Engineers Australia

Quantifying quantitative rock fall risk – How many rocks? How far?

Garry Mostyn and David Piccolo


AGS Landslide Risk Management Guidelines require that quantitative risk assessment be undertaken where there is a risk to life. When it comes to rock fall risk, a meaningful quantitative risk assessment often requires quantification of both boulder generation rates and travel distances. Three case studies are discussed each presenting methods of quantifying risk with respect to boulder falls. The common element being targeted data collection to minimise the need for untestable assumptions resulting in either dangerous or overly conservative assessments.

Case 1: A cliff regression model to estimate the boulder generation frequency and travel distance distribution.

A quantitative risk assessment of a development in the Pittwater Local Government Area required an estimate of the travel distance of boulders falling from an adjacent cliff line. A Commission of Inquiry had received lay and expert evidence to the effect that boulder falls presented an extreme risk to the development.

Case 2: SEL Greystanes

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 as part of assessing whether reshaping of the slopes or other support was required. Industrial and commercial development was proposed to be immediately adjacent to the toe of the quarry slopes. The risk analysis addressed risk during the construction phase and in perpetuity (agreed to be 100 years).

Case 3: Widening Sandstone Cuttings Adjacent to Live Rail

In July 2013, the speakers were involved in a project which comprised the construction of a new railway track adjacent to two existing tracks in Sydney, Australia. At a number of locations, the construction of the additional track required widening of existing cuttings comprising shale and sandstone. At two locations, referred herein as Cuts A and B, the existing cuttings comprised steep medium to high strength sandstone up to 12 m high with the toe located as close as 0.75 m to the existing rail envelope.

About the Speakers

Garry Mostyn, BE (Hons), MEngSc, BA (Geology), FIEAust, AIAMA, CPEng, Grade 3 Arbitrator, Chief Engineer, PSM

Garry Mostyn has been an active member of several national and international code and practice committees and been involved at the highest levels of the Australian Geomechanics Society and the International Society for Rock Mechanics. He is a frequent speaker at professional society meetings and symposia both in Australia and overseas. He has authored or co-authored over 70 journal and conference papers in his fields of expertise. He has been a member of the Standards Australia drafting committee for AS3798, the Earthworks Code, since its inception in the 1980s and is the chair of the drafting committee for AS4678, the Earth Retaining Structures Code. His current practice includes significant involvement in earthworks of all kinds and litigation/expert advice regarding geotechnical matters.

David Piccolo, BE Civil (Hons), MEngSc, Principal Engineer, PSM

David Piccolo has worked on a wide variety of geotechnical projects including the detailed consideration of many aspects of earthworks and their control. His special fields of competence include earthworks, site investigations, analysis and geotechnical advice for civil engineering infrastructure. David has worked on a number of slope stability projects on rock and soil slopes, both in slope risk assessments, and slope remediation. He also has experience in other areas of practice including urban tunnelling and soft ground design.

David was awarded the best paper in Australia for geomechanics (AGS 2008) and the civil engineering (Warren Medal 2009) for his paper “Experiences with post-construction retesting of engineered clay fills”, co-authored with Brian Burman and Garry Mostyn. He placed first in the national young geotechnical professionals conference in 2006 and was selected to present his paper “Engineering Issues relating to filling of deep pits for residential and light industrial development” at the international young geotechnical professionals conference in Alexandria, Egypt in 2009.

Engineers Australia members participating in AGS technical sessions can record attendance on their personal CPD logs. Members should refer to Engineers Australia CPD policy for details on CPD types, requirements and auditing guidelines.