Applications of the GSI System to the Classification of Soft Rocks

Abstract

The chapter deals with the geotechnical classification of weak and complex rock masses. The term “weak rock mass” instead of “soft rock” is generally used in this chapter to highlight better the nature of the examined geomaterial. The weak rock masses that are examined in this study are generated by tectonical compression or weathering either the parent rock is initially soft or not. Cases, where the decreasing of the quality is expressed on the rock mass scale and not necessarily on the primary low intact rock strength is thus presented. Use of the GSI rock mass classification system and the associated m, s and a parameter relationships linking GSI with the Hoek-Brown failure criterion provides a proven, effective and reliable approach for prediction of rock mass strength for surface and underground excavation design and for rock support selection. The need for geological definition of rock mass properties required as inputs into numerical analysis, allowing for characterisation of even the most problematic of weak and complex rock masses. Back-analyses of tunnels, slopes and foundation behaviour using the approach attest to its reliability. New or revised GSI charts for weak and complex rock masses are presented in this chapter. Specific key engineering geological characteristics that differentiate various igneous, metamorphic and sedimentary rock units one from each other and generates weak (soft) forms are presented.