8.3 Dynamic Soil Properties
Important elements in a seismic response analysis are: input motions, site profile, static soil properties, dynamic soil properties, constitutive models of soil response to loading and methods of analysis using computer programs. The contents include: earthquake response spectra, site seismicity, soil response to seismic motion, seismic loads on structures, liquefaction potential, lateral spread from liquefaction, and foundation base isolation.
The properties that are most important for dynamic analyses are the stiffness, material damping, and unit weight. These properties enter directly into the computations of dynamic response. In addition, location of the water table, degree of saturation, and grain size distribution may be important, especially when liquefaction is a potential problem. One method of direct determination of dynamic soil properties in the field is to measure the velocity of shear waves in the soil. The waves are generated by impacts produced by a hammer or by detonating charges of explosives and travel times are recorded. This is usually done in between bore holes.
Figure 3: Relation between number of blows per foot in standard penetration test and velocity of shear waves