Evaluation Nonlinear Soil Response In Situ

Evaluation of nonlinear soil properties is an important concern in geotechnical earthquake engineering. Typically, nonlinear properties are expressed in terms of the nonlinear reduction in shear and constrained moduli with strain and the nonlinear increase in material damping in shear and constrained compression with strain. At this time, there is essentially total dependency on laboratory testing to evaluate nonlinear soil properties. The accuracy and limitations involved in modeling in situ properties with laboratory evaluated properties remains to be studied. In an attempt to evaluate nonlinear soil properties directly in the field, an in situ test method is being developed at the University of Texas that dynamically loads a soil deposit while simultaneously measuring strains, soil properties, and pore water pressures. Initial testing with this method has focused on vertically loading an unsaturated sandy soil, evaluating the magnitude of induced strains, and assessing the variation of constrained modulus (in terms of compression wave velocity, VP) with effective vertical stress and vertical strain. Preliminary results show that the test method can be used to: (1) evaluate the increase in small-strain VP with increasing vertical effective stress, (2) induce nonlinear compressional and shear strains, and (3) evaluate the nonlinear reduction in VP with increasing vertical strain