Geomechanical monitoring of an underground bulk mining operation using a novel distributed optical fiber strain sensing method

As mines continue to deepen and become more expansive, active monitoring of larger volumes of rock mass will become more critical to calibrate numerical simulations and to ensure the safety of underground workers. Monitoring larger volumes of rock mass requires low-cost sensors that are simple in construction and installation. In this study, a novel hybrid optical fiber cable (HOFC) designed for use in distributed optical fiber sensing (DOFS) via grouted boreholes was deployed to monitor a bulk mining operation in an underground metal mine. The HOFC was successfully used to monitor approximately 2.7 × 10 m3 of rock mass above excavations surrounding a pillar removal area in which six large pillars were removed simultaneously. A total of six measurement boreholes (maximum depth of 22 m) were used to measure strain along the optical fiber during the pillar removal operation using the HOFC, allowing for 70 individual strain measurement points, which were constructed for under one U.S. dollar each. Monitoring of the excavation area took place over a 44-day period after pillar removal. Extensional strains were noted in the areas closest to the removed pillars, while areas of compression were noted directly above the remaining pillar in the area. The results of the case study demonstrate that a low-cost optical fiber strain sensing network can be rapidly installed in a large excavation area and can provide highly sensitive strain measurements in a manner that would be cost prohibitive using other methods. Fig. 1 (a) HOFC with installed anchor point. (b) First anchor attached to breather tube for insertion into a measurement borehole. (c) The installed sensor ready for grouting.(d) Schematic diagram of the installed HOFC. Background Despite concerted efforts by engineers, researchers and mine operators, fatalities in the underground mining industry due to rock failures continue to persist. In addition, global trends in mining indicate that decreasing ore grades coupled with increasing demand will result in deeper underground excavations, and an economic favorability toward bulk mining practices such as room-and-pillar removal, block caving and open stopping. Trends such as these indicate that underground excavations will be excavated faster, become more expansive, be exposed to higher stresses, and thus will require engineers and mine operators to enhance their ability to ensure the safety of workers and operational efficiency.A clear gap that exists in the currently utilized geomechanical monitoring technologies is the ability to monitor large underground areas in real time. As mines continue to deepen and become more expansive, active monitoring of larger volumes of rock mass will become more critical to ensure the safety of underground workers. Monitoring larger volumes of rock mass requires low-cost sensors that are simple in construction and installation. A monitoring solution that provides intuitive and actionable data is required, especially when a large area is to be monitored. Fig. 2 3-D excavation map with measurement boreholes and relevant geologic contacts. Methods An installation plan was devised to provide geomechanical monitoring coverage to two main access points into a large room-and-pillar removal stope. A series of 12 boreholes were drilled throughout the stope with a maximum depth of 27 m. The installation plan in this case study was designed for use with a double-channel BOTDR interrogation system. The HOFC was deployed and installed in all 12 measurement boreholes (Fig. 1).Micro-anchors were installed at the interval bonded sections, which were spaced at 1.5-m increments along the length of the cable. After installation of the HOFC in each borehole, grouting operations began, and ceased after grout was noted ejecting from the exposed end of the breather tube, which indicated that grout had reached the toe of the borehole. A detailed description of the installation procedures can be found in Nowak et al., 2022 [1]. Results A total of six mine pillars were simultaneously removed from the measurement site on May 5, 2022, via blasting of the ring-drilled boreholes. The DOFS network installed around the pillar removal area has generated a large amount of data that can be interpreted to provide insights into the geomechanical deformation that results from bulk mining practices. Strain signals, both positive and negative, are generally of a lower magnitude with increasing distance from the pillar removal area. Positive strains indicate a lengthening of the cable, while negative strains indicate a shortening of the cable and likely indicate compressional strain in the rock mass (Fig. 2). Conclusions A novel HOFC is used with distributed strain sensing to quantify geomechanical deformation after a bulk mining/pillar removal operation at an active underground metal mine. The use of DOFS allowed for simple and rapid sensor installation over a wide area. The cost of installed sensor was significantly lower than what would have been achievable using comparable instrumentation, such as borehole extensometers with a similar sensor density. The HOFC overcomes several limitations facing DOFS installation in brittle media where direct access to the sensor is not possible, such as within grouted boreholes. The HOFC allowed for measurement of rock mass deformation after pillar removal mining in a high-stiffness rock mass environment where little deformation was expected. High extensional strains detected in an access point to the pillar removal stope allowed the mine operator to select a different access point, thus ensuring operational continuity.The use of the HOFC for DOFS in the pillar recovery area allowed for the simple collection of data over a large area from a single or few access points, in a manner which is impractical or excessively expensive using comparable sensing technologies, such as borehole extensometers. This technique allows for the detection and localization of failure depths, which would complement the distributed surficial monitoring of LiDAR scanning techniques for mine safety. Through the collection of high-precision, high-density rock mass measurements during bulk mining operations, the research team aims to aid in the calibration or validation of mine-scale numerical models, which, when properly calibrated through monitoring data, can be useful for forward analysis of more complicated mining scenarios. Selected reference Nowak S, Sherizadeh T, Esmaeelpour M, Guner D, Karadeniz KE (2022) Hybrid fiber optic cable for strain profiling and crack growth measurement in rock, cement, and brittle installation media. Sensors 22(24):9685

Geomechanical monitoring of an underground bulk mining operation using a novel distributed optical fiber strain sensing method

Authored by: Samuel Nowak, Taghi Sherizadeh, Mina Esmaeelpour, Paul Brooks, Dogukan Guner, Kutay Karadeniz, Gunnar Wurst and Alper Kirmaci

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To view this article you must be a member of SME/UCA.

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