Mechanisms and measurements of destressing in underground mines: A state‑of‑the‑art review

Mining operations in the 21st century will be conducted at increasing depths within very high stress environments. Destressing and preconditioning are traditional control techniques that have already been used in past decades but will be relied upon on a regular basis under those conditions. Destressing activities are currently designed based on an empirical approach, and the use of numerical modeling is limited to a back analysis of previous applications. In this study, an extensive overview is conducted of destressing in underground hard rock mines and collieries with stiff formations, as well as caving operations where preconditioning is applied. The underlying mechanisms proposed by numerous authors are categorized into several schools of thought, and measurements obtained from case-study mines are summarized based on the purpose of destressing, operational depth, and the mechanisms involved. The various summary tables provide an updated database for empirical design and can be used as a foundation for calibrating numerical models where destressed rock mass is simulated. Background Destressing and preconditioning are techniques that have been used in the mining industry for the past 75 years. They consist of specialized blasting and full excavations that either create a weaker rock mass or remove it altogether, respectively, thus creating a stress shadow in the immediate vicinity. With the future of mining lying at further depth within very high stress environments, the need for destressing and preconditioning will increase significantly. Numerical modeling is a powerful tool that is regularly used in the industry, and its computational power can be harnessed to assess the impact of destressing. However, it is currently used mostly to back analyze previous applications of the technique, and to deduce its impact on the rock mass properties. In this study, an extensive literature review is conducted of field measurements and observations from the application of destressing and preconditioning in underground hard-rock mines, coal mines with stiff geologic formations, and caving operations. In addition, mechanisms put forward by multiple researchers are categorized to provide a basis for future numerical analysis. Finally, field data from case-study mines are summarized to provide a robust empirical database that can be used for design purposes. Methodology More than 350 references were obtained and reviewed for the preparation of this literature review. The focus was on hard-rock mine environments, but collieries with stiff geologic formations were also included as well as caving operations where preconditioning is commonly used. Case-study mines were categorized based on their geographic area first, followed by the type of destressing adopted — blasting or excavations — and the depth of implementation. In the following stage, theoretical postulations and various mechanisms proposed by numerous researchers were categorized. These were then compared to field measurements and observations from mines and other projects. The data are presented in summary tables based on the mechanisms involved, where the depth and region of destressing are also indicated. In addition, changes in rock mass properties such as deformation modulus, compressive strength, seismic wave velocity, stress decrease, closure and seismic activity are summarized. Destressing and preconditioning applications are categorized based on ore production and development excavations in hard-rock mines, with similar summaries for coal and caving operations. Results The geographical distribution of destressing and preconditioning applications was found to extend to Canada, the United States, South Africa, Australia, Sweden, Finland and Poland. Coal mines in the Czech Republic, Poland, China and the United States were also observed to have implemented the technique extensively. A summary timeline of the various geographic regions and the destressing approach used is provided in Fig. 6 in the full paper. In addition to South African mines, preconditioning was adopted especially in caving operations — mostly in Australia and Chile — to improve fragmentation and ore flow, as well as to control seismicity. Researchers agreed that destressing resulted in a reduction of rock mass properties such as deformation modulus, compressive strength and stiffness. They also noted that stress transfer to adjacent regions occurred as a result, which translated into a reduction in its magnitude, the release of stored strain energy, and deformation. Two main mechanisms for destressing were postulated by numerous authors throughout the decades. The first was the creation of new fractures or extension of existing networks, which was mostly held by North American researchers, as summarized in Table 1. The second was slippage of rock blocks along pre-existing fractures, put forward by many South African and Scandinavian authors, as presented in Table 2. Around the turn of the century, a balanced approach was proposed by others, which allowed for both mechanisms to potentially occur based on the specific geologic environment where destressing was applied. Field measurements and observations from case-study mines around the world provided support for both mechanisms, and these are summarized in table form, where the depth and region of application within the mine are indicated. Other effects on rock mass properties, such as modulus changes, strength and stress reductions, closure rates, and a decrease in seismicity and seismic velocity are also tabulated for reference purposes. One of the key outcomes of this work is the development of an empirical database where the objective of destressing is used as a categorization tool. Hence, all the case-study mines are tabulated separately depending on whether destressing was used for stoping (ore production) or development purposes. Descriptions of the location, depth, mine name and country are provided for each of the two categories, in addition to separate tables for coal and caving mines. Finally, a single figure is presented, providing a graphical indication of the depth — ranging from approximately 100 to 3,650 m — at which destressing or preconditioning took place at these mines, as well as the mechanisms involved in the process (Fig. 21 in the full paper). Conclusions This literature review provides a comprehensive evaluation of publications on destressing and preconditioning during the past 75 years. It categorizes their application in underground mines based on the mechanisms invoked, various types of measurements obtained and the operational purpose of the exercise. As empirical design processes are used for destressing, the multiple summary tables that were generated can be used as an ideal database in this regard. In addition, they can provide a baseline for numerical model calibration where destressed rock masses are simulated. References A list of all references is available in the full paper. The numbers indicated in the tables in this abstract refer to those in the full paper.

Mechanisms and measurements of destressing in underground mines: A state‑of‑the‑art review

Authored by: Shahé Shnorhokian and Samar Ahmed

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