Résultat de la recherche
Experimental evaluation of a distributed Brillouin sensing system for detection of relative movement of rock blocks in underground mining
Underground mining in highly stressed, hard rock masses is commonly accompanied by seismic events that are located close to mining, near active stopes and other excavations, most frequently within one diameter of the excavation boundary. However, as the extraction ratio increases or mines progress to greater depths, remote events have been observed at large distances from active mining, where the direct mining related stress changes are negligible. Events in different mining blocks cannot be explained by models involving stress redistribution processes alone.
Numerical modeling of strain transfer from rock mass to a fibre optic sensor installed inside a grouted borehole
Strain measurements in underground excavation are usually done locally, with extensometers or similar devices placed within 10-15 meters of adit or stope faces, mainly to gage development of the EDZ (excavation damaged zone) and assess its evolution and impact on local safety (rock falls, rapid deterioration of wall condition...). However, the calibration of three-dimensional stress analysis models used to assess excavation geometry and sequencing requires strain (displacement) measurements in those parts of the rock mass that are in the elastic domain for some or all of their active design life. Recently developed distributed fibre optic sensors are now being used to measure local linear displacements continuously in a large rock mass volume in real mining conditions in Canada. Grouted inside a borehole and therefore encased in a material of far lower stiffness that the rock mass, an optical fibre may register strains different from those actually occurring in the rock mass. A number of factors affect the process of rock mass strain conveyance through the grout to the fibre. This paper reports a study that simulates the borehole-grout-fibre interaction to find how the strain transfer takes place and whether there are any issues serious enough to warrant alterations in installation procedures and grout materials.
Experimental evaluation of a distributed Brillouin sensing system for measuring extensional and shear deformation in rock
Distributed Brillouin sensing systems (DBSs) have growing applications in engineering and are attracting attention in the field of underground structures, including mining. The capability for continuous measurements of strain over large distances makes DBSs a promising monitoring approach for understanding deformation field evolution within a rock mass, particularly when the sensor is installed away from excavation damaged zone (EDZ). A purpose-built fiber optic sensing cable, a vital component of DBSs, was assessed in laboratory conditions. A test program was performed to observe DBSs response to various perturbations including strain and joint movements, including opening and shearing of joints. These tests included assessment of the strain-free cable response and the application of extensional and lateral displacement to various sensing cable lengths (strained lengths), from 1 m down to 1 cm. Furthermore, tests were done to evaluate the time-dependent behavior of the cable and to observe the effect of strain transfer using a soft host material (e.g. a soft grout) under lateral displacement. The noise level of the DBSs range was ±77 le, determined through repeated measurements on an unstrained cable. Stretching test results showed a clear linear correlation between applied strain and Brillouin frequency shift change for all strained lengths above half the spatial resolution of the DBSs. However, for strained lengths shorter than half the spatial resolution, no strain response was measurable and this is due to the applied internal signal processing of the DBSs to detect peak Brillouin gain spectrum and noise level. The stability with time of the measurements was excellent for test periods up to 15 h. Lateral displacement test results showed a less consistent response compared to tension tests for a given applied displacement. Although the Brillouin frequency shift change is correlated linearly with the applied displacement in tension, it shows a parabolic variation with lateral displacement. Moreover, the registered frequency response (correlated with strain) of the system decreased significantly when the sensing cable was embedded in a sand-filled tube compared with direct cable displacement.
Monitoring mining-induced rock mass deformation using distributed strain monitoring based on fiber optics
Triggered seismicity remains one of the main geomechanics hazards that affect safety in deep mining. Mobilization or propagation of existing faults can potentially release large amounts of seismic energy that in turn may trigger rockbursts and falls of ground, threatening worker safety and generating production delays. Cur-rent modeling approaches, typically based on stress analyses, do not fully succeed in capturing such seismically triggered mechanisms as the main seismic event location may be beyond the volume of rock affected by the mining induced stress perturbation. However, the displacement field generated by the excavation process may explain the triggering of far-field seismicity. Deeper understanding of the mining-induced defor-mation field is the motivation for the development of mine-scale deformation monitor-ing techniques. Innovative deformation sensors developed for structural monitoring based on fibre optic technology allows distributed measurement of strain at high spa-tial sampling rates over large distances. This paper presents the results of the testing of one of these systems in an active mining context. The system selected is based on Brillouin Optical Time Domain Ana-lyses (BOTDA) and allows for a spatial resolution of 10 cm. It has been tested in the laboratory and installed in five boreholes piercing through an actively mined, 25 m thick, 1000 m deep, sill pillar. Benchmarking of the system against extensometer re-sults was successful in a qualitative manner. The high spatial resolution of the fibre optic system brings valuable additional insights to rock mass deformation processes.
Laboratory-scale strain and temperature response of a distributed optical fiber sensor
Distributed optical fiber sensors (DOFSs), used initially in structural health monitoring for high-rise buildings and bridges, are attracting attention in the field of underground structures, including mining. Designed for long-term study of deformations, DOFSs are more efficient when installed away from excavation damaged zone (EDZ) in a borehole filled with a grout mixture to measure elastic strain field responses to excavations. The DOFS sensing cable, as a component of a complex compliance system, i.e. rockmass and grout, is being assessed through laboratory work. A test program is underway to observe DOFS response to various perturbations including strain and joint displacement. Initially, tests on unstrained sensors are performed in order to assess measurement repeatability and noise-to-signal ratio at both local and global scales. Then, the various lengths of the cable, from 1 m down to 1 cm, will be stretched up to 0.5% strain. In other tests, the same lengths of the cable will be exposed to shear displacement, such as might occur in the vicinity of a joint or fault that experiences shear. The results from these tests will answer uncertainties and questions regarding the scaling factor between straining sections over a full sampling window, i.e. spatial resolution, and a partial sampling window, i.e. validity of calibration factors provided by the supplier, and assessing effects of coating and plastic protective layers of the sensor. Issues such as shear deformation responses of cable and bending direction of the cable are being evaluated. Initial results on unstrained cable to assess measurement repeatability showed variability in length assessment between successive readings. This variability particularly impacts the data interpretation from the strain sensors since these sensors present locally large Brillouin frequency gradients which results in locally large variability in differential readings. Our detailed experimental results will be presented in the paper.