INFLUENCE OF VISCOUS COUPLING IN PROPAGATION OF ELASTIC-WAVES IN SATURATED SOIL

The interactions between the solid and fluid phases of saturated porou s media are due to inertial, viscous, and mechanical coupling. In part icular, viscous coupling plays a key role because it makes wave propag ation dispersive. The effects of viscous coupling on harmonic problems were described in detail by Riot, but the implications in transient p roblems have not been fully analyzed. Therefore, a detailed analysis i s carried out on the effects of viscous coupling on the mechanics of t ransient wave propagation, by considering the propagation of simple sh aped driving pulses (a step pulse, a single sine, and a single triangl e), for both constant and frequency-dependent viscous coupling. Partic ular attention is paid to the interpretation of dynamic soil test meas urements, because of their importance in the current practice of soil investigation, both in laboratory and in situ. Results show that it is possible to identify two extreme kinds of transient behavior: in the first, the porous medium behaves as a two-phase medium in which the ve locity of propagation corresponds to null viscous coupling; in the sec ond, the behavior corresponds to a one-phase medium with velocity of p ropagation corresponding to infinite viscous coupling. There is a grad ual transition between these two extreme behaviors, but it extends for quite a narrow range of values of travel length and of the coefficien t of permeability for a given frequency content of the driving pulse. Such conclusions are very useful in the interpretation of dynamic meas urements and should enhance the comprehension of the mechanics of disp ersive wave propagation in saturated porous media.