Field measurement of the linear and nonlinear constrained moduli of granular soil
Traditional field seismic measurements have been performed for more than 50 years to determine the small-strain shear and constrained moduli of geotechnical materials under existing conditions. Field measurements to characterize the nonlinear response of the constrained modulus have received essentially no attention in the engineering community. This study was undertaken to characterize the in-situ response of the linear and nonlinear constrained moduli in one testing method. In this dissertation, a field method is presented which uses large shakers to impart vertical sinusoidal excitations directly above an embedded sensor array. This methodology essentially performs parametric studies on the constrained moduli of geotechnical materials in-situ over a wide range of axial strains. In this study, embedded sensor arrays at two different locations were constructed. A staged loading sequence was used to determine the constrained compression wave velocities between sensors in the linear, i.e. small-strain, and nonlinear strain ranges. Constrained moduli were determined using the mass density of the soil and the constrained compression wave velocities. The axial strains generated between sensors were estimated using a displacement-based method. At both sensor arrays, the method successfully measured in the field: (1) the variation of the small-strain constrained compression wave velocity with increasing confining pressure and (2) the effect of axial strain on the constrained moduli of soil in various states of stress. The field measurements indicate that, at lower levels of confining pressure, the constrained modulus increases slightly with increasing axial strain, but then decreases with increasing axial strain. However, in other cases, the constrained modulus increased with increasing axial strain and showed little or no tendency to reach a "peak" value. The nonlinear stress-strain behavior of the constrained modulus is quite complex and appears to be a function of several factors, including the amount of overconsolidation and cementation in the soil and the locations of the sensors in the array. Therefore, while the results of this study indicate that the proposed field method can be successfully used to investigate the constrained modulus, more work is required in this area to fully quantify the response of the constrained modulus in the nonlinear strain range.