Mechanisms Of Penetration And Heat Transfer Of Thermal Cone Penetration Testing

Thermal cone penetration test(T-CPT)is based on the static penetration technology,adding heating elements such as thermocouple,and then measuring the mechanical and thermal response of the probe in the process of penetration simultaneously.Thermal cone penetration technology consumes less time and has high accuracy.Through data interpretation,the mechanical and thermal characteristics of soil layer can be analyzed.As a convenient and quick in-situ test technology,T-CPT avoids the sampling disturbance of laboratory test and overcomes the defects such as high cost and long test time of in-situ thermal response test.It is of great significance to the development and utilization of geothermal energy.At present,most of the related studies of T-CPT are in situ experiments,and empirical formulas or simplified analytical solutions are proposed for inversion of soil thermal physical property parameters.However,many factors such as probe size and stress disturbance are ignored in the empirical formula and simplified solution.Therefore,this paper studies the T-CPT data interpretation method,heat transfer under the consideration of probe size and penetration-heat transfer coupling problems.The main research contents and achieved results are as follows:(1)A method of T-CPT probe and its penetration test were proposed to measure the thermal response of heating and cooling stages after the penetration was suspended to a certain depth.Considering the influence of probe size,specific heat capacity of soil and different heating/cooling cycles,the semi-analytical solution of heat conduction under the transient column heat source hypothesis is presented for one-dimensional heat conduction in the probon-soil system.Particle swarm optimization(PSO)is used to identify the parameters of inverse problems with complex boundary conditions.The results show that the data interpretation method can improve the prediction of thermophysical parameters under the condition of limited calculation amount.(2)The heat transfer of T-CPT in soil was simulated by ABAQUS finite element software,and the influence of the internal structure of T-CPT probe on the heat transfer was analyzed.The results show that with the increase of the thickness of the outer wall of the probe,the retrieved thermal property parameters of soil are lower than the actual values.When the length of the heating module of the probe is limited,the length of the heating module must be at least greater than 200 mm to meet the assumption of infinite length in the heat transfer analysis,and this minimum length guarantee needs to grow appropriately with the increase of the length of the heating and the thickness of the outer wall of the probe.(3)The penetration and heat transfer coupling analysis model of thermal cone penetration was established,and the change law of penetration resistance and deformation with penetration depth was studied.The tracking particle method is used to solve the problem of node displacement output caused by mesh redivision,and the displacement and strain laws of soil during the probe penetration process are obtained,which provides an effective way to study the penetration mechanism of soil element trajectory and strain path.The influence of the stress generated by the probe penetration on the heat dissipation test and the influence of the temperature field generated by the heat dissipation test on the stress state of the original soil are studied by numerical simulation.The results show that the thermodynamic coupling can affect the stress and displacement state of soil,and the more obvious the thermal response is,the greater the influence will be.Finally,the effects of heating time and compactness on the interpretation of T-CPT thermal physical parameters and the effect of single factor on the penetration resistance of the probe tip were studied by physical modelling tests.