Unloading effects in the dynamic response of granular soil

Abstract

Experimental studies are described in which two different types of air shock are used to induce stress waves in an elastically confined, cylindrical specimen of sand, the objective being to permit direct observation of the effects of unloading on the dynamic response of granular material. In one case, “pure” unloading waves are generated by suddenly subjecting the end of a prestressed specimen to pressures below atmospheric. In the second type of test, the specimens are impacted at one end by an air shock which has the form of a sharp front followed immediately by a rapid exponential decay of pressure. The decay of the shock pressure generates unloading waves which cause attenuation of the wave as it propagates in the granular material.

As a result of the tests, it is suggested that seismic-wave velocity may be correlated to the modulus of unloading cycles in a quasi-static test and that, when strainrate effects are included, a relatively simple model may be used to predict the dynamic response of granular materials.