Considerations in the Optimisation of Bench Face Angle and Berm Width Geometries for Open Pit Mines

William Gibson, Ian de Bruyn and Diane Walker
Sunday, January 1, 2006
First presented: 
The South African Institute of Mining and Metallurgy International Symposium on Stability of Rock Slopes, 2006
The inclination of any pit wall composed of stacks of benches is limited by the overall quality and structural complexity of the rock mass in which the slope is situated. For a bench stack angle that satisfies the stability criteria, various combinations of bench height, bench face angle and berm width can be used in the slope design. The bench face angle and berm width geometries must be chosen to fit the bench stack angle but must provide a berm that will be capable of catching the volume of failed wedge material from the bench faces above. Other considerations for bench height and berm width are equipment size, ore selectivity and blasting design.
This paper studies the effects of different slope configurations on slope performance as it relates to wedge stability, volume of rock safely contained on berms, and rockfall. Optimisation of the geometry is carried out based on the need to minimise the volume of failed material which limits the effectiveness of the berm. This will reduce the number of rocks that can continue to fall further, reaching geotechnical safety berms or pit ramps, that put safety of personal and equipment at risk.
For general study it is most effective to investigate and compare the performances of each of a wide range of geometrical combinations of bench height, bench face angles and berm widths that fit a variety of different bench stack angles. These analyses were conducted for a predetermined bench stack height and three different bench stack angles.

Feature Author

William Gibson

William Gibson has over 30 years’ experience in geotechnical, mining and civil engineering projects as well as in development of computer programs.  His expertise in open pit mining includes slope stability analysis for Chuquicamata, Esperanza and Pelamberes (Chile), Olympic Dam (Australia) and Oyu Tolgoi (Mongolia).  He has expertise in the analysis of interaction between open pit and underground mining operations.  William’s experience in underground mine design includes analysis of block caving mines, mining sequence, open stoping, stability of pillars and interaction of block caving operations with mine facilities.  William is experienced in the analysis of static and seismic conditions, having undertaken seismic risk assessments in parts of South and Central America and Canada.  His computer programming experience includes finite elements and discontinuous deformation analysis programs; and programs for rock fall 3D analysis (RFall_3D), probabilistic wedge stability analysis (MWedge_P), rock mass strength assessment and gravity flow analysis (MFlow) and generation of artificial earthquakes for dynamic analysis (Crono). 

Principal Consultant (Geotechnical Engineering)
MSc (Geotechnical Engineering)
SRK Perth
Ian de Bruyn

Ian de Bruyn has 19 years’ experience in the fields of engineering geology and geotechnical engineering over a wide range of projects in both the mining and civil engineering sectors. He has particularly strong expertise in geotechnical assessment and provision of design parameters for open pit mining operations, and has worked on projects involving very large pits in challenging rock mass and hydrogeological conditions. He is well versed in rock mass characterisation and domain modelling for input into fragmentation analysis, selection of mining method and support design for underground mines. Ian’s projects have involved site investigation, characterisation, analysis, evaluation, design, risk assessment and reporting at all levels – from conceptual through pre-feasibility, feasibility and working design. He has also performed geotechnical assessments for due diligence studies. Ian has expertise in civil engineering site investigations / evaluations, including extensive work on karst-related ground instability, and site characterisation for foundation design for civil and mining infrastructure. Ian is currently the team leader of the SRK geotechnical group in the Perth office.

Principal Consultant (Geotechnical Engineering)
BSc Hons (Engineering Geology), Pr.Sci.Nat., MAusIMM
SRK Perth
Diane Walker

Diane Walker has 18 years’ experience as an engineering geologist in both mining and civil engineering projects. Her principal area of expertise is geotechnical investigation, rock mass characterisation and geotechnical domain modelling for open pit and underground mine design. Diane has extensive experience in investigation programme design, project management, geotechnical data analysis and provision of geotechnical design parameters. Her projects have included conceptual studies, feasibility studies and support for operating mines, as well as geotechnical assessments for due diligence studies. In addition, she has performed a planning and management role for geotechnical studies and design outcomes for the iron ore operations of a mining major in Western Australia. Diane has also conducted geotechnical soils investigations and site characterisation for civil and mining infrastructure projects.

Principal Consultant (Geotechnical Engineering)
MSc (Engineering Geology), Pr.Sci.Nat., MAusIMM
SRK Perth
SRK Worldwide