Australian Geomechanics Society Newcastle, New South Wales – 2000 Meetings

• February, 2000

  The mysteries of blast induced ground vibrations

  Prof. Gour C. Sen, University of Wollongong

  There are often anomalies between the expected level and the actual monitored level of blast induced ground vibrations. The main theme of this talk highlights the effect of the degree of confinement of an explosive charge on the magnitude of the shock wave transmitted through the ground. An analysis of the energy distribution of an explosive charge will also be discussed in order to support this hypothesis.

• April, 2000

  Shear Strength and Stiffness of Silty Sand

  Prof. Rodrigo Salgado, Associate Professor of Civil Engineering, Purdue University

  The stiffness and shear strength of clean sands have been studied extensively. However, natural sands generally contain significant amounts of silt and/or clay. The mechanical response of such soils isdifferent from that of clean sands. This lecture addresses the effects of non-plastic fines on the small-strain stiffness and shear strength of sands. A series of laboratory tests was performed on samples of Ottawa sand with fines content in the range of 5% to 20% by weight. The samples were prepared at different relative densities and were subjected to various levels of mean effective consolidation stress. Most of the triaxial tests were conducted to axial strains in excess of 30%. The stress-strain responses were recorded, and the shear strength and dilatancy parameters were obtained for each fines percentage. Bender element tests performed in triaxial test samples allowed assessment of the effect of fines content on small-strain mechanical stiffness.

• July, 2000

  Tunnels and Mines - Imaging Ahead of the Working Face with Seismic Reflection Tomography

  Dr Bob Whiteley, Director, Resources Division, Coffey Geosciences Pty. Ltd.

  Economic and safe rock excavation is critically dependent on detailed understanding of rock conditions beyond the working face. The latest advance has come in the area of seismic reflection tomographic imaging implemented from the opening during operations. The new technology allows three dimensional (3D) images of rock structure and condition to be rapidly obtained ahead of the excavation. The method is called True Reflection Tomography (TRT™) and is analogous to shining a torch beam into the rock ahead of the working face. Since its introduction last year TRT™ has proven to be an effective and non-intrusive tool for imaging ground features along and adjacent to tunnels and mine developments in complex geologic settings. Currently, TRT™ has an detection range of 50m to 120m along the alignment, and a peripheral range of 20m to 30m either side of the opening. The technology uses readily available acquisition equipment and a portable computer with specialist processing software, and represents a breakthrough in tomographic imaging for the mining and tunnelling industry.

  Some recent projects will be discussed including the Northside Storage Tunnel in the northern suburbs of Sydney.

• October, 2000

  Tomago Mineral Sands Mining Project

  Stephen Jones, Principal, Douglas Partners Pty Ltd

  This project was recently acknowledged by the Association of Consulting Engineers Australia (ACEA) in their Engineering Excellence Awards for 1999. For the past 20 years Douglas Partners have been advisors the water authority (initially HWC, and subsequently DLWC) in monitoring the effects of mineral sands mining on the Tomago Sandbeds aquifer. The Sandbeds are also a major source of potable water for the Newcastle region.

  The project is believed to be the longest running continuing environmental consultancy in Australia, with Project Manager Stephen Jones having been involved in the project for the entire 20 years. Stephen will discuss the use of the Sandbeds for two competing resources: drinking water and mineral sands (mainly rutile and zircon). The project has seen many changes in technology and mining practices. The history of mining activities and the geochemical effects of mining on the groundwater quality will be presented.

  Mining finished in late 1998, and decommissioning of the mine site and separation mill is nearing completion. A post mining monitoring regime has been established for both groundwater and revegetation, in order to ensure a satisfactory environmental outcome, and monitoring will continue for several years to come.

• November, 2000

  Geotechnical Analysis

  Professor David Potts, Imperial College, London

  The design of any geotechnical structure of significance requires the quantification of safety and in some cases movements. To achieve this the designer must perform calculations using some form of analysis. At present there are several forms of analysis available, all of which involve simplifications and approximations. This lecture will begin by briefly comparing these forms of geotechnical analysis. It will then examine the role of advanced numerical analysis concentrating on their advantages, their ability to accurately predict reality and their shortcomings. Examples of the application of numerical analysis to real engineering problems will be used to reinforce the points made. The lecture concludes by identifying the areas of future development.