Equivalent properties for layered media

For a significant class of soil-structure interaction problems, soil media have nonuniform properties. This includes both layered sites and soil continua with depth-dependent stiffness characteristics. An approximate method to characterize such sites is proposed as an aid to preliminary analysis.

Although numerical solution techniques exist for nonuniform media analysis, there are also appoximate methods based on uniform media. To aid in applying the latter method to a nonuniform medium, its properties are approximated by those of an equivalent uniform medium. In vertical motion, the nonuniform medium is modeled by a series of springs as influenced by stress level with depth. The spring system is then replaced by a single spring and hence, a single uniform half-space. The results of this type of analysis are extrapolated to degrees of freedom other than vertical motion. The method is applied to both stiffness and damping, giving a different set of equivalent uniform properties for each degree of freedom in either stiffness or damping.

An example problem indicates that the proposed method yields reliable results for impedance values. The approximation is acceptable within the limits of geotechnical accuracy. The only unconservatism occurs for values of translational damping because the method does not consider layering effects on radiation damping. A blanket damping reduction factor is recommended to account for those effects.