Influence Of Overburden Pressure On Thixotropy Of Structural Clay In Zhanjiang Formation

In recent years,driven by the development of the ASEAN Economic Circle,a large number of engineering projects have begun to be constructed in the Beibu Gulf region.A structural clay of Zhanjiang Formation with thixotropy is widely distributed in the Beibu Gulf region,and its special properties have caused difficulties for local engineering construction.The overlying pressure has a significant impact on the thixotropy of soil mass.The different magnitude of overlying pressure results in different degrees of thixotropy of structural clay in the Zhanjiang Formation,resulting in differences in the strength recovery degree of structural clay foundation in the Zhanjiang Formation.Exploring the impact of overlying pressure on the thixotropy of structural clay in Zhanjiang Formation can help to deeply understand the essential reasons for the different degrees of recovery of foundation thixotropy strength caused by overlying pressure,and provide a basis for evaluating the stability of foundations with thixotropic clay.In this paper,the basic physical and mechanical properties of Zhanjiang Formation structural clay from three different locations were tested to obtain the physical and mechanical properties of the undisturbed soil,and then the remolded soil corresponding to the three types of undisturbed soil was subjected to different overburden pressures(200k Pa,250k Pa,300k Pa,350k Pa,400k Pa),and the direct shear tests were carried out at different resting times(0d,7d,14d,28d,63d,91d),The change law of thixotropic strength ratio and thixotropic recovery strength ratio with overburden pressure was explored.Then,the structural clay of Zhanjiang Formation at different thixotropic periods under different overburden pressures was tested by SEM,and the obtained SEM images are analyzed qualitatively and quantitatively by PCAS image analysis software to explore the evolution law of soil microstructure with overburden pressure.Finally,the micro mechanism of the influence of overburden pressure on the thixotropy of structural clay in Zhanjiang Formation was clarified from the macro and micro perspectives.The main conclusions of this paper are as follows:(1)The sensitivity of structural clay in the Zhanjiang Formation at three different locations is high,medium,and low,respectively;The initial consolidation pressure P_c is around 300k Pa.(2)Under the same overburden pressure,the shear strength,thixotropic strength ratio and thixotropic recovery strength ratio of structural clay of Zhanjiang Formation with high,medium and low sensitivity increase with the increase of resting time.The increase rate is the fastest in the early thixotropic period,the slower in the middle thixotropic period,and the slowest in the late thixotropic period until gentle.The higher the sensitivity of the soil,the greater the rate and amplitude of the increase of its shear strength,thixotropic strength ratio and thixotropic recovery strength ratio with the increase of the resting time.At the same resting time,the shear strength of structural clay of Zhanjiang Formation with high,medium and low sensitivity increases with the increase of the overburden pressure,showing a positive correlation,while the thixotropic strength ratio and thixotropic recovery strength ratio gradually decrease with the increase of the overburden pressure,and the reduction is large when the overburden pressure is greater than 300k Pa in the middle and later resting time.The higher the sensitivity of soil,the higher the thixotropic strength ratio and thixotropic recovery strength ratio with the increase of overburden pressure,but the shear strength is not necessarily greater,and there is no specific correlation.(3)At the same resting time,the probability entropy,fractal dimension of particle distribution and surface porosity of structural clay in Zhanjiang Formation gradually decrease with the increase of overburden pressure,and the larger the decrease is when the overburden pressure is greater than 300k Pa.Under the same overburden pressure,the probability entropy,fractal dimension of particle distribution and surface porosity of structural clay in Zhanjiang Formation gradually decrease with the increase of resting time,and the lower the decrease with the increase of resting time under the higher overburden pressure.(4)The essential reason why the thixotropy of structural clay in Zhanjiang Formation weakens with the increase of overburden pressure is that the higher the overburden pressure,the slower the increase rate of the orientation of soil particles,the increase rate of agglomeration and the decrease rate of inter-particle porosity of structural clay in Zhanjiang Formation.