Stress measurement

At El Teniente Mine, situated in the Andes Mountains of central Chile, stress tensor measurements have been conducted at over 150 sites using different stress measurement techniques. However, definition of the local and regional stress field is difficult due to the large mining footprint and surface crater, characteristic of caving operations, the effects of stress redistribution due to mine voids and mountainous topography. As part of the investigation and planning for the future mining the stress tensor database has been extended to include deeper stress measurements made by the Acoustic Emission (AE) technique. The stress tensors in the El Teniente Stress database are analysed and measurements thought not to be greatly affected by mining are selected and compared in an attempt to define the contemporary stress field.

El Teniente Mine is situated in the Andes Cordillera of central Chile. For mine engineering purposes, a large stress tensor database comprising rock stress tensor measurements using different stress measurement techniques has been compiled. The effects of stress redistribution due to mine voids and mountainous topography make definition of the local and regional stress fields difficult. An attempt is made to reconcile measured stress tensors with structural geology, surface displacements and seismicity on a regional scale in preparation for numerical modelling and future mine planning.

In this study acoustic emission (AE) techniques were investigated to determine in situ rock stress from rock cores. The stresses were estimated from the AE behaviour and compared with the values calculated using conventional Hollow Inclusion (HI) cells. Stresses in three different directions such as normal to bedding and parallel to the strike and dip of bedding were determined from the rock cores. In almost all the tests the Kaiser effect was clearly observed, so the stress could be determined from the AE signature in the first loading. The deformation rate analysis (DRA) was also used to estimate the stresses. The results from DRA were consistent with those estimated from AE method. In general, the stresses calculated from core testing were within 10% of the values determined using the conventional HI cells.

The Perseverance Deeps Pre-feasibility Study is looking at an extension of the existing Perseverance Mine through the delineation of additional reserves from a depth of 1.1 km to 1.4 km below surface. Knowledge of the in-situ stress field in the proposed Perseverance Deeps block is seen as critical in terms of effective stress management for both safety and economic production purposes. To date and over several years, CSIRO HI, HFRAC, WASM AE and borehole surveys have been used to measure and define the stress field.

From a review of the different stress measurement techniques, it was found that there was a relatively consistent relation between stress magnitude and depth for the principal stresses for CSIRO HI, HFRAC and WASM AE results. The review also found that there was a preferred orientation of the major principal stress above 1100 m depth, but with a clear rotation of the orientation below a depth of 1100 m. The trend below 1100 m is reasonably consistent and supported by three-measurement methods. The results of the review demonstrate that without the use of the WASM AE stress measurement technique and borehole breakout survey, engineering design work for the Study would have been based on an incorrect assumption (ie the stress orientation at depth) that would have had a significantly negative impact on the mining design.

Reliable estimation of in situ stress is a major step in the analysis and design of underground excavations in rock, particularly for evaluating the stability of underground structures and influenting processes to prevent failure or collapse. A technique that allows the estimation of stresses using oriented core that can be drilled at depth has been studied over the last six years at the Western Australian School of Mines (WASM). The technique is based on acoustic emission (Kaiser effect) and has been used to estimate the in situ stresses from more than fifty mine sites in Western Australia. After introducing this method, two case histories are used to to compare in situ stress profiles established by CSIRO HI Cell (overcoring) and AE methods.

A low-cost methodology that allows the estimation of in situ and induced stress using oriented rock core specimens has been investigated. The technique can be used to determine the stresses either during the early stages of a project, even in undeveloped areas of a mine, or to measure in situ and induced stress within active mine workings, such as stopes and pillars. The research aim was to compare the experimental results estimated by the Acoustic Emission and Deformation Rate Analysis methods with those estimated by conventional HI cell measurements. Data was collected from a number of sites with different geological environments and in most cases the core was obtained from the same hole in which a conventional stress measurement had been carried out. The studies have focused on the determination of the full stress tensor from a single oriented cored rock. In all cases, the rock core specimens recollected similar in situ stress values to those estimated using conventional overcoring methods.

The experimental procedures for a new Acoustic Emission technique for rock stress tensor measurement are presented together with results obtained from implementation at approximately 20 trial sites spread across Australia. Two examples are selected where chance has allowed WASM AE results to be interpolated and compared with stress tensor results obtained from reasonably close overcoring exercises.

Reliable evaluation of in situ stress is an important phase in the analysis and design of underground excavations, particularly for evaluating the stability of underground structures to prevent failure or collapse. A technique for the estimation of the full stress tensor based on acoustic emission (the Kaiser effect) has been developed and tested at the WA School of Mines. The result obtained compare well with the hollow inclusion over-coring method. The advantages of the technique are that it is relatively cheap, non-time consuming and is applicable to in situ stress measurements at depth in remote regions.

Reliable estimates of in situ stress is a major step during geologicla regime definition within a rock mass characterization process. A technique that allows the estimation of stresses using oriented core that can be drilled at depth, and from remote locations has been studied over the last eight years at WASM. The technique is based on acoustic emission (Kaiser effect) and has been used to estimate the in situ stresses from more than sixty mine sites around the world. Over 150 individual stress measurements have been carried out to date. After introducing the method and typical results, a methodology for the requirements of sample selection based on orientation core is presented.

The in situ stress field has been measured, using two separate stress tensor measurement techniques, at six mine sites located in the Kundana mining region along a 10km strike of the Zuleika Shear Zone. The Zuleika Shear Zone is an extensive regional shear in the Archean Yilgarn Craton of Western Australia. The stresses measured by the two techniques are compared and then considered in the context of a larger database of stress measurement results taken from within the Yilgarn Craton. The results allow comparison between stresses accommodated by the craton en masse and by its extensive crustal scale faults and shears.

A technique that allows the estimation of stresses using oriented core that can be drilled at depth, and from remote locations, has been developed and widely implemented over the last ten years at the Western Australian School of Mines (WASM). The technique allows the determination of a representative and detailed knowledge of the in situ stress field during the early stages of a project (such as mine feasibility studies), even in areas where development access is not yet available (below current large open pits or existing block caves). The technique is based on Acoustic Emission monitoring of microstructure mobilization and over the last decade it has been used to estimate the in situ stresses for more than 80 mine sites located within 13 countries. In excess of 240 full stress tensor determinations have been carried out to date. This paper briefly introduces the method and presents typical results for a future block caving mine at a depth exceeding 1500m. Furthermore, the WASM database of results for the last decade is presented and the results compared to other techniques that measure the complete stress tensor.