There are several geological hazards that are encountered during the life of a quarry or open pit mine. Most of these hazards are included in a mine’s Ground Control Plan (GCP). The GCP ensures adequate geotechnical planning, safe mine operation and reclamation goals.
Some measures of consideration in the GCP should include geological structures and their influence on wall stability; strength of the rock mass; rain or water inflow issues; surface drainage; mine dewatering procedures; geometry of the pit; pit wall stability; and appropriate drilling and blasting procedures for final walls.
The first step toward preventing wall instability and rock-fall hazards is to begin with sound engineering and planning to ensure the long-term stability of highwalls. Equally important is the need for ongoing refinement, based on geological changes within an active mine.
As mining advances and mine design standards undergo operational changes, it is crucial any geotechnical assumptions be reviewed and modified by an experienced engineer or geologist to continuously ensure the integrity of the final highwall design and reduce the likelihood of highwall instability and rock-fall hazards.
It is important to re-evaluate modes of failure potential with relevant geological characteristics such as discontinuities (spacing, continuity, shear strength); bench orientations relative to the highwall face; and bench properties that contribute to rock mass behavior as new geological information becomes available.
Communication
Regular meetings with mine personnel to formally review changes in geological conditions and observed ground-control issues helps to develop early recognition protocols. It is far more effective to make required modifications to current mine operational practices, such as blast size or reinforcement, and mine plan changes prior to highwall failures, because remediation to a hazard is commonly less expensive before a failure.
Geotechnical engineers and geologists can design overall slope angles and bench configurations to minimize extensive loss of catch bench width and thus minimize rock-fall hazards. Train all onsite mine personnel in recognizing hazardous highwall conditions and environmental factors that decrease highwall stability.
Highwall investigation techniques and protocols serve as our best method to minimize hazards before they become an issue. It should be noted that unexpected rock falls that do not exhibit early signs of displacement are commonly not identified by individuals as a hazard.
A heightened awareness and close monitoring of environmental and site changes throughout the year, especially during spring and fall rains, is invaluable. Additionally, proactive steps can be taken to reduce the chance of surface ground failures and rock falls.
Mitigation techniques
Rock-fall mitigation techniques have been used with several ground-control-related issues and continue to incorporate and develop appropriate remediation measures. This is important because the consequence of a highwall failure can pose hazardous conditions and threaten the long-term viability of an operation.
Several methods exist to reduce the hazards related to ground instabilities from a highwall, including scaling, provision of adequate catchment area and ground reinforcement. Scaling is an operation where rocks on the highwall are checked for stability with a miner’s bar and what is commonly referred to as air bags. These air bags were originally developed for the fire-and-rescue industry as a means of lifting heavy objects using air pressure to inflate the bags.
There are several sizes that can be used in the scaling operation, and the amount of force exerted ranges from 5 to 90 tons of pressure. If loose blocks are encountered, they can be removed from the highwall under a controlled environment.
A common approach to dealing with rock-fall hazards is to provide an adequate catchment area at the base of the highwall. An experienced geotechnical engineer or geologist can assist in providing requirements of an adequate catchment area with tools such as the Rockfall Catchment Area Design Guide.
Berms must be properly sized and located to contain failing material. When measures are needed to handle rock-fall hazards beyond scaling and providing an adequate catchment area, many other remediation tools are available. These include the use of dowels, tensioned anchors, draped mesh, pinned mesh, rock-fall fences, hybrid systems, and berms.
Selecting a ground reinforcement method relies heavily on a good working knowledge of the local ground conditions in determining the most effective design.
There are several variations in mesh designs providing different load capacities. Their application includes areas of highly weathered rock near high-traffic areas such as access ramps and portals. This provides an adequate measure to reduce the risk to mine personnel and infrastructure by controlling rocks as they fall from a highwall.
Control of the rock fall reduces its energy and contains the rocks to a catchment area where they can be collected and removed in a safe manner. Minimal maintenance is required if designed properly. The type of material used for the mesh design should be scrutinized to make sure it can manage potential rock falls without tearing and creating gaps in the system.
Flexible rock-fall barriers, also called rock-fall fences, contain falling rocks from moving into protected areas. These systems can be installed easily and quickly but regular maintenance will be required depending on the frequency of the rock-fall events. There are two predominant types of rock-fall barriers on the market: standard flexible barriers and hybrid barriers.
Standard rock-fall barriers are designed as flexible systems that dissipate the energies and immobilize the blocks upon impact. The drawback of this type of system is that once the barrier has been impacted and the falling rock immobilized within the system, the ability to contain and immobilize future rock falls is diminished.
The only way to assure that the system will continue to work as designed is to remove the rock that is contained in the system, replace any damaged parts and reset the system. With hybrid flexible barriers, the net is not attached to the bottom cable and the netting is extended out downhill to allow the system to capture the falling rock, dissipate the energies and then release the rock.
Monitoring systems
There are many new technologies emerging. Technologies such as remote sensing should not replace basic geotechnical methods of investigation, but should be used in combination with tried-and-true methods. An active attention to early warning signs from highwalls such as tension cracks, loose material or abnormal water flow can signal a greater geohazard event is imminent.
Most highwall monitoring systems measure the relative displacements to monitor slope behavior and maintain a safe operation.
Highwall surface monitoring systems can include radar systems, robotic total station systems and surveying target prisms.
Subsurface measurements can incorporate inclinometers, time domain reflectometry and borehole extensometers to collect data on rock mass displacement.
Some of these monitoring methods can be difficult to implement with limited access along the highwall. As mining advances, it is imperative that this monitoring system be relocated to those areas of concern. There is a growing trend for radar monitoring systems at mine sites to identify specific areas of instability along highwalls, waste dumps and tailing dams.
Wall failures do not occur without warning if the failed area is being well monitored. Early detection of wall failure allows mine operators to plan and implement appropriate actions with sufficient notice such that the effect of the failure on mine safety and productivity is minimal.
For more information, please contact GeoStabilization International.