Quantitative risk assessment for urban redevelopment adjacent to quarry Slopes

The redevelopment of Boral’s Greystanes Quarry in Western Sydney involved a risk assessment of existing shale and dolerite quarry slopes, excavated over 40 years and up to 50 m high, to assess whether reshaping of the slopes or other support was required. Immediately above the crest of the quarry slopes are three Sydney Water Corporation (SWC) reservoirs which were considered to be critical assets in Sydney’s water supply. Immediately adjacent to the toe of the quarry slopes was to be industrial and commercial development.

The risk analysis addressed risk during the construction phase and in perpetuity (agreed to be 100 years).

The hazards addressed included overall slope failure, bench scale failures, toppling failures, boulder fall, erosion and rill development, vibrations resulting from blasting, flyrock resulting from blasting and accessing the slope for maintenance.

The methodologies adopted in the analysis included AGS2000 and the “Draft Capital Investment and Risk Assessment Framework”, Technical Services Branch, February 2006 (State Water Framework 2006). This document has been adopted by the Department of Commerce NSW and Goulburn Murray Water in their risk assessment for major dams.

As part of this work a geological, geotechnical and hydrogeological model was developed, the historical slope performance was assessed, quantitative data regarding rockfalls, toppling and wedge failures were gathered and used as input to the quantitative risk analysis.

A batter management plan with limited constraints was developed. No artificial support was required and only minor regrading of the weathered profile was required. On this basis, the QRA indicated acceptable risk to life, the SWC Assets and the proposed development within the quarry.

The work was undertaken by PSM, reviewed by GHD and verified by SMEC as the independent verifier for SWC.