Experimental Study On The Static And Cyclic Behavior Of Saturated Soft Clay Under Complex Stress Path

Thick soft clay layers are widely distributed in the southeast of China. A lot of important projects, such as motorways, high-speed railways, airport runways, have been or will be constructed on the soft clay subgrade. Strain accumulation and strength degradation of the subgrade clays are induced by the static load of embankment and the cyclic load of traffic loading, leading to large deformation and unstability. These types of catastrophe cause great economic loss and even threat to the safety of humans. Hence, long-term service performance of important high-speed traffic engineering is urgently needed to be improved and it is necessary to develop effective catastrophe control technology for the soft clay subgrade. The key to solve this problem is to study the static and cyclic behavior of saturated soft clay under complex stress paths.In this study, high quality samples of natural Wenzhou soft clay are firstly obtained by thin-walled tube cutting method. A series of experimental studies are then conducted on them through GDS triaxial apparatus and hollow cylinder system. The detailed research contents are as follows:1. According to the actual stress conditions of natural soft clay subgrade induced by the static load of embankment, undrained shear tests on isotropic consolidated specimens and stress paths triaxial tests or shear tests under different major principal stress angles on K0consolidated specimens have been conducted. In this section, the characteristics of soil structure, dependency of stress path, and anisotropy of soft clay are researched. The influence of soil structure on the shear strength, stress-strain relationship and development of effective stress path is investigated. A formula for pore pressure of K0consolidated soft clay specimens under different stress paths in p-q plane is established. The uniqueness relationships of p-q-e are verified. The stress-strain relationship and pore water pressure evolution of K0consolidated soft clay specimens under different major principal stress angles are analyzed. The failure envelop in the τzθ-(σz-σθ)/2plane is obtained. The non-coaxial phenomenon between the increment of major principal stress and major principal strain is unveiled.2. One-way cyclic triaxial tests with number of50000under different confining pressures and cyclic stress ratios have been conducted to study the long-term cyclic behavior of soft clay induced by traffic loading. The pore pressure evolution, stress-strain hysteresis loops, resilient modulus degradation and permanent strain under large number of cycles are investigated. The long-term pore pressure-strain model is established based on the relationship between the distance of peak point in effective stress path to the CSL and the axial strain. An empirical equation for long-term resilient modulus of saturated soft clay is established, providing guidance for the chosen of the value of modulus in numerical analysis. Besides, an empirical equation for long-term permanent strain is established. The physical meanings and the determination methods of the two parameters, εp,1000and λ, in this equation are given. The long-term permanent strain prediction model is obtained by further regression analysis on the equation parameters.3. The steady resilient modulus and permanent strain after50000under different confining pressure and cyclic stress ratio have been researched. Results show that for CSR<0.32, the values of steady resilient modulus decrease with the increase of CSR. At the same time, the values of permanent strain increase slowly and almost linearly with the increase of CSR. While for CSR>0.32, the values of steady resilient modulus almost remain unchanged (Mr,50000/P0=90) and reach the "asymptote strength". At the same time, the values of permanent strain begin increasing rapidly. The linear relationship no longer exists and the values of permanent strain increase exponentially. Thus CSR=0.32is a critical value for Wenzhou saturated soft clay, which can be called "allowable cyclic stress ratio". When CSR is below this value, the long-term permanent strain of saturated soft clay specimens can be controlled below2%. The proposed allowable cyclic stress ratio can solve the problem that the threshold cyclic stress ratio is too conservative while the critical cyclic stress ratio may lead to large settlement when they are used in pavement engineering.4. According to the vibration performance of traffic loading, soil structure of soft clay and drained condition of subgrade, cyclic triaxial tests with constant confining pressure under different influence factors on isotropic consolidated natural Wenzhou soft clay have been conducted. The influence of soil structure, cyclic frequency, drained condition and intermittent cyclic loading on the pore pressure and strain evolution, stress-strain hysteresis loop development, modulus degradation and strain accumulation are investigated. The conclusions provide a lot of data for deeply understanding the cyclic behavior of soft clay under actual engineering conditions.5. Cyclic triaxial tests with the coupling of cyclic confining pressure and cyclic deviator stress have been conducted through GDS dynamic triaxial apparatus to simulate the characteristic of coupling cyclic deviator stress and cyclic horizontal stress in traffic loading. A computational formula for the total pore pressure (elastic pore pressue, residual pore pressure and transmit pore pressure) in VCP tests is obtained based on the pore pressure formula proposed by Yasuhara and Skempton theory. The influence of cyclic confining pressure on the cyclic strain, pore pressure and effective stress path are researched. Results show that the influence of cyclic horizontal stress can not be ignored when the cyclic behavior of saturated soft clay are investigated under traffic loading.6. Comparative experimental researches between cyclic torsional shear tests (CST) and cyclic triaxial tests (CT) have been conducted by GDS hollow cylinder system to simulate the characteristic of variation for both amplitude and direction of major principal stress in subgrade soft clay under traffic loading. The cardioid stress path in the2τzθ-(σz-σθ) plane induced by traffic loading is simulated by user-defined waveforms of cyclic vertical stresses and cyclic torsional stresses. Under a single cycle, the major principal stress angle is changed form-90°to90°. Results show that under the same vertical stress level, the cyclic rotation of principal stress axle in CTS test can accelerate the pore pressure accumulation and the specimen degradation. Furthermore, the generation of resilient and permanent strain is accelerated. With the increase of CSR, the differences between CTS and CT are widened. A permanent strain equation considering principle stress rotation is established by introducing the parameter of η to modify the CSR based on the permanent strain prediction model obtained by the long-term cyclic triaxial tests.