A legacy slope failure in Penlee Quarry - a warning to others

Abstract

Penlee Quarry is a large quarry in West Cornwall that has been in operation since the late 1880s. It was a major producer of aggregate, but since 2003 under new ownership, quarry operations have concentrated on maintenance and preparatory works for the recovery of armourstone and the eventual construction of a marina. The western face of this quarry was excavated between the 1950s and 1970s and is akin to other legacy slopes found at several older British quarries. The slope is up to 90m in height, has little benching and has shown increasing signs of instability since 2005. Initially instability was evidenced by rockfall and more recently by serious collapses that have indicated the need for appropriate geotechnical design of a new replacement slope. This paper sets out background and historical data and then considers investigations into the underlying mechanisms and rock structures that have contributed to instability and are relevant to the design of measures to overcome the potential for future significant ground movements. Methods to remotely assess the controlling joint sets are discussed and the rationale behind the excavated solution to facilitate future workings is outlined. High, over-steep rock faces with limited, ineffective benching and excessive bench heights that may be found in some older quarries, as at Penlee, are likely to become a matter of increasing concern. In addition the potential for major air blast or flow slide phenomena needs further investigation in these legacy slopes some of which are present in Southwest England.