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Experimental and numerical investigation of hydraulic stimulation as a risk mitigation technique for fault slip rockburst hazards in deep underground mining

University of British Columbia

Fault slip and associated rockbursting present a significant risk to the safety of personnel and infrastructure in deep underground mines. Sudden displacement can occur on pre-existing fault planes when mining alters the stress regime present at depth, with the resulting energy release causing potentially extensive damage to mine workings. Recently, hydraulic stimulation, defined here as the injection of fluid in proximity to a fault, has been explored as a method of reducing fault slip risk. This has the potential to release the built-up stress and strain energy driving slip events before mining is advanced and workers are exposed; however, field trials to date have yielded varying degrees of success. In response, a two-part investigation has been conducted to examine the effectiveness of hydraulic stimulation in preventing fault slip for a range of geometric characteristics. In the first investigation, a laboratory testing procedure was developed in which pressurized water was injected through cylindrical granite specimens containing different offset saw cuts subject to triaxial loading. A state-of-the-art servo-controlled system was used to measure displacements, stress drops and moment magnitudes. Results indicate that heterogeneous fault surfaces produce slip events with greater moment magnitudes than smooth surfaces, and tend to respond less effectively to injection. Three-dimensional (3D) post-test specimen imaging demonstrates that variations in moment magnitude and stress release are linked to the breakdown of asperities and accompanying cohesive strength loss that occurs during shearing. This explains why stimulation is less effective on highly irregular fault surfaces, such as those containing asperities or rock bridges, since fluid pressure acts to reduce frictional strength only. In the second investigation, a numerical model of a hypothetical mining sequence was created to determine the effect of hydraulic stimulation on the frequency and severity of mining-induced slip. Though injection was effective at mitigating slip in planar fault models, increasing fault strength was observed to correlate with larger, more damaging slip events. Additionally, when fault heterogeneity was explicitly incorporated, stimulation treatments were less effective as slip and fluid propagation were impeded by intact rock segments, illustrating the importance of understanding fault geometry before the successful implementation of field-scale treatments.

Text

2018-04-18

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/65492

Geological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/