Experimental Study On Dynamic Characteristics Of Saturated Soft Clay Under Principal Stress Axis Rotation

Traffic load is a kind of dynamic load with long cumulative acting time and many acting times.In engineering design,its action on foundation soil is usually simplified as the horizontal and vertical forces,and the continuous cyclic rotation of the principal stress axis of foundation soil caused by traffic load is seldom considered,thus the deformation of foundation soil will be underestimated,and even engineering accidents will occur.Therefore,it is of great significance to explore the dynamic characteristics of saturated soft clay under the condition of continuous cyclic rotation of principal stress axis for the safety construction of transportation infrastructure in soft clay region and the prevention of engineering disaster.In this paper,the saturated soft clay in Tianjin area is taken as the research object.Through cyclic triaxial and cyclic torsional shear tests,the dynamic characteristics of soft clay under the coupling changes of principal stress axis direction and magnitude caused by traffic loads are systematically studied.The effects of different intermediate principal stress coefficients and deviatoric stress levels on the non-coaxial characteristics of soft clay are analyzed.The softening and deformation characteristics of saturated soft clay under different cyclic stress ratios,shear stress ratios and loading frequencies were investigated.Based on these,a cumulative plastic strain model considering different factors was established,and its applicability was verified.The main contents and achievements of the research are as follows:(1)Cyclic triaxial and cyclic torsional shear tests under different intermediate principal stress coefficients and deviating stress levels were carried out to study the development law of rotation Angle of principal stress and strain components with large principal stress direction,and to analyze the change of principal strain increment direction and non-coaxial Angle.The experimental results show that the principal stress changes periodically with the rotation Angle of the principal stress axis in the coupling change of amplitude and direction caused by traffic load.When the rotation effect of principal stress axis is considered,the values of strain components are obviously larger.In the process of principal stress axis rotation,saturated soft clay has obvious non-coaxial characteristics,and the non-coaxial Angle fluctuates with the increase of rotation Angle,and the cycle period is about 90°.When the rotation Angle is the same,the non-coaxial Angle decreases with the increase of the intermediate principal stress coefficient.The value of deviated stress affects the non-coaxial Angle,but does not affect the fluctuation of non-coaxial characteristics.(2)Cyclic triaxial and cyclic torsional shear tests under different cyclic stress ratios,shear stress ratios and loading frequencies were carried out to study the effects of different factors on stress-strain hysteresis curves,elastic modulus and softening index,and to establish a prediction model of softening index.The experimental results show that the rotation effect of principal stress axis will increase the degree of softening.With the increase of cyclic stress ratio and shear stress ratio,the axial elastic modulus decreases and the softening index increases,but the change law is opposite with the increase of loading frequency.According to the traditional triaxial stress path,the axial elastic modulus attenuates not significantly with the increase of vibration times.After 5000 cycles of rotation,the softening index is 0.92.When the loading path is heart-shaped stress path,the softening index decreases to 0.45.(3)An improved empirical model was used to analyze the cumulative plastic strain of samples under different stress paths,and the effects of cyclic stress ratio,shear stress ratio and loading frequency on the deformation characteristics of saturated soft clay were investigated.The results show that the cyclic rotation of principal stress axis accelerates the accumulation of axial plastic strain,which increases with the increase of cyclic stress ratio and shear stress ratio,and decreases with the increase of loading frequency.For Tianjin soft clay in the frequency range of 0.5HZ-4.0Hz,a cumulative plastic strain prediction model was established based on Monismith power function model,which considered three factors including cyclic stress ratio,shear stress ratio and loading frequency.The applicability of the prediction model was verified by combining the existing test data.