Geomechanical characterisation of discontinuous greywacke from the Wellington region based on laboratory testing

ABSTRACT

Greywacke rock masses are the most prominent (∼20%) bedrock type across New Zealand. This paper reviews the publicly available laboratory testing data on the geomechanical properties of greywacke from the Wellington region and compares the results with two recent studies. This includes compilations of density, unconfined compressive strengths (UCS), tensile strengths, Poisson’s ratio, Young’s Modulus, and m_i factor. This paper also presents the first published geomechanical data from a cataclasite zone in the greywacke of the Wellington region. The overall results presented in this paper highlight the influence of both weathering and discontinuities on the geomechanical properties derived from the testing of core samples. The results are discussed in the context of sampling bias, scale effects and recent developments in the geomechanical characterisation of discontinuous rock. Two different approaches for characterising the rock mass geomechanical properties of greywacke at the project-scale are presented. The first approach uses the geological strength index (GSI) as the main parameter to capture the change in rock mass character with depth while the UCS remains constant. The second approach uses the UCS as the main parameter to capture the change in rock mass character with depth, while the GSI value remains constant.

KEYWORDS:

• Intact rock
• deformability
• strength
• discontinuities
• weathering
• discontinuous rock mass

Acknowledgements

We would like to thank M. Pender from the University of Auckland along with S. Read and N. Perrin from GNS Science for discussions about previous testing programmes of greywacke rocks in the Wellington region. Help with the sample preparation and sample testing from Z. Bruce of GNS Science is gratefully acknowledged. The authors would also like to thank A. Wolter, L. Seelig, associate editor M. Brook, and two anonymous reviewers for their constructive comments which improved the manuscript. It’s our Fault and the Ministry of Business Innovation and Employment funded the Stability of Land In Dynamic Environments Project.

Data availability statement

The data that support the findings of this study are openly available in Zenodo at https://doi.org/10.5281/zenodo.4263346.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by Ministry of Business, Innovation and Employment.