Mike Sandy Principal Geotechnical Engineer with AMC reports that his company “continues to improve its capabilities in numerical modelling and seismic monitoring. Recent developments include significant upgrades to numerical modelling hardware and software to allow very large problems to be analysed in practical timeframes, with substantial benefits for clients. “AMC has considerable experience in modelling using all the major stress analysis codes including Abaqus, FLAC3D, 3DEC and MAP3D. Having access to and experience in a wide range of software allows us to use the model most suited to the client’s requirements. With growing interest in the use of advanced modelling in mining studies and to assist decision making in established operations, AMC has built a strong modelling team that includes some of the leading international experts in this field. Enhancements have also been made to some of the in-house geomechanics software developed by one of our Principal Geotechnical Engineers, William Gibson, including MFLOW, RFall3D and FracRock.

“MFLOW is a stochastic particle flow model and is used in caving projects to assess dilution and optimise the draw sequence for improved ore recovery. In a recent sublevel caving example, metal recovery was improved by 6% over the base case as a result of optimised draw.

“RFall3D is used to carry out deterministic and probabilistic analysis of rock falls from slopes and can be used to assess the effectiveness of catch berms and catch fences. As its name suggests, the analysis is fully three-dimensional, and can include catch fences of varying heights. Different coefficients of restitution can be incorporated to simulate the effects that different material types have on rock fall trajectories. This includes deliberately-placed ‘soft beds’, which are one of the options for arresting rock falls. 

“FracRock is a program for generating discrete fracture networks (DFN) in a rock mass model. Using standard rock mass characterisation data, FracRock can generate a rock mass model in which the joints are represented as ellipses. This type of model is used to reproduce the jointed structure of a rock mass that can then be used as input into programs like 3DEC. 

“Demand for advanced geotechnical modelling has increased significantly in recent years, partly due to growing awareness of the need to represent ‘yielding’ behaviour in some mining situations, but also as the cost and ease of developing and running very complex numerical models improves.

“In situations where large amounts of yield are expected, such as in caving or the subsidence over a longwall mining operation, or where pillar or abutment stresses approach the rock mass strength, there is a need to represent this behaviour in the model.

“In high yield environments, the development of rock mass damage around excavations is usually of key interest as this is closely linked to support requirements, dilution and in some cases, seismic hazard. Accurate modelling of conditions near development requires the use of suitably fine ‘meshes’. This in turn can result in very large numbers of elements, particularly for extensive mining operations.

“To provide a cost effective service and to allow large models to be run in practical timeframes, AMC has recently upgraded its dedicated geotechnical computing hardware. This, along with improvements to software, allows very large, detailed models to be constructed in acceptable timeframes.

“Some problems require a hybrid approach, using the best features of several different modelling packages to get the most cost-effective outcome for the project. In other cases, the results of analyses using one package are used as input for subsequent analyses. In a recent example, Abaqus was used to investigate and quantify the development of rock mass damage associated with the retreating mining ‘front’ in a sublevel caving operation. This information was then used as input to a draw optimization study using MFLOW.

Managing mine seismicity

“AMC has been providing a range of seismic management services to the mining industry for more than ten years. This service is based around two main areas: assisting clients with data acquisition, including leasing and installation of portable seismic monitoring systems, and seismic analysis. Analysis is sometimes combined with numerical modelling studies to investigate the link between mining-related stress changes and seismicity. This in turn can be used to assess the seismic hazard associated with future mining plans.

“Alternative mining sequences are sometimes capable of significantly reducing seismic hazard. These alternatives can be investigated using suitably calibrated numerical models. Areas that will require upgraded support or other seismic hazard management options can also be identified.

“The backbone of AMC’s seismic monitoring leasing business has been the ISS Pacific stand-alone ‘QS’ portable seismic system. These units are rugged, versatile and easy to install. A typical array of six geophones and a central data acquisition QS box can be installed in two to three days. The coverage provided is usually adequate for smaller operations or for specific investigations such as the seismic response to the recovery of a crown pillar.

“AMC is currently upgrading its seismic hardware to the new ‘GS’ boxes, following their recent release by ISS Pacific. Apart from being a more robust item, the GS box software uses a Windows platform. The benefits of this include:

• Easier programming when the seismic array is relocated.
• Simple data transfer through the use of a USB memory stick.
• The option to connect two or more GS boxes, should increased seismic coverage be required.

AMC currently provides a routine monitoring and seismic analysis service to four mines in Australia with two additional projects likely to come on stream within the next few months. Seismic hazard analyses have been conducted at a number of other mines in recent years, including several seismically ‘challenged’ projects.