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Simplified 3D modelling of bench-Berm design

SRK News | Issue 54: Rock Engineering and Slope Stability

A4   |   Letter

Effective bench-berm management is critical for safety at operating open pit mines. SRK Vancouver’s Geotechnical Group developed a simple and practical 3D modelling tool that can be used to quickly assess the stability of bench scale failures and the minimum berm width within a given design without advanced numerical knowledge. The user inputs basic information on the bench face (height, orientation, angle, and number of benches), rock mass (density), and joint sets (dip, dip direction, persistence, spacing, etc) within a simplified 3DEC code to create the blocks and wedges calibrated from site mapping. With this entry the user can create a discrete fracture network that can be calibrated from known discrete features. Multiple representations of the potential failure blocks are then created from the joint set information on several bench faces to help maximise the inter-ramp angle without being limited by block shape or size. This tool can allow a quick estimation of potential back break, bench face stability, developing multi-bench failure mechanisms and progressive wedges, bench carry capacity, and spill volume.

The main advantages of this approach are: 1) multiple benches can be modelled, 2) the block shape is not limited to a wedge but rather the intersection of measured joints, 3) the program is based on an accepted industry program, 4) bench scale concerns to multi-bench scale instabilities can be modelled in one program, 5) individual blocks can be removed to represent scaling or sliding, and 6) mapped geological features can be generated within the model at their mapped location. Limitations of the approach are: 1) the user must gain access to the 3DEC program from Itasca Consulting Group, 2) model sizes must be kept relatively small to allow for multiple representation, 3) blocks are kept rigid, which does not allow for internal deformation, and 4) breaking of rock bridges is not accounted for.

Bruce Murphy:

3DEC representation of pit slope (top), with effective bench shown (above) after displaced blocks are removed

SRK Africa