Upper Paleozoic to Mesozoic-age greywackes are widespread throughout New Zealand. This paper describes the characteristics of the greywacke rocks based on field mapping, laboratory testing and rock mass classification from sites around the country. The rocks comprise hard sandstones, interbedded sandstones and mudstones, and mudstones. Where unweathered, intact rock materials have unconfined compressive strengths generally above 100MPa and moderate to high modulus ratios. The rock masses, which are typically closely-jointed and commonly tectonically disturbed, have an unusual combination of very high intact strength and joints with low persistence. The effect of these properties on rock mass deformability and strength is illustrated by estimation of dam foundation deformability from tiltmeter measurements and assessment of critical foundation failure mechanisms from estimates of defect and global rock mass strengths.
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) where they are the bedrock of many of the country's engineering projects and also an important source of roading and concrete aggregate. Greywacke rocks are commonly closely-jointed as a result of their complex geological history. This paper is part of a long-term New Zealand research project into the engineering properties of unweathered greywacke. Studies at three sites (Aviemore Dam, Belmont Quarry, Taotaoroa Quarry – Fig. 1) have involved engineering geological mapping, laboratory testing of intact rock and assessment of rock mass properties (Read et al. 1999, 2003, Richards & Read 2006, Read & Richards 2007). Postgraduate studies involving mapping, laboratory testing and analysis of earlier in situ testing (e.g. Cook 2001) also form part of the project. This paper summarises geotechnical characteristics of greywacke rock masses and discusses influences of defect characteristics on their strength and deformation properties.
Greywacke sequences (Begg & Mazengarb 1996) consist of interbeds of:
Sandstone – coarse to medium grained, and medium to dark grey. Individual grains are poorly sorted angular quartz and feldspar, plus fragments of metamorphic and igneous rocks. The intergranular filling is clay minerals formed during induration or slight metamorphism.
Mudstone – layers of clay, silt or mud, generally dark grey to black, sometimes red from iron minerals. Proportions of mudstone to sandstone vary between localities.
For example, at Waitaki near Aviemore, mudstone is the dominant lithology, while elsewhere (e.g. Karapiro near Taotaoroa) sandstone dominates. More massive beds of both lithologies may be up to tens of metres thick, although more cyclic deposition can result in interbedding with discrete beds < 0.5m thick. Figure 2 summarises intact compressive strength and deformation parameters for greywackes from a number of sites. Sandstones, which have moderate modulus ratios, have strengths > 100MPa with stronger rocks being coarser grained. Mudstones, with moderate to high modulus ratios, are generally weaker than the sandstones and strong to very strong.