Description:

Abstract: One of the most common techniques employed in ground improvement is the addition of cements or cohering agents to soft clays or low relative density sands in order to enhance their strength and stiffness. Typically cementing agents such as ordinary Portland cement, lime, gypsum etc have all been used for improvement of soil strength. While optimal design strategies have evolved predominantly based on macro / continuum scale experiments an understanding of the micromechanics is paramount if design has to span multiple length scales. It is well documented that the amount of cohesion, confining pressure, and void ratio / density control the ensemble mechanical response of any cohesive-granular material. However, at the microstructural scale – the particle arrangement (fabric) and the cohesive bonds control the behavior. The results of a series of photoleastimetry experiments which were performed to investigate the microstructural manifestations of this artificially introduced cementation between particles will be presented. In this series of photoelasticity experiments we carefully parse the influence of fabric and the amount of interparticle cohesion. The fabric influence was controlled by recreating the exact same fabric for all the experiments (i.e. locking in the fabric), enabling a critical examination of the influence of interparticle cementation on the ensemble. A series of biaxial experiments were performed on these model cohesive granular materials with cohesive diads placed in random locations throughout the ensemble. These cohesive diads show interesting properties and act as sinks to force chains. This locked in fabric allows a clear delineation of changes in several other microstructural facets such as coordination number, particle fabric etc. The experiments reveal very interesting facets of the landscape of force propagation in the presence of cohesion, and provides extraordinary insights into the microstructure of composite soils, and sedimentary rocks. It also provides an ab-initio understanding of the stress-force fabric relations in materials such as cemented sands. Complimentarily, a series of in situ compression tests on the cemented sands were conducted inside a tomograph, allowing us an exploration of the particle scale mechanics. We propose fabric chains akin to force chains, to understand the propagation of contacts in a cohesive granular specimen under confined compression. The results of photoelastimetry and tomography offer insights at multiple length scales on the mechanics of cohesive granular materials.

Bio: Tejas is a professor of geomechanics at the Indian Institute of Science, Bangalore, INDIA. He is currently the Edward Curtis Visiting professor at Purdue University for this academic year. He got his PhD at Purdue University, followed by a postdoc in Cambridge University, and at the Center for Offshore Foundation Systems at the University of Western Australia.