Rock Reinforcement Options in Underground Mines
Last Updated: 3/8/2018 10:52 a.m.
Rock failure and rock falls are ever-present dangers in underground mines. It’s especially so where rock structures in subsurface excavations are left unsupported. This includes side wall and roof portions of mining tunnels and shafts. To counteract this, mining engineers have developed several ingenious methods to reinforce rock faces. This gives mine operations various options for safe underground mining support systems.
Reinforcement for mine walls and roofs works on a basic principle — to build support systems for mobilizing and conserving inherent strength already energized in the rock’s mass. The idea is to take remaining rock structures in mines and make them self-supporting. That includes intentionally left rock pillars and arches as well as roofs of mine cavities and their supporting walls. It also includes reinforcing loose rock sections that could suddenly dislodge and fall.
“Rock support ” or ground support is the industry term for designing mine enclosures that hold themselves intact. This approach uses natural forces in geological masses and combines them with stabilizing devices. Rock support describes how materials and procedures are utilized to enhance structural stability. They maintain load-bearing capacity and transmit gravity forces through excavation solid boundaries.
A rock support system’s primary objective is maintaining a rock mass’s strength while mining operations hollow out cavities to harvest valuable ore. Much of the remaining strength is due to the rock mass’s natural density and ability to withstand tension and compression forces. Hardrock mines have different open span tolerances than soft rock conditions. Each ground type has individual reinforcement requirements and techniques.
Geologists and mining engineers are intimately familiar with each mine’s particular strengths and weaknesses. They design rock support systems to utilize natural features and enhance them with underground rock reinforcement equipment. They also incorporate rock support as part of their underground mine emergency and escape planning. In every mine, supporting rocks is vital.
Importance of Rock Support
Most underground mines turn into a honeycomb of shafts and passageways. This would be easy to stabilize if the three-dimensional maze were uniform. But the reality is miners have to follow ore veins and nature didn’t design the earth to make it easier on miners. All it takes is one weak section of a mine’s cross-section to fail, and the entire mass could collapse or implode.
This makes it extremely important that every single piece of a mine’s support system can support its bearing weight. As a mine deepens, the force of gravity increases. Lower load-bearing structures take enormous weight, and the further the mine excavation extends, the greater the force pressure becomes. Often, that’s more than the supporting rock’s load bearing characteristics can handle. Fortunately, engineers calculate for this and devise reinforcing controls to assist rock support.
Mines have always used reinforcement tools and strategies to some degree. Mining is a dangerous operation but is much safer now due to advanced rock support engineering models and rock support reinforcement systems. Catastrophic collapse or cave-in isn’t anywhere near the risk it once was. However, loose and isolated rocks are always subject to falling unless you contain them with a reinforcement system.
In fact, the most prominent risk in today’s underground mines is falling rock. This isn’t just granulated pebbles or small stones. Most mine walls, backs and roofs are carefully scaled before general work takes place. Scaling isn’t an exact science, though miners err on the side of caution. They anticipate when large sections are potentially unstable and put extensive effort into reinforcing areas where rocks need support.
Underground Rock Instability Causes
If underground rock were stable, there would be no need for reinforcement. But, that’s not how underground conditions are. No matter what type of ore an operation is mining, there will always be risks of collapse or falling rock once the earth has been disturbed.
Some geological conditions are far more stable than others. Rock density and structural uniformity are the two most significant factors in determining if excavated structures are self-supporting or if they require reinforcement. But it’s not just the static conditions that determine how safe a mine structure is. Dynamic influences are present as well.
The combination of static and dynamic conditions cause mine instability. It’s often multiple conditions that dictate what sections need support. Some dangerous situations are natural, and mining activities create others. These are the main factors that make underground mines unstable:
- Natural seismic activity like earthquakes and tremors
- Mining-induced activity like blasting and drilling
- Water course and seasonal load changes
- Naturally deteriorating ground conditions
- Corrosion of natural rock
- Corrosion of reinforcement metals
- Inappropriate ground or rock support
- Inadequate ground or rock support
- Failure to properly reinforce structural members
- Failure to properly reinforce for falling rock
Two Strategies for Underground Mining Rock Reinforcement
The underground mining business has two reinforcement strategies. One is active support. The other is passive rock support. In most underground mines a combination of active and passive reinforcement designs are applied. Active reinforcement deals with excavating the mine passages so that each integral remaining rock structure is actively carrying weight and distributing it as evenly as possible. Passive reinforcement deals with potential hazards that are capable of dislodging or causing secondary failure.
Drilling deeper into active and passive supports, the distinction between them also covers the management of existing load forces, either with natural rock or reinforcement devices. Active support deals with constants forces and supports the formations. Passive support reacts to developing and changing ground forces, such as a collapsing section of rock. In other words, active reinforcement controls existing pressure while passive reinforcement is in place as insurance in case something changes.
Many modern miners refer to their safety and support strategies as primary and secondary. It’s fair to say active rock reinforcement is primary support while passive reinforcement is secondary support. One is the first support system in place or line of defense against structural failure. The other follows as secondary protection against incidental disengagement of falling debris.
Primary reinforcement support happens in conjunction with the excavation process when load bearing points are developed. It’s designed or installed as digging progresses and immediate dangers become evident. Secondary support gets put in place at a later stage. It’s there for long-term control of rock faces and roofs where dislodged rock pieces can suddenly fall and cause injury or equipment damage.
Both types of reinforcement support have their own approaches. Historically, underground mines had few options. That’s because technology was limited and geological knowledge was nowhere near what engineers have today. Safety standards were also poor. There wasn’t the strong emphasis on worker safety like what’s in place at modern sites.
Underground Mining Technology Advancements
Mining technology has made massive advancements in the past century. Some of it occurred in active or primary structural stability, but most of the new tools and equipment are passive and secondary. A real benefit in modern mining is the ability of engineers to integrate active and passive reinforcement approaches into a combination where the whole is greater than its parts.
The original active supporting devices were wooden timbers shoring up roofs and preventing sidewall cave in. These braces were weak and could only support limited spans. Tunnels were narrow and low, not allowing the open spaces now found in many underground mines. Wood also faced rot in wet mine conditions. This led to premature collapse and presented a compounded danger.
A breakthrough occurred in the 1920s when miners used the first mechanical rockbolts in U.S. underground mines. Rockbolts allowed cracks and fissures in supporting rock columns and roof caps to be joined together through compression. Workers drilled holes through opposing rock sections and installed threaded rod. Large nuts and washers then twisted and clamped down on compromised rock support sections making them into one solid structure.
Active Reinforcement Examples
Rockbolts, the earliest example of active, primary mining reinforcement tools and techniques, have evolved. Some have sophisticated approaches and see use in millions of individual bolt applications in underground mines worldwide. Here are the main types of active underground mining reinforcement techniques:
- Mechanical Rockbolts:Mechanical bolts are the earliest active rock reinforcement devices. They’re also called point anchor bolts and are straightforward rods inserted into predrilled holes in rock faces. Nuts and bolts compress the rod and transfer forces into the rock composition, stabilizing it.
- Friction Rockbolts: Friction bolts work by being driven into predrilled holes that are slightly smaller in diameter than the metal rod or dowel. This creates a strong friction force that transfers to the rock mass giving it integral stability. They’re common in light-duty applications as they’re fast and easy to install.
- Grouted Rockbolts: Grouted bolts are set in holes slightly larger than the rod diameter through the use of an expanding grout mixture. This works like a friction bolt where forces all along the bolt transfer into the rock. Common grouts include expanding epoxy resin and cement based products.
- Cable Bolting Rigs: Cable bolting rigs, wire rope sections that have threaded ends, are typical in larger applications where flexibility is required. They’re capable of being inserted into drilled holes or wrapped around unstable rock massed. Tightening the end bolts creates a clamping force.
- Strapping: Strapping uses metal bands placed across rock laps or seams. Bolts are set through the strap face and tapped into the rock surface. Straps are often used with hydraulic supports.
- Hydraulic Props: Hydraulic jacks work with straps and serve as modern day timber shoring similar to the posts and beams used in the past. Advantages include their ability to be adjusted when necessary and the fact that they won’t rot like wood.
Passive Reinforcement Examples
What all active reinforcement systems have in common is that they exert pressure on rock structures. They work to actively support existing rock structural elements that can’t support entire loads on their own. Passive reinforcements don’t use much force. They’re in place to catch something if it accidentally falls. Here are the main types of passive underground mining reinforcement techniques:
- Shotcrete: Shotcrete is a specially designed, liquefied concrete product that’s blown or shot onto rock faces under high pressure. It’s sometimes known as gunite and consists of cement powder, fine aggregate and water. This pressure application forces shotcrete into rock imperfections and crevices, sealing it and preventing erosion.
- Steel Sets: Steel sets act like straps except they’re not mechanically connected to rock faces through bolts. Steel sets are like passive beams that are held in place by screw jacks and sit passively against a rock surface in case there’s some movement.
- Mesh: Mesh is another commonly used passive underground mining reinforcement method. Welded wire mesh and chain link mesh are the most utilized varieties. The main difference is that welded wire mesh is quite rigid while chain link is flexible. Both types are fastened to rock faces with bolts and washer plates. Mesh is exceptionally effective at retaining loose material while letting moisture and gasses dissipate.
- TM International Blast Shields: Blast shields and blasting mats haven’t frequently seen use in underground mining reinforcement applications until recently. They’ve been a mainstay in protecting workers and machinery from flying rocks and debris during blasting operations, but now mining engineers are finding how much reinforcement value there is in blast shields.
Reinforcement Value in Blasting Mats and Blast Shields for Underground Mines
Rock support blast walls for underground mines have excellent reinforcement value. Blast shields use the same concept as chain link mesh. TM International is a New York company that’s been supplying Safe Pass blast shields and Mazzella blasting mats to the construction industry for decades. These high-strength products are ideal for passively shoring up mine structures where shotcrete is impractical and welded wire mesh isn’t adequate.
TM International blast walls are multi-purpose systems. They serve to protect mining interests during excavation works where explosive blasting incidental strikes need securing. Mining operations can then repurpose blasting mats for use as rock face reinforcement devices that contain loose debris. This can be on both temporary and permanent installations. Blast shields and walls can be moved along as works progress, unlike shotcrete, which is permanent.
Blast mats and shields are made from flexible steel cable that’s incredibly strong. Steel cable mesh is far stronger than chain link and welded mesh, and it’s much more portable. That makes blast shields the perfect option for underground mining reinforcement.
Blast shields are also the best option for specific location protection in underground mines. You might place them in high-risk areas where workers congregate like elevator or lift entrances, lunchrooms, offices and first aid stations. Blast shields and blasting mats provide reinforcement protection that can’t be achieved with conventional systems.
Industry testing proves just how durable TM International products are. They’re constructed of 5/8-inch galvanized IWRC wire rope that withstands up to 6,500 J/cm2 of energy from an explosive blast. They also reduce peak blast pressures by between 50 and 80 percent. Additionally, blast shields are lightweight making them easy to transport and install. They’re also completely fireproof and highly resistant to corrosion.
Many surfaced-based industrial companies use TM International blast shields and blasting mats for their protection. Now it’s time for the underground mining industry to realize the full potential and exception reinforcement value for sub-surface protection. For more information on TM International products, call 718-842-0949 or contact TM International online.