NUMERICAL AND PHYSICAL MODELING OF STRESS PATH AND CONFINING PRESSURE DEPENDENT NONLINEAR BEHAVIOR OF GRANULAR SOILS

Abstract

This dissertation describes the development of an empirical correlations related small strain shear modulus and application, development of strength corrected normalized reduction curve, and simulation of stress path dependent soil behavior and application to calculate of contact pressure below shallow foundation. The first part of the thesis describes the development of an empirical correlation between small strain shear modulus and axial stress at failure. Building upon a previously reported novel hypothesis suggesting a unique linear relationship between the small strain shear modulus and major principal effective stress at failure, a new empirical equation was proposed to estimate the drained shear strength from measured shear wave velocity. A series of drained triaxial tests on clean sands and natural granular soils were performed to measure the shear strength of granular soils. Shear wave velocities were measured using bender elements installed at the top and bottom caps. Although a strong correlation between maximum shear modulus calculated from the shear wave velocity and effective stress at failure is observed, it is shown that the correlation can be improved by additionally including the confining stress. A unique relationship between maximum shear modulus, effective stress at failure, and confining stress is shown to exist for all granular soils tested. The predicted friction angles from the proposed empirical relationship exhibited good agreement with the measured values. The applicability of the equation is further validated through measured data on silty sand from a published study. Also, the proposed correlation was applied to the field conditions in Korea and Japan to predict the secant friction angle depending on shear wave velocity profile, effective overburden pressure, and soil type. The calculated secant friction angle was compared with the secant friction angle predicted by the previous proposed empirical equation to estimate the applicability. The second part of the thesis describes the development of the strength corrected normalized shear modulus reduction curve. The newly proposed correlation between small strain shear modulus and shear strength at large strain was applied to adjust the shear strength of the nonlinear curve. The strength corrected normalized modulus reduction curve was fitted to Darendeli (2001)’s nonlinear curve up to a shear strain of 0.1%. The shear modulus at larger strain level was calibrated using a curve fitting parameter to match the target strength. The proposed procedure is very robust because the nonlinear curve can be constructed from shear wave velocity. A suite of 1D nonlinear site response analyses was performed using the proposed nonlinear curves based on the correlation between shear modulus and shear strength. It is shown that the shear strain greatly decreases when the strength correction is applied and unrealistic soft soil behavior can be avoided using the strength correction. The third part of the thesis describes the effect of stress path on the stress – strain response of granular soil. A series of drained triaxial test were performed, where the stress paths were varied. The laboratory tests reveal that the soil response is stress path dependent. We use one elasto-perfectly plastic and two plasticity constitutive models. The input parameters of the stress path dependent soil constitutive model were determined through the CID test results and the measured shear wave velocities at various relative densities and confining pressures. The triaxial test was modeled under the same conditions as the laboratory test through stress path dependent soil constitutive model, and the results were compared directly with laboratory test results. It is demonstrated that only one of the models provides an accurate estimate of the stress path dependent soil response. We use the constitutive model to calculate of contact pressure below shallow foundation. The calculated stress-strain curve below the shallow foundation is shown to depend on the confining pressure, capturing the stress dependency.