Numerical modelling of underground excavations – uncertainties caused by use of empirical based downgrading methods and in situ stresses

Mahdi Zoorabadi

Numerical modelling techniques are not new for the mining industry and civil engineering projects. These techniques have been widely used for rock engineering problems such as stability analysis and support design of roadways and tunnels, caving and subsidence prediction, and stability analysis of rock slopes. Despite the significant advancement in the computational mechanics and availability of high speed computing hardware, the input data and constitutive models remain the main source of errors affecting the reliability of numerical simulations. The problem with input data has been deepened by the introduction of empirical-based methods such as GSI classification to downgrade rock properties from laboratory scale to field scale. The deformability modulus and strength parameters are the main outputs of these downgrading techniques. Numerical modelling users simply apply these downgrading methods and run the model without considering the real mechanics behind the stress induced failure and deformation around underground excavations. Contrary to the commonly used downgrading methods that produce a constant modulus for rock at all depths, the rock modulus is stress dependent and varies with depth. In addition to this, the mechanism of stress induced displacement is not similar to the deformation of a continuum model simulated with equivalent rock properties. The magnitude and orientation of in-situ stresses are two other important parameters that have significant impacts on stress induced rock fracturing. The impacts of these two parameters have also been neglected in many practical cases. This presentation will discuss this old-fashioned topic in more detail including the known facts and mechanics which numerical modelling users ignore due to the unquestioning acceptance of downgrading methods. It also covers the influence of the stress magnitude and orientation on stress induced rock fracturing.

About the speaker

Mahdi ZoorabadiPrincipal Geotechnical Engineer, Golder Associates

Mahdi (BSc, MSc, PhD) is a Principal Geotechnical Engineer at Golder Associates providing consulting services in the areas of ground control and rock engineering in both open pits and underground mines. Mahdi has experience in consultancy and research in a broad range of rock mechanics, rock engineering and mining geomechanics fields. He is a MAusIMM CP (Geotech) and RPEQ. Mahdi is a competent expert in the fields of rock testing, rock stress measurement, groundwater studies, monitoring and instrumentation, 2D and 3D numerical modelling, mine induced seismicity, rock and coal burst risk assessment, open pit slope stability (both metalliferous and coal mines), rock cavability, stope designing, mine subsidence, mechanised rock breakage and ground support design, managing geotechnical investigations for pre-feasibility, feasibility, and design studies. Mahdi is a highly experienced numerical modeler who developed his own constitutive model for numerical simulation of stress induced rock fracturing, rockburst, outburst and coalburst. Mahdi has developed his own code for stability analysis of structural controlled failures of rock slopes and probabilistic stability analysis and uses IMS Combined Vintage & Trace package for analysing mine induced seismicity. He is actively involved in research activities through collaboration as adjunct senior lecturer with school of mining engineering, UNSW, supervising PhD and Master students. He has also been granted several research funds for outburst mechanism, coalburst risk management, downgrading rock mass strength, and true triaxial strength of the rocks.

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