The microstructural response of granular soil under uniaxial strain

Soil mechanical behavior is described and phenomenological (macroscopic) and microstructural (particulate) theories, developed to predict the constitutive response of granular media, are discussed. Special emphasis is given to the concept of fabric in granular media and how the concept relates to observed mechanical behavior. This report describes an experimental effort to quantify changes which occur in a cuboid specimen of granular silica material under uniaxial strains of up to 10 percent. The material tested was a poorly graded ottowa sand with specimens consisting of either 0.5- or 0.75-mm-diam. granules. The displacement fields perpendicular to the loading axis are generally not directly proportional to applied macroscopic strain as typically predicted by a local theory. Rather, the three 'zones' existing in a specimen under uniaxial strain are (1) a zone in which relatively high local displacements (and strain) occur (near the loading cap), (2) a zone behind this 'front' wherein the grains move directly in relation to the advance of the loading cap, and (3) a zone which has not yet responded to the advancing cap. Sidewall friction, although not negligible, appears not to play an overriding role in the grain micromotions.