The development and application of a 3D geotechnical model for mining optimisation Sandsloot open pit platinum mine South Africa.
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Date
2003
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Abstract
Detailed geological knowledge is often a major unknown factor in open pit mining and design, and
therefore poses a significant risk in the mining venture. As the knowledge of the geology improves
so the risk of unforeseen conditions reduces and therefore safety and productivity can be increased.
Historically, geotechnical methods and information have predominantly been used exclusively for
pit slope optimisation. This research documents the procedures and developments undertaken to
compile a comprehensive geotechnical database, and the application of the geotechnical data to
open pit mining, beneficiation and planning. The utilisation of the geotechnical information has
been enhanced through the novel development and application of a computerised, 3D geotechnical
model.
Sandsloot open pit was developed to extract the Platreef pyroxenite orebody, which is hosted within
the Northern Limb of the Bushveld Complex. Sandsloot is currently the world's largest open pit
exploiting Platinum Group Metals. Interaction of the basic magma with the footwall sediments of
the Transvaal Supergroup and varying degrees of assimilation has resulted in a unique suite of
hybrid rock types. These various rock types provide significant engineering geological challenges.
Geology and the detailed understanding of its properties are fundamental to the optimal design and
successful operation of any mine. Extensive fieldwork was conducted to collect geotechnical
information, both from exploration boreholes and in-pit mining faces. Over a 5-year period,
geotechnical data were collected from 29,213 m of exploration core and 6,873 m of exposed
mining faces. Extensive field and laboratory testing was undertaken in order to define the complete
set of geotechnical properties for each rock type in the Sandsloot mining area.
The geotechnical information relating to each borehole and facemap was stored in the Datamine®
software package. The information was collected in the form of rock mass rating (RMR), uniaxial
compressive strength (DCS), fracture frequency (FF/m) and rock quality designation (RQD). The
architecture of the database was developed along the principals used for generating an ore reserve
model.
One of the novel applications was the development of a computerized 3D, geotechnical model in
Datamine®. The geotechnical parameters, namely RMR, DCS, FF/m and RQD, were modelled for
each rock type, using geostatistics, to generate a 3D model. The data were interpolated between
exploration boreholes and exposed mining faces and the modelling was constrained using
wireframes separated by rock type. The result is a 3D model containing 15 m3 model blocks
populated with interpolated geotechnical information. The dimensions of the model blocks are
linked to the mining bench height of 15 m. The model can be queried to give predictions on rock
mass conditions for any planned mining area, as is the case with the ore reserve model, which
provides predictions on platinum grades.
The crux of the innovative research is the practical application of the 3D geotechnical model. This
was achieved through the development of both a fragmentation and a slope design model, which
read the interpolated geotechnical information. These models provided an engineering tool to optimise mining and milling perfonnance.
Rather than viewing the drill and blast department as an isolated cost centre and focussing on
minimising drill and blast costs, the application of the model concentrated on the fragmentation
requirements of the milling and mining business areas. Two hundred and thirty-eight blasts were
assessed to detennine the optimum fragmentation requirements for ore and waste. Based on the
study a mean fragmentation target of 150 mm was set for delivery to the crushing circuit and a
mean fragmentation of 230 mm was set for waste loading from the pit.
The mine operates autogenous mills, which are sensitive to the fragmentation profile delivered.
The harder zones occurring in the ore zone have a major impact on the plant's perfonnance. The
geotechnical parameters in the model were related to Lilly's Blastability Index, and in turn to
required explosive volumes and the associated drill and blast costs. Having defmed the
fragmentation targets, the Kuz-Ram equation was used in the fragmentation model to predict the
explosive volumes required to ensure consistent mining and milling perfonnance. The
geotechnical model is used to predict changes in geotechnical conditions and therefore the blasting
parameters can be adjusted in advance to ensure the milling and mining fragmentation
requirements are met. Through the application of the fragmentation model over an eighteen-month
period the loading and milling efficiencies improved by 8.5% and 8.8% respectively, resulting in
additional revenue ofR29 million for PPL.
Based on the mining rock mass rating (MRMR) values within the geotechnical model a stable slope
design model was created in order to calculate optimum inter-ramp angles. From a slope design
perspective the model was used to target data-deficient zones and highlight potentially weak rock
mass areas. As this can be viewed in 3D, the open pit slopes were designed to accommodate the
poor quality areas before they are excavated. It also follows that competent geotechnical zones can
be readily identified and the slope optimised accordingly.
Due to the detailed geotechnical infonnation being available in three dimensions, the open pit
slopes were designed based on a risk versus reward profile. As a significant geotechnical database
was available, more accurate and reliable designs were generated resulting in the overall slope
angle increasing by 3 degrees. This optimisation process will result in a revenue gain of R900
million over the life of the mine. The revenue and safety benefits associated with this design
methodology are substantial and have potential application to all open pit mining operations.
The research has enabled detailed geotechnical infonnation to be available in three dimensions.
This information can be readily accessed and interpreted, thus providing a powerful planning and
financial tool from which production optimisations, feasibility studies and planning initiatives can
be implemented. The development and application of a 3D geotechnical model has added a new
dimension to the constant strive for business improvement and reflects a novel and successful
approach towards the application of engineering geology at the Sandsloot mining operation.
Description
Thesis (Ph.D.)-University of Natal, Durban, 2003.
Keywords
Strip mining--Mathematical models., Mathematical optimization., Platinum mines and mining--South Africa., Theses--Geology.