Characterization of a highly heterogeneous flysch deposit and excavation implications: case study from Auckland, New Zea
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ORIGINAL PAPER
Characterization of a highly heterogeneous flysch deposit and excavation implications: case study from Auckland, New Zealand B. Fleetwood 1 & M. S. Brook 1
&
G. Brink 2 & N. R. Richards 1 & L. Adam 1 & P. M. Black 1
Received: 10 February 2020 / Accepted: 10 June 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Engineering geological characteristics of the rapidly growing Auckland urban area are of increasing interest due to ongoing and planned tunnels and excavations. Auckland is mainly underlain by Miocene-age East Coast Bays Formation (ECBF), a highly heterogeneous flysch deposit, and is also affected by lava flows from the Late Quaternary Auckland Volcanic Field (AVF). Therefore, excavations in the region can encounter highly variable rock mass. Here, the geological strength index (GSI) and rock mass rating (RMR) were applied to outcrop in coastal cliff exposures located on Auckland’s North Shore, to evaluate the engineering geological properties of the ECBF. Samples were also extracted for laboratory analysis of elastic wave velocities, intact strength parameters and thin section analysis of microtexture. Specific issues identified include lenses of the heavily zeolitized Parnell Volcaniclastic Conglomerate, and marked strength contrasts within the weaker ECBF siltstone and sandstone. The ECBF sandstone, where uncemented, is much weaker and is highly permeable, while the presence of smectite clay may be problematic for tunnelling if it disaggregates, forming a ‘sticky spoil’. The presence of larger-scale defects has been problematic for tunnelling, including the presence of low shear strength clay seams along ECBF bedding planes, and bedding planes intersecting tunnel crowns. Keywords Rock mass classification . GSI . Geotechnical properties . Flysch . Auckland
Introduction Globally, over the last two decades, considerable emphasis has been placed on the use of rock mass classification systems to summarize and facilitate the characterization, classification and knowledge of rock mass properties (e.g. Palmstrom and Broch 2006; Russo 2009). This is despite some authors (Pells 2008; Aksoy 2008) questioning the apparent oversimplification of some rock mass classification systems. Moreover, other researchers have also highlighted the potential limitations of rock mass classification systems when implemented by inexperienced practitioners (Pells 2008). Nevertheless, rock mass classification systems can provide quantitative data and
* M. S. Brook [email protected] 1
School of Environment, University of Auckland, Private Bag 92019, Auckland 1010, New Zealand
2
Tonkin & Taylor, 105 Carlton Gore Road, Newmarket, Auckland 1023, New Zealand
guidelines for engineering design, and therefore present an extension from the original abstract description of rock mass structural geological parameters (Cai et al. 2004). In particular, heterogeneous rock masses such as flysch formations have received attention because of their complex mechanical behaviour, which can make design
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