ABSTRACT:
In the design of open pits and shallow foundations engineers are often faced with materials that can be classified as rock but do not fit the current set of criteria used to assess their field strength (saprolites, regoliths, duricrusts (ironstones and carbonate caps), coralline limestones, etc.). These materials can be thought of as transition materials between rock and soil and typically exhibit low “intact” strength, which overshadows the presence of any macro features (such as relic structure) in the control of their field behaviour characteristics. A methodology for the estimation of strength parameters for these rock-soil transition materials is presented with guidance for practical application. A hybrid continuum-discontinuum model is employed to verify the approach. The methodology includes a progressive linearization of the Hoek-Brown strength envelope (a ? 1) in the transition zone as well as a progressive improvement of the controlling “s” and “m” parameters to the intact values (s ? 1, and m ? mi).
1 INTRODUCTION
1.1 Background Rock mass classification systems, including the GSI system, for rock mass strength estimation (Marinos & Hoek 2000) are based on the principle that structure within a rock mass (joints, fractures, joint surface alteration and degradation) acts to reduce both the cohesion and frictional properties of the intact rock, represented by a degradation in “s” and in “m”respectively in the Hoek-Brown non-linear criterion. While these systems serve the rock mechanics practitioner well, there is a class of materials at the low strength end of the rock competence scale which fall outside of the spectrum of materials classically treated as rock masses. These low intact strength materials in field conditions comprise saprolites, regoliths, laterites, duricrusts, among others. However, often they are not so weak that they classically have been thought of as soils and have therefore been dealt with by rock mechanics approaches.
