A number of large ore deposits which are mined initially by large open pit methods, also have considerable vertical extents. At some point, a decision has to be made as to whether to continue deepening a pit, or to change to low cost, high productivity underground mining, generally by caving methods. Currently, several large open pit mines are planning to make the transition to underground caving operations. These include Bingham Canyon in the USA, Chuquicamata in Chile, Grasberg in Indonesia, Venettia in South Africa, and Argyle, Mount Keith and Telfer in Australia. At the time the research reported in this thesis was carried out, the Palabora Mine in South Africa was undergoing a transition but there was still very little published information on this case available in the open literature. However, this case study was able to be used to validate two of the predictive models developed in this thesis.
In spite of the importance to current and future mining of this topic of the transition from deep open pits to large caving methods, there are neither sufficient direct experiences nor the design methodologies available to enable such projects to be engineered with confidence. A literature review shows that there are particular needs to develop an improved understanding of the response of the rock mass in a transition and the design methodologies required. These issues are particularly important given the likely depths of potential future pits and the sizes of the subsequent caving blocks that will be required to produce the necessary high tonnages from underground.
Accordingly, the topic of the transition was chosen for this thesis. The research reported was focussed on predicting the most probable rock mass response to the transition from open pit to underground mining by caving methods. The analysis is concentrated on the impact of geometry (defined by the undercut width, the ore column height, the depth of the undercut and that of the pit), the stress field (defined by the in situ stress field and the stress redistributions caused by the pit and the block cave), and the rock mass strength, on the vertical propagation of caving, on the stability of the surface crown pillar, and on the subsidence in the transition phase. These are considered the three key stages of the evolution of the caving process in a transition.
First, a method is developed for evaluating the likelihood of caving propagation in strong rock masses in a transition project. The concept of the caving propagation factor, CPF, is developed and used to evaluate the likelihood of cave propagation and, conversely, of cave stalling. This method is validated by back analyses of four documented successful and unsuccessful caving case histories. In terms of mine planning, this method enables the undercut level to be established and, therefore, the most mineable block or panel height to be determined in terms of the likelihood of successfully connecting the cave to the pit bottom.
Secondly, two methods are developed for evaluating the minimum thickness of the surface crown pillar and the allowable time for simultaneous open pit and underground cave mining in a transition project. A surface crown pillar in this context is defined as the rock mass between the pit bottom and the cave back position. As would be expected, its thickness changes with time as the cave propagates. Also, based on the results of a benchmarking study and the qualitative interpretation of the results of numerical model analyses, average draw rates are defined for different rock mass qualities and fault conditions. The rate of draw also influences the rate of cave propagation. Draw rates are used in this methodology to develop a design chart that can be used to estimate the time available for simultaneous open pit and underground cave mining. The Palabora case is back-analysed to validate the design methodology. The results show good agreement between the observed performance at Palabora and the estimates made from the design charts developed to evaluate the minimum thickness of the surface crown pillar required for safe simultaneous surface and underground mining operations, and the time available for safe simultaneous open pit and underground cave mining.
Thirdly, two methods are developed for predicting subsidence in a transition project. In a transition, the subsidence phenomena are more complex, and the resulting crater and the extent of the influence zone are expected to be much larger than for the comparable case of mining the same orebody by caving alone. This is because of the potential for slope failures to occur as the toes of the slopes become undercut when the cave connects to the pit bottom. A limit equilibrium analysis, based on Hoek's basic hangingwall caving model, is developed specifically to estimate the angle of break in a transition. This analysis is supplemented by a numerical analysis used to estimate the extent of the influence zone. In a transition, accurate prediction of these two parameters is considered crucial as they influence the locations of surface and underground infrastructure for the underground caving phase of mining. The limit equilibrium method allows either the Mohr-Coulomb or the Hoek-Brown criteria to be used for the rock mass strength. If the Mohr-Coulomb criterion is used, the values for the cohesion and angle of friction are part of the input data. If the Hoek-Brown criterion is used, the values of the uniaxial compressive strength of the intact rock and the Hoek-Brown material constants are required as input data. The approach developed calculates, for each case, equivalent values of cohesion and angle of friction. The factor of safety is evaluated using these equivalent values. Subsidence prediction charts are then developed for different conditions and characteristics of the rock mass. The resulting charts can be used to evaluate the subsidence by defining the angle of break and the influence zone for a case of transition from open pit to underground mining by caving methods. Finally, the case of the Chuquicamata Mine, Chile, is used to demonstrate the application of the design methodologies and guidelines developed in this thesis. The design methodologies will be used for the pre-feasibility engineering of Chuquicamata's transition project which, in the next three to four years, will assess the feasibility of making a transition from a large open pit to an underground caving operation.
These methodologies and guidelines are aimed at project managers, project engineers and geotechnical engineers involved in pre-feasibility studies of a transition project from open pit to underground cave mining or simultaneous surface and underground mining by caving methods. They are not intended to replace the detailed engineering analyses required in subsequent project stages.
Two international conference papers have been published from the thesis work (see list of publications).