Deep mining, seismicity and reducing riskHazard awareness and risk mitigation for front line crews in underground mine development
Hazard awareness and risk mitigation for front line crews in underground mine development
By Sandra V. L. Smith
April 08, 2021
Courtesy of Sandra V. L. Smith
It is no coincidence that a workforce with a safety mindset has a safer workplace, fewer incidents and fewer delays. As underground hard rock mines are getting deeper, they are experiencing more seismicity and ground control challenges. It is essential to understand how depth of mining can change the work environment for front-line crews – the one thing that does not change as mines get deeper is that the highest risk work is at the face. As hard rock mines have gotten deeper, we have learned some lessons about hazards and awareness, from the early design stage through to construction and developing excavations.
Mining and seismicity
Seismicity in underground mines is directly related to the mining process, as mining promotes local stress changes. In typical conditions, underground rock is very stable in a pre-mining state – where there is no mining, there are no seismic events.
Characteristics that are unique to each site, such as rock strength, local geological features (faults, shear zones, etc.), depth of mining, sequence, rate of advance, fracturing and excavation geometry, among others, inform the design and operational decisions in order to maintain stable excavations. Rock failures may still occur where ground stresses are greater than 0.3 to 0.5 times the strength of the intact rock, but the majority of these can be controlled when the ground support system is designed for the specific environment.
Rock stresses increase at depth and, as the mine matures, there are more excavations and active faces experiencing stress changes. High stresses promote seismic events and the released energy of high-magnitude events often causes damage to existing openings. Brittle rocks, especially where there are contrasting rock strengths at contact zones, experience more seismicity.
While large seismic events are potentially damaging, they are relatively rare. Workers are most likely to experience small, more frequent events, associated with strain-bursts or gravity-induced groundfalls initiated by stress changes in fractured or jointed rock. Workers need to understand those changes to recognize visual cues underground and maintain work habits that support safe operations. Continually observing and communicating changes at the face is the first line of defence.
Whether it is shaft sinking, new development or rehab work, every project requires diligence in comparing designs to the actual site conditions, and revising the designs through a managed process to suit the work and the site. The goal is to recognize hazards and risks in order to keep people safe, avoid damage and minimize rehab work and schedule delays.
A standard approach to evaluate risk is to identify the potential consequences with the likelihood of occurrence and the controls that need to be in place to reduce risk. Understanding the local site conditions, stresses, rock quality, excavation geometry, sequence and rate of advance is the first step. The three tiers of risk mitigation should be applied at every stage of site development:
Elimination or substitution: Removing people from exposure to risk is most effective. Using remotely operated equipment distances people from hazards, while barricades or exclusion zones keep workers from entering hazard areas. Establishing site-specific re-entry protocols following blasting activity is also common practice.
Engineering: Excavation sequence or design changes, reducing the rate of advance, upgrading ground support systems or applying de-stress techniques are among the engineering and operations controls that are successful in reducing damage and overall risk. Variations of dynamic ground support systems have been effective in controlling damage to excavations. An integrated system can include stiff and dynamic ground-support elements that act together to retain and reinforce the fractured rock and absorb energy of the seismic event.
Administrative controls: Administrative controls include hazard awareness training to develop an understanding of the local conditions and observing changes in the rock stability as development advances. Ongoing communications regarding workplace observations is critical so that the mine owner and supervisors, engineering and design teams can gain a better understanding of the local conditions and fine tune best practices to reduce risks.
The hazard awareness training program at Cementation – a four-time gold medal recipient of Canada's Safest Employers since instituting the program in 2014 – is aimed at sharing information, providing examples of what to look for during development, possible actions to take and the importance of ongoing feedback to improve future designs, protocols or processes based on the specific site experience. While mining activity can produce seismic response, it’s important to understand local changes in rock mass behaviour so that decisions for safe mining are based on actual conditions and suited for the site.
Recognizing hazards is key to making decisions that minimize risks throughout the life cycle of an underground construction project. It is more than a checklist, it is a commitment to safety in design, carried through to the working face. We need to continually monitor and report changing conditions and apply lessons learned, even more so with high stresses in deep mining.
Sandra V. L. Smith is a mining engineer who specializes in engineering analyses for ground stability, support systems, geomechanical rock mass quality, site assessments, mitigation options for surface and underground hazards and mining in high stress conditions. She is currently working towards a PhD focused on stability and seismic energy analysis as a planning tool to reduce risk associated with rock mass failure.