Study of the micro-mechanical behaviour of the Opalinus Clay:: an example of co-operation across the ground engineering disciplines

The high degree of scientific cross-fertilisation possible between the three geo-engineering disciplines soil mechanics, rock mechanics and engineering geology, is demonstrated by means of a micro-mechanical model of the Opalinus Clay. After a brief review of Terzaghi's effective stress principle and the importance of micromechanical models in general, a conceptual study of a micromechanical model of a claystone is presented in some detail. The model is based on the Particle Flow Code (PFC) developed by Itasca Corp. It introduces into the model the pertinent composition and structure of the Opalinus Claystone established in the local engineering geology of Switzerland and SW Germany. This includes elongated clay platelets, various layers of densified water around the platelets, free water in the pores and a specific texture of the platelets after consolidation. The model is numerically subjected to a series of loading stages. It is shown that the micro-mechanical model reproduces a number of features which have been known for a long time in soil and rock mechanics but which are often intractable in conventional generic models. The features include non-linear stress-strain curves with pre-failure damage and post-failure strain softening, a non-linear increase of the particle contacts with loading, distinct clustering of deformations, clustering of micro cracks leading to the development of shear bands and hysteresis in cyclic loading. It is concluded that micro-mechanical models are promising tools for further development of our understanding of the mechanical behaviour of geological materials. They offer an excellent opportunity for scientific co-operation between engineering geologists and soil and rock mechanics engineers.