Experimental Study On The Static And Dynamic Behavior Of Anisotropic Sands Involving Rotation Of Principal Stress Axes

In embankment and slope engineering, not only the magnitude of principal stress of subgrade soil varies, but also the major principal stress axes rotates. This kind of "inclined" consolidation induced anisotropy involving the rotation of principal stress axes, on one hand leads to the arrangement of sand particles, and on the other hand results in the presence of various initial consolidated stress states, including the initial vertical deviatoric stress and the initial torsional shear stress. Under this complicated conditions, the deformation and strength behavior of soil would become more complex when subjecting to the further complicated loading involving the rotation of principal stress axes, such as wave loading and traffic loading. Hence, to simulate the complicated stress conditions induced by the "inclined" consolidation and further conduct the static and cyclic tests under complicated stress paths in the laboratory can provide the experimental facts and physical interpretation for modelling the behavior of sands under compliates stress paths.In this study, a series of experimental tests based on the isotropically and anisotropically "inclined" consolidated soils are carried out under various complicated stress paths involving the rotation of principal stress axes using the hollow cylinder apparatus. The major research contents are listed as follows:(1) A series of static drained shear tests based on the isotropically and anisotropically "inclined" consolidated specimens are conducted to evaluated the effects of the anisotropy, including both the inherent anisotropy informed in the sample preparation process and the anisotropy induced by the "inclined" consolidation, on the stress-strain relationship, peak shear strength, secant modulus, and non-coaxiality characteristics. The relationship between the incremental peak stress ratio, the initial secant modulus and the difference between the consoidated directions of principal stress and principal axis directions of stress increment are established.(2) Through the comparative analysis among the static shear tests under various consolidation conditions, the phase transformation stress ratio (q/p’)phase independent on the shear directions and consolidation conditions is obtained. Combined with the development of the stress-strain non-coaxiality, the effects of non-coaxiality on the stress dilatancy behavior is evaluated and the deviation between the actual stress-dilatancy curve and Rowe’s stress-dilatancy curve by introducing a non-coaxiality factor.(3) According to the shear tests under the rotation of principal stress axis on the isotropically consolidated specimens, the effects of stress ratio on the development model of volumetric strain are analyzed. The consistent phase transformation stress ratio (q/p’)phase with the value obtained from the static shear tests is achieved. When the stress ratio is lower than (q/p’)phase, volume change tends to contract during the rotation, and vice versa. On the other hand, the impact of non-coaxiality on the stress dilatancy behavior during the rotation through the introduction of non-coaxiality factor.(4) Based on the shear tests under the continuously rotation of principal stress axis on the anisotropically "inclined" consolidated specimens, the effects of the consolidation induced anisotropy on the development of strain components, the magnitudes of strain increments and non-coaxiality during rotation are examined. It is noted that the deviation between the actual direction at which the magnitudes of strain increments reach the peak and the direction～70°, can be interpreted as the maximum of non-coaxial angle. The effects of rotation cycles on the development mode of volume change is also studied, specimens contract accumulatively and turn to dilate within some range of rotation from the 5th cycles of rotation, and the dilation range of rotation becomes wider as the increase of the number of rotation cycles.(5) The cyclic drained tests based on the "inclined" consolidated subgrade soils beneath the low embankment are conducted to evaluate the effects of various initial consolidation stress states, including both the initial vertical deviatoric stress and the initial torsional shear stress, on the vertical permanent deformation behavior. The test results indicate that the magnitude of the vertical permanent strain in the first loading cycle εp,1 grows linearly with the increase of the initial vertical deviatoric stress, and is barely affected by the initial torsional shear stress. The increase of both the initial deviatoric stress and initial torsional shear stress can accelerate the growth of permanent strain when qv0>75kPa and τ0>OkPa, while the average growth rate of strain is hardly affected by the initial vertical deviatoric stress when qv0≤75kPa. A modified model accounting for the combined effects of initial vertical deviatoric stress and torsional shear stress has been proposed to predict the vertical permanent strain based on Barksdale logarithmic model.(6) The effects of various initial consolidation stress states induced by the "inclined" consolidation on the cyclic volumetric strains indicate that there exists a threshold value for the initial vertical deviatoric stress (qv0=75kPa), which divides the volumetric response into contraction and dilation during the cyclic loading. The presence of the initial torsional shear stress promotes the dilative behavior of soils, the dilation increases with the increase of τo. When accounting for combined effects of qv0 and τ0, the Critical Consolidated Line (CCL) originating from the K0 consolidated stress state, and with the inclination angle (α△σc)ccL in reference to the horizontal axis in (σz-σθ)-2σzθ plane is established. The contractive volumetric strain is induced when α△σc>(α△σc)CCL,otherwise the dilative volumetric strain is produced.(7) A series of "apple-shaped" dynamic cyclic stress path tests and conventional dynamie stress path tests are conducted on K0-consolidated saturated sand. Comparisons between the results under the two different stress paths indicate that the continuous rotation of principal stress axes induced by the traffic loading would accelerate the accumulation of the vertical permanent strains and aggravate the degradation of the resilient modulus. And with the increasing of CSR, the discrepancies in the deformation characteristics under different stress paths become more apparent. Finally, a modified formula of resilient modulus is proposed to evaluate the principal stress rotation effects based on Uzan model by introducing the torsional cyclic stress ratio CSRt.