Resonant Column Behavior of Unsaturated Near-Surface Sands

It is well established that the small-strain shear modulus is a function of the effective confining pressure. However, soil fabric is the principal factor that dictates soil behavior under low-to-zero confinement and is not accounted for in classical calculations. In order to better understand the behavior of unsaturated near-surface sands, a series of soil-water characteristic curve (SWCC) and resonant column tests was conducted to characterize the potential saturation-stiffness and confinement-stiffness relationships of a cohesionless sand at confining pressures less than 100 kPa. The results of SWCC testing indicated that suction does not significantly influence the overall stiffness of the specimen. Resonant column testing showed that irrespective of initial density state, the stiffness characteristics of a soil-controlled matrix under 0-kPa confinement can be approximated by a mean state (resonant frequency of 35.4 Hz; shear wave velocity of 130.3 m/s; shear modulus of 31.8 MPa) for most geotechnical problems. As the confining pressure is increased for samples below 70% saturation, there is an abrupt stiffening of the sample, increasing stiffness characteristics by as much as 223%, as the specimen transforms from a matrix-controlled sample to a confinement-controlled sample. This transformation is not observed for saturations in excess of 70%. For this material, the transition occurred at 22 kPa, which coincides approximately with the 1-m overburden definition of near-surface soils.