Experimental Investigation And Constitutive Modeling Of Anisotropic Sand Under Complex Static And Cyclic Loading Conditions

As typical natural materials,sandy soils are widely used in engineering constructions and are often under an anisotropic stress condition.Thus these soil elements sustain an initial static shear stress before being acted upon by cyclic loading,which may involve varying amplitudes and sequences of stress components,leading to complex consequences on the liquefaction response and constitutive behavior of sand.It is the fundamental subject of soil dynamics,and also the crucial premise of dynamic stability analysis for important projects.To investigate the undrained monotonic and cyclic responses of saturated sand and their interrelationships,a series of triaxial tests that can simulate the stress conditions combining both static and cyclic shear stresses were performed.Based on the experimental observations on the combined influences of static shear stress and cyclic loading irregularity,a sand plasticity model was developed to address the stress anisotropy and fabric evolution of soils within the framework of the anisotropic critical state theory.The main research results are as follows:(1)A series of undrained triaxial compression and extension tests were conducted under different static shear stress conditions.The elastic deformation characteristics,stiffness degradation,instability,and phase transformation responses,were presented and analyzed.Triaxial compression and extension tests on both loose and dense sand show distinctly different behaviors,which is dominated by the inherent fabric anisotropy.The initial inclination of the effective stress path varying with the preloading history is related to the stiffness anisotropy;while at intermediate to large strains,the undrained stiffness tends to decrease in triaxial compression but grows in triaxial extension with the static stress ratio.Moreover,the undrained strength and brittleness are found to be influenced by the initial stress states,and the stress ratio at which the instability occurs is not unique,and can be correlated with the proposed state index.(2)A series of undrained cyclic triaxial tests were performed covering a broad range of initial static and cyclic deviatoric stresses.The axial strain development,pore pressure response,and liquefaction susceptibility of sand with varying initial states were analyzed,and interpreted using the concept of dissipated energy.The results indicate that different stress conditions lead to three types of cyclic behavior:flow liquefaction,cyclic mobility and residual deformation accumulation,which may also be accompanied by flow deformation.By comparing the available test data obtained from the same sand with varying initial densities and confining pressures,the static shear effect on cyclic resistance was found to be dependent on the state of the sand,and the triggering conditions of flow deformation under cyclic loading can be interpreted with the instability response of sand under monotonic loading.In addition,the residual pore pressure generated in the sand may reach a limiting value depending on the static stress conditions and can be interpreted from the critical state soil mechanics.The concept of dissipated energy was then employed and a unique relationship between the residual pore pressure and dissipated energy was obtained for sand with varying static and cyclic stress amplitudes.(3)A systematic experimental work concerning the cyclic loadings with various types of irregular time histories were conducted under triaxial conditions,to investigate the combined effect of the loading amplitude and sequence on the liquefaction behavior of saturated sand.Both the conventional equivalent approach and the energy-based method were applied in liquefaction assessment under irregular loading conditions.The results indicate that the earlier the higher-amplitude stress pulses arrive in a time history,the larger pore pressure generation and thus the lower liquefaction resistance the sand samples display.The conventional equivalent approach involving uniform stress cycles is found to produce poor performance in quantifying the liquefaction susceptibility,while the trends of pore pressure and axial strain versus the energy dissipation are barely affected by the characteristics of the irregular loading patterns.Moreover,the normalized energy required to trigger initial liquefaction is not affected by the loading sequence and amplitude.There exists an approximately unique relationship between the residual pore pressure ratio and the accumulated energy,which is loading path independent.(4)An anisotropic plasticity model was established to address the effect of fabric evolution on the responses of sand within the framework of the anisotropic critical state theory,and then a bounding surface model with reasonable mapping rule was developed.In this model,the interplay of the fabric and its evolution with the loading stress path were explicitly considered.The state of sand that underwent different initial stress conditions was described by a modified state dilatancy parameter.The obtained monotonic and cyclic test results for both loose and dense sand that underwent a varying initial static shear stress conditions are compared with the model simulations with a satisfactory performance.Moreover,the significant sequence effect of random loading indicates that the soil behavior is generally path dependent,and can be explained from the perspective of the kinematic hardening concept that adopted by this model.