Cohesionless soils exhibit a loss of stiffness and strength when subjected to earthquake or explosive loading, followed by a gradual recovery as confining pressures and/or cementitious bonds redevelop. If the soil experiences further shock loading during this period of reduced stiffness, the soil experiences large shearing strains, and may liquefy. However, quantitative measures of this behavior are not available. This research program is a laboratory and field investigation to observe the changes in soil stiffness as a function of the soil's properties, stress history, and magnitude of disturbance, in order to determine the increased risks associated with secondary disturbances. The laboratory phase of the program uses two types of resonant column apparatus to monitor soil stiffness: a Hardin-type device modified for impulse loading, and a recently-developed resonant column-torsional shear device. The former apparatus can generate high pressure, short- duration impulse loads typical of explosions, while the latter device will simulate earthquake-type ground motions. Stiffness losses following detonation of conventional explosives will be monitored during the field testing. Subsurface dynamic pore pressures and ground motions will be measured and related to the changes in the soil stiffness as determined by cross-hole shear tests.