The impact of a PEER funded research project "Prediction of Seismic Compression of Unsaturated Backfills" is highlighted below. The project Principal Investigator (PI) is John McCartney, Professor and Department Chair, Department of Structural Engineering, UC San Diego. The Research Team includes Wenyong Rong, Post-doctoral Researcher, UC San Diego.
Seismic compression is defined as the accrual of permanent contractive volumetric strains in soils during earthquakes and has been recognized as a major cause of seismically-induced damage in earth structures. Although backfill soils are typically in an initially dense state and are expected to have minimal settlement under static or traffic loading, they may still experience volumetric contraction during earthquakes. Even small backfill settlements can have a negative impact on the functionality of transportation systems and can lead to high repair costs. Most approaches for seismic compression prediction are semi-empirical, which have been shown to result in variable predictions, and do not necessarily consider the impacts of unsaturated conditions. Accurate predictions are challenging for unsaturated soils, as the degree of saturation and matric suction (the difference between pore air and water pressures) will change during volumetric contraction and will affect the effective stress and dynamic soil properties (e.g., the shear modulus, damping ratio). Generation of pore air and water pressures depend on the bulk fluid modulus and on the initial degree of saturation in the soil.
Figure 1: Preliminary simulations of seismic compression of unsaturated sand having an initial degree of saturation of 0.4 highlighting evaluation in key hydro-mechanical variables: (a) Applied sinusoidal cyclic shear stresses; (b) Calculated volumetric strain; (c) Calculated pore air and water pressure; (d) Calculated matric suction; (e) Calculated degree of saturation; (f) Calculated mean effective stress