Estimating rippability of Brisbane Tuff using quantitative and qualitative characteristics and a modified approach to an existing rippability rating method

Abstract

[Abstract]: The aim of this research paper is to assess the rippability of Brisbane Tuff (welded ignimbrite) using both quantitative and qualitative information collected from geotechnical borelogs and rock strength testing for the North–South Bypass Tunnel (NSBT) project in Brisbane. Rock properties were quantified into specific ratings and used in an existing rating method (“The Estimation of Rock Rippability” F. MacGregor et al. 1994) to estimate productivity of rippability for general bulldozer types, using regression equations. Some qualitative classifications such as ‘weathering / staining’, were modified for the Brisbane Tuff based on interactions between weathering characteristics and strength for this rock type. These modified ratings were then re-entered into F. MacGregor et al. 1994 equations, to give an alternative estimation of rippability based on these proposed variable dependencies. Correlations between point load index (PLI or Is 50) and unconfined compressive strength (UCS) were also determined specific to the data set used for this research, and compared to findings with existing literature on Brisbane Tuff. This UCS / PLI correlation enables use of the much faster, cheaper point load index test Is (50) to correlate a UCS (MPa) value, which forms the backbone in several F. MacGregor et al. 1994 regression equations used in estimating rippability of rock. Results and information collected were part of a data set collected by the author whilst working for Golder Associates, and is a stand alone assessment of rippability unique to the limited data set used, and not necessarily relevant to any advice given to the client for the North-South Bypass Tunnel (NSBT).