Experimental Study On Dynamic Behaviour Of Saturated Sand Under Complex Stress Paths

Sand is the most widely existing granular material as is the foundation of extensive constructions bearing various complex dynamic loading such as traffic loading and earthquake. Traffic loading is kind of long-term cyclic loading with both the magnitude and the direction of principal stresses varying with time due to the motion of vehicles. With the increase in both traffic loading and vehicle speed, road pavements are more prone to gradual degradation such as fatigue cracking, rutting, and excessive vertical permanent. Earthquake is kind of short-term loading. Shear wave progagates causing shear stress under the level ground. With strength reduction and large deformation, liquefaction leads to significant costs and threaten life security. Hence, to study the dynamic properties under complex stress paths will promote the catastrophe control technology.In this study, experimental studies are carried out to investigate soil properties under various complex stress paths using hollow cylinder apparatus and dynamic triaxial apparatus. The main contents and conclusions are listed as follows:(1) Simultaneous increases in shear stress level and rotation of principal stress directions are commonplace within the significantly stressed zone beneath a shallow footing subjected to vertical static loading (loading or unloading). A series of experiments are carried out using hollow cylinder apparatus. Four kinds of stress paths are considered to study the volumetric strain, maximum shear strain, the major principal stress increment direction and strain increment direction: under fixed principal stress directions with increasing deviatoric stress ratio (shear loading with fixed principal stress directions); principal stress rotation with constant deviatoric stress ratio (pure principal stress rotation); simultaneous increase in deviatoric stress ratio and principal stress rotation; multilinear simultaneous increase in deviatoric stress ratio and principal stress rotation.(2) Vertical and shear stresses vary with time during traffic loading, leading to principle stress rotation. A series of tests with and without principle stress rotation are carried out using hollow cylinder apparatus with different levels of confining pressure and vertical stress. Tests results indicate that more permanent deformation is cumulated with principle stress rotation. Larger permanent deformation cumulated as the confining pressure or cyclic vertical stress ratio increased when the principal stress rotation was coupled into the test. A predicted model is proposed considering principle stress rotation.(3) Vertical and horizontal stresses vary with time during traffic loading. A series of constant and variable confining pressures tests are carried out to investigate the vertical permanent deformation, accumulated volumetric strain and stress strain hysteresis loop according to previous three comparison methods using cyclic triaxial apparatus. Tests results indicate that for most cases, larger permanent deformation cumulate when variable confining pressure is taken into consideration. The factors affecting vertical permanent deformation are analyzed and a model for predicting permanent deformation is established considering variable confining pressure.(4) Different sample preparations generate different fabrics. A series of variable and constant confining pressures stress paths are carried out using cyclic triaxial apparatus to investigate the effect of fabric on soil dynamic properties. The effect of fabric on permanent deformation is analyzed considering various relative densities. Meanwhile, the effect of fabric on shear velocity under different stress paths is discussed using bender element test. Tests results indicate that larger permanent deformation cumulated with moist tamping method compared with dry deposition method under same stress path and relative density.(5) According to the stress state in level ground under earthquake, undrained and drained monotonic as well as undrained cyclic torsional shear loading tests are carried out on saturated Nanjing sand at various densities in a hollow cylinder apparatus. The phase transformation line and failure line are obtained in monotonic torsional shear test. The developed modes of stress path in cyclic torsional shear tests are analyzed as well as the relationship between developed modes and phase transformation line.