Slope Stability Workshop

BRUCE McCLURE, CMPA’s General Manager reports on the slope stability workshop conducted in February.

THE CMPA conducted a slope stability workshop on 17th February 2011 at Holcim’s Oaklands Junction quarry.

Over the last couple of years a number of quarries have suffered material slips and collapses from faces, stockpiles and dam walls. These events have highlighted the need for quarry owners and managers to be mindful of slope stability requirements on their sites. The CMPA decided to hold this workshop to update and inform participants on many aspects of slope stability specifically pertaining to extractive industry sites.

The workshop commenced with Ian McLeod, Manager Mineral & Extractive Operations Earth Resources Regulation Branch, Department of Primary Industry (DPI) talking about the proposed Geotechnical Guidelines that the DPI intends to release in Victoria.

The Guidelines are currently with the Technical Review Board of the DPI but will be placed on the DPI website for general comment shortly. It is understood that these Guidelines will be formally adopted (following consideration of all comments) and become the required practice in 2012.

All quarry sites in Victoria will fall within four Risk Categories, the categories being as follows:

• Category 1 Sites – Low geotechnical risk
• Category 2 Sites – Medium geotechnical risk
• Category 3 Sites – High geotechnical risk
• Category 4 Sites – Declared

There are currently no Category 4 quarry sites in Victoria.

Provided that members’ quarries fall within either the proposed Category One or Category Two Risk categories, the guidelines will provide extractive industry members with a means of ensuring that their business complies within the limits set without needing detailed investigation. They will need to satisfy a sound understanding of the ground conditions, a successful history of the suitability of the design in similar circumstances in the past and sites may opt to use prescriptive geotechnical designs.

If a quarry falls into Category Three or above then the fundamental requirements for geotechnical stability will only be satisfied by a site specific geotechnical assessment of ground conditions.

The second guest speaker Mr Lesley Munsamy Manager Geotechnical & Spatial Vic., GHD discussed Operational Geotechnical Engineering and Slope Stability. Lesley has considerable experience in this subject and spoke about dealing with issues both here in Australia and overseas. He explained the various types of slips and the factors that are involved which results in the
actual slips. Lesley explained in detail the mechanisms involved and highlighted many of his points with very graphic slides.

Lesley also discussed in detail the importance of regularly examining within the quarry sites how actual slope stability is actually being considered and dealt with. He stressed that it is important to have stable slopes for the following reasons:

• Safety – protect our people and machinery
• Economics – extract the maximum ore, no loss of ore, no cost of failure and rehabilitation cost
• Environmental/Social – minimal impact to our communities, rivers, roads, rail and other infrastructure

The last phase of the workshop dealt with surface and groundwater management issues and their impact on slope stability.

Basil Natoli from Bell Cochrane and Associates discussed surface water management on quarry sites. Basil explained how in recent times there have been a number of incidents in both quarries and mines resulting from significant rainfall events. The incidents resulting from the recent rains have included:

• Embankment collapses – both quarry excavations and constructed embankments i.e. dams and dumps.
• Inundation of excavations and fixed mobile plant setups.
• Loss of access to plant and stockpiles.

The impact of these rainfall events has been exacerbated by the preceding prolonged drought which has in many sites caused the ground to dry out and surface shrinkage cracks to form. This has allowed the recent rainfall to penetrate faster and deeper into embankments contributing to loss of their stability.

The final guest speaker was Alexis Valenza Geosciences Team Leader – Groundwater Engineer Hyder Consulting who gave a presentation on a groundwater management. He discussed the four zones of water in the ground, the range of water level variations, surface water and groundwater interaction and the groundwater management tools available to Hydrologists.

Alexis also discussed some of the tools which are available to Hydrologists and these include:

• Surface Water and Groundwater level baseline monitoring
• Hydraulic testing (pumping test, slug test, pit trial testing).
• Laboratory testing (granulometry, permeameter test).
• Analytical and numerical modeling.
• Design of the groundwater monitoring network.
• Maintenance of the monitoring network and monitoring regime over the quarry development.

SITE VISIT

Following the formal presentations the group travelled down into the actual quarry hole where David Jones of Holcim, Lesley Munsamy and Basil Natoli commented on a number of pertinent points relating to the geology of the site and the reasons as to why the quarry was being operated in its current configuration.

The CMPA would like to thank David Jones and the other staff from the Holcim, Oaklands Junction quarry who made our visit to their site enjoyable and very informative.

The CMPA would also like to make a special mention of our sponsor for this workshop C.K. Prowse and Associates. Their assistance was very much appreciated.

The second slope stability workshop will be held on 24 March 2011 in Traralgon.

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