Rock Mass Classification of highly variable Lismore Basalts and its application to shallow cover tunnels

摘要

As a result of the nature of basaltic lava flows they are subjected to a substantial degree of variability.rnThere has often been a significant amount of time between individual flows which has allowed soilrnhorizons to develop in variable locations. This creates significantly weaker horizons between eachrnflow. The rubbly nature of basalt flow surfaces has also exacerbated the weathering betweenrncompetent flow cores which makes determination of classification for the rockmass challenging.rnTherefore a classification system has been developed and engineering parameters of the LismorernBasalt formation derived based on the available field, sub surface investigations and tests results,rnsupplemented with structural geological mapping and 3D geological modelling along a tunnel sectionrnincluding portals. The encountered strata have been classified into engineering rock mass units usingrnan empirical classification system such as the Rock Mass Quality Index (Q-System) and typical activernand passive support systems determined. This process has been assessed and the relative merits ofrneach approach are commented on in this paper.