temperature-stress paths by means of direct shear equipment that can realize heating and cooling functions,and studies the effects of different stress histories,different temperatures and cyclic loads at different temperatures.The results show that under different stress states and stress histories,the irreversible shrinkage or expansion of clay during heating process is one of the main mechanisms of OCR dependence on the behavior of pile-soil interface under nonisothermal conditions.The strength of the pile-soil interface under the heating-cooling cycle is relatively complex.In the process of increasing the number of temperature cycles,the mechanical behaviors of the contact interface between the overconsolidated soil and the naturally consolidated soil and the concrete show different rules.For normally consolidated clay and heavily over-consolidated clay,the interfacial shear strength increases and decreases monotonically respectively.However,in the experiment of moderately and lowly over-consolidated clay,it is found that the interfacial strength is non-monotonically variable.Similarly,under the heating condition,the peak shear strength of the normally consolidated soil gradually decreases with the increase of cycle times,while the over-consolidated clay increases slightly and then decreases.The thermal coupling behavior of saturated clay should be the result of the volume deformation caused by temperature load,the change of particle arrangement at the interface and the excess pore water pressure.In terms of theory research,based on the theory of thermodynamics and the establishment of the hyperelasticity energy method,the elastic potential energy model introduced in elastic invariants of higher order term and nonlinear term,considering the state of elastic modulus with confining pressure and the influence of compactness,considering bonding properties of clay particle materials at the same time,established the saturated soil thermo-elastic coupling nonlinear hyperelastic constitutive model.Under the framework of hyperelasticity theory,the thermo-elastic coupling mainly depends on the thermal expansion coefficient of the elastomer,which depends on the thermal expansion property of the mineral components of the geotechnical material and the nonisothermal interaction between the mineral and adsorbed water.Therefore,it can be concluded that the non-linearity of the thermo-elastic deformation of rock and soil is mainly due to the temperature dependence of the thermal expansion coefficient of bound water and mutual transformation between bound water and free water in the process of temperature change.From the perspective of elastic stability and thermodynamic equilibrium stability,the ultimate shear strength capacity and strength criterion of geotechnical materials are obtained theoretically.At the same time,based on the same thermodynamic energy method,the potential energy and energy dissipation models of the pile-soil interface are established in this paper,and the elastic-plastic constitutive relationship of the interface which can take the temperature effect into account is derived.In this model,the non-isothermal effect of pile-soil interface behavior mainly depends on the change of the elastic thermal expansion coefficient and the interface density under the action of temperature load.