ABSTRACT:
Greywacke rocks, which are widespread throughout New Zealand, are composed of strong to extremely strong, sandstones, interbedded sandstones and mudstones, and mudstones. The rock masses are closely-jointed and often tectonically disturbed as a consequence of their complex geological history. Local experience indicates that the combination of high intact strength and low defect persistence in a closely jointed rock mass is not well-suited to multi-parameter rock mass classification systems such as Rock Mass Rating (RMR) and Tunnelling Quality Index (Q). A preferred approach is a comprehensive engineering geological description to characterise the greywackes for input to a specifically developed descriptive classification system. This step provides a direct linkage to classifications like RMR or the simpler Geological Strength Index (GSI) classification system, which is based on defect spacing and quality only, as a required input for the Hoek- Brown failure criterion.
1 INTRODUCTION
In New Zealand, the term ‘greywacke’ is applied to the very well indurated to slightly metamorphosed, interbedded mudstones and muddy sandstones belonging to the Torlesse Supergroup. These Upper Paleozoic to Mesozoic-age basement rocks are widespread throughout New Zealand (Fig. 1). Many of the country’s engineering projects are sited in these rocks and they are also an important source of roading and concrete aggregate. Greywacke rocks are commonly closely-jointed as a result of their complex tectonic and geological history. This paper is part of a long-term New Zealand research project into the engineering properties of unweathered greywacke. Three dam or quarry study sites (Aviemore, Belmont, Taotaoroa – Fig. 1) have been engineering-geologically mapped, laboratory testing carried out to determine the properties of the intact rock and rock mass properties assessed (Read et al. 1999, 2003, Richards & Read 2006).
