Experimental Study On The Effect Of Temperature Cycling On Shear Properties Of Cohesive Clays

In geothermal resource development,nuclear waste disposal and other projects,the impact of temperature changes on the mechanical properties of soil mass needs to be considered.For example,the energy pile will experience cyclic rise and fall in service,resulting in changes in the mechanical properties of soil around the pile,such as shear strength,which will affect the safety and stability of the project.Therefore,it is of great academic value and practical significance to study the impact of temperature cycles on the shear properties of soil mass.In this paper,temperature controlled triaxial apparatus was used to carry out temperature cycling tests of saturated clay under different conditions,studied the response of excess pore pressure,shear strength and effective stress path of clay to temperature,and carried out micro test research through mercury intrusion porosimeter,thermogravimetric analyzer and other instruments,and obtained the similarities and differences between the hardening effect of temperature cycling on soil and the volume hardening mechanism of mechanical loading unloading.The main work and conclusions are as follows:(1)Conduct temperature cycling tests on kaolin under different effective confining pressures,overconsolidation ratios,and temperature amplitudes,followed by consolidated undrained shear tests.The test results show that under the effective consolidated confining pressures of 100kPa,200kPa and 400kPa,the soil samples show similar characteristics to the over consolidated soil after temperature cycling.The consolidated undrained shear strength increase by 25.8%,22.1%and 14.8%,respectively.The critical friction angle remains basically unchanged.Temperature cycling can accelerate the softening rate of pore water pressure of overconsolidated soil in the shear stage,but has no effect on the residual friction angle and the effective stress path shape of overconsolidated soil.Temperature cycling can significantly enhance the shear strength of weakly overconsolidated soil with OCR=2,but has little effect on the shear strength of strongly overconsolidated soil with OCR=8.At temperature increments of 20℃,30℃,and 40℃,the temperature cycling effect has a significant improvement effect on the strength of soil,and the increase amplitude gradually increases with the increase of temperature increment.(2)By analyzing the consolidated undrained shear test results of different types of clay before and after temperature cycling,and comparing with the undrained shear test data of overconsolidated soil samples,it is found that with the increase of montmorillonite content in mixed clay,the peak pore pressure of soil samples in the undrained shear stage gradually decreases,and the strength curve also transits from strain hardening to strain softening.For the mixed clay with 20%and 40%montmorillonite content,the increase of the shear strength of the soil sample has a good correspondence with the plastic volume strain generated during the temperature cycle.For kaolin samples,the peak strength of the soil sample after temperature cycling is similar to that of the overconsolidated soil sample with OCR=1.5,but there are significant differences between the two in terms of excess pore pressure,effective stress path,and plastic volume strain increment,indicating that the hardening effect of temperature cycling on kaolin is different from the volume hardening mechanism of overconsolidated soil.(3)The scanning electron microscope test,mercury intrusion test and thermogravimetric analysis test were carried out for the normally consolidated and overconsolidated kaolin at room temperature and the normally consolidated kaolin after temperature cycling.By enhancing the strength of the water connection,the resistance that needs to be overcome for the mutual movement of particles increases.The increase in strength of kaolin caused by overconsolidation is due to the discharge of pore water in the soil,the reduction of macropores,and the closer proximity between soil particles,enhancing the interlocking effect and friction force between the soil.