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What are Rock Bolts?

what-are-rock-bolts?

Rock bolts were a game changer for safety in the mining and excavation industries. These long trusted anchors have undergone continual advancements, are simple to install, cost effective, and work well to keep rock in place and people and equipment safe.

Mining and the building of infrastructure are fascinating subjects when it comes to American history. Many people moved to the United States seeking fortune. Once they got here, there was a massive move to the west. Men flocking to the mines built many parts of our country, allowing some to strike it rich, while others were sent to their death.

Mining, building roads, and tunneling is dangerous professions. Before rock masses were stabilized, rockfalls and cave-ins were responsible for many deaths. Early methods for rock support included initially using timber and then lining the area with brick or masonry, which had high rates of failure.

Present-day processes for mining and tunneling look little like they did back in the late 1800s. Once steel became available, this became the primary means for support. There have been many improvements and advancements to make mining, road building, and tunneling safer. One such improvement has been the use of rock bolts.

Rock bolting helps with corrosion protection to stabilize rock so that it will not fracture or collapse on workers, travelers, or adventurers. Today, there are many applications of rock bolting. In addition to mining and tunneling, these bolts are used in concrete dams, bridge abutments, rock cliffs, steep slopes, and stabilizing road cuts. Rock climbers also use rock climbing bolts to stay safe while climbing.

What are Rock Bolts?

Rock bolts are threaded bars made of steal, which are inserted during excavation to stabilize the rock that will stay in place so that workers and machinery can be protected. A drilled nail or anchor is inserted into the rock, grouted in place, and post-tensioned by using compression with faceplates.

This tension is used to distribute the load evenly onto the surrounding rock. The process is similar to using soil anchors or a soil nail, with these bolts working to hold fractured, jointed, or unstable rock masses together. This provides passive resistance and creates a more stable rock surface, protecting those on the ground.

Rock Bolt Timeline

The first recorded uses of rock bolts come from the 1890’s. More thorough documentation of their use started in the 1920’s. During this time, the St. Joseph Lead Mine, located in Missouri in the Unites States, has extensive records of using rock bolts. Over time, these bolts continued being widely used in mines, but also in civil tunneling projects in both the United States and Australia in the 1940s.

In 1947, engineers in Australia started to experiment with expanding anchor rock bolts that were four meters long. The engineers were working on a complex for hydroelectricity and irrigation in the south-eastern area of Australia while experimenting with the rock bolts. Today, rock bolts are used in a huge variety of excavation projects. Research continues and new materials and practices are emerging to make these bolts even more effective.

How are Rock Bolts Used?

The first step in installing steel rock bolts is to work with a professional to determine the best length, type, and spacing for the rock bolts. Different circumstances and rock or rock strata will have different reinforcement requirements. Once all of the planning has been completed, the first step is to drill holes in the walls and roof of the rock in the decided-upon pattern.

The steel rock bolts can then be inserted into the holes, grouted in place with resin or cement grout if necessary, and the bearing plate can be put on and the bolts are tensioned. The installation is simple, quick, and relatively inexpensive. In many cases, each bolt can be installed in less than one minute.

The process can even be fully mechanized. After the bolts have been inserted, sometimes chain link, lattice girders, or other wire mesh material are stretched over the surface to further reinforce the are or to catch any falling pieces of rock from shear displacement. This is not always necessary.

What are the Types of Rock Bolts?

Grout-Anchored Rock Bolt

A grouted rock bolt is a threaded bar made of steel, or rebar bolts. Once drilling is done and this steel bolt is inserted, cement grout or resin grout can be used as the grouting agent. For reinforcement, a cable bolt made of steel wires intertwined as a steel rope is placed with the grout in the borehole. A bearing plate is used on the fully grouted rock bolt on the surrounding rock face to create tension and stabilize the rock.

Cable Bolts

Cable bolts can be used for temporary or permanent rock reinforcement. These bolts allow for tensioning without tensioners. They have a free length or threaded anchorage instead of a barrel and wedge system that is often found. This allows for a more flexible system, even when using longer cable bolt lengths. Cable bolts have a low bolt weight, but a high load-bearing capacity. Installation can be completed with cement cartridges, resin cartridges, cement grout, or injection resin. These are great for conditions with limited space and work well for the reinforcement of roadways and gateways.

Hollow Rock Bolts

These steel bolts have a hole in the center and were made to ensure a reliable grouting process. The hole is used for grout, eliminating entangling tubes. This type of bolt can have the same cone and wedge anchor mechanism. When using the bolt facing downward, the grout is applied in the hole and then a short tube is inserted to get rid of any trapped air. When using the bolt facing upward, a short tube is used for grouting and the central hole is left clear for a breathing tube.

Self-Drilling Rock Bolt

These types of bolts are made out of a hollow bar that is fully coarse threaded. It can be used in roof and wall bolting, in the ground, for installation with grout, and can be placed temporarily or permanently. A self-drilling rock bolt acts as the drilling rod (it is not necessary to pre-drill a hole) as an anchor rod and a grouting pipe. A splicing system is available to create a longer tendon length. These bolts can be found with a T (trapezoidal) or R (rope) for tread profiles and have diameters from 25 to 76.

Mechanically Anchored Rock Bolts

This type of bolt uses an expansion shell instead of grout. This is much like how a drywall anchor works. As the bolt is rotated, the wedge that is attached to the bolt shank gets pulled into a conical shape. The shell is anchored with both friction and interlock. The mechanical anchor can be tensioned with a hanger or faceplate. This will distribute the load more evenly along the rock face to provide rock reinforcement.

Friction-Anchored Rock Bolts

The friction-anchored rock bolt is one of the newest advancements in rock bolt techniques. A radial force is used against the borehole wall along the entire length of the bolt. This creates a frictional resistance to sliding after bolt installation.

MDX Bolt

If working in an area with seismic rock conditions, the MDX bolt will provide more ground support stability. Seismic events make it even more dangerous to be working under the ground and it is very important to use a rock bold that can handle the squeezing ground.

The MDX bolt can be installed simply in a single pass and has huge expansion and load-bearing capabilities. The design enables to bolt to handle significant loads with a small amount of displacement. These bolts have a spacial wedge that can expand up to 60mm. This provides a solid anchor in the wall without needing grout or resin.

Fiber-Reinforced Polymer Composite Rock Bolts

When working in places with harsh environmental conditions, such as in coal mines, steel bolts deteriorate quickly. They can sometimes fail within a matter of days. In these tough conditions, fiber-reinforced polymer (FRP) rock bolts can work better because of their corrosion resistance.

This makes them more suitable for conditions that have harsh chemical and alkaline environments. These bolts are non-conductive, light weight, easy to install, durable, are dimensionally stable under thermal loading, have an anti-static rating, and can also be cut without creating sparks.

Dowel

A dowel is simply an untensioned steel rod that is placed in the borehole after drilling in the rock mass and grouted in place. It is not threaded, does not have an anchor head, and needs ground displacement for activation. When joints become subject to displacement, the dowels will withstand both shear and tensile stresses. The ratio of the shear and tensile stresses will depend on the grout material and the make-up of nearby ground support.

Other Reinforcements Used With Rock Bolts

Sprayed Concrete

This fast hardening materials is sprayed over a tunnel once rock bolts are installed to provide more stabilization. This comes in two forms, gunite, which is a dry mix, and shotcrete, with is a wet mix. It is more efficient to use shotcrete, as it has a lower rebound rate. Gunite is normally only used when high strength is needed in small areas.

Gunite is left dry until it hits the nozzle, when water is added to the mix just before it is applied to the substrate at a high velocity. Shotcrete is used over larger areas and requires a mechanized nozzle, as the mixture is heavy and can’t be carried by a worker. This material is premixed and consists of cement, aggregate, and water. When it is sprayed, the shotcrete is mixed with air and accelerators just before application. In some instances, fibers are added to the shotcrete to make it more strong and durable.

Lattice Girders

Providing immediate support, lattice girders are triangular steel frames that are lightweight. These can be used in addition to shotcrete and are most commonly found in crown heading excavation. This product can be used along with strata bolts inserted through the lattice girders to provide extra forward support.

I Beam Girders

I beam girders are heavy steel frames. They can be used in an arch, spanning the entire width of the tunnel. These are used when rock formations are weak, but they will degrade shotcrete quality because of their shape.

In What Applications are Rock Bolts Used?

Rock bolts have been used during underground excavation in the mining and tunneling industry for well over 100 years. They provide temporary roof support and also keep walls secure. Today, rock bolts are used in many other applications.

These include reinforcing concrete dams, bridge abutments, rock cliffs, steep slopes, and stabilizing road cuts. When driving through winding mountain roads with steep rock faces on either side of the road, rock bolts can often be found. These bolts are protecting the cars and passengers as they travel through the area.

Planning for Rock Support

When preparing to build a tunnel, there are many variables to consider. There are four major groupings for tunnels, depending on what material they pass through. These are soil, weak rock, soft rock, and hard rock. Each method requires different methods of excavation and ground support.

Those planning the project also need to take into consideration whether the tunnel will be used temporarily for a purpose such as extracting ore, or for a long period of time in a place where people will be regularly present.  An example of this would be a new subway tunnel.

When it comes time for the ground support, engineers must understand the geological composition of the area, take into account the purpose of the project, and plan their system based on the type of support that is needed, how strong it needs to be, and how soon it will be installed after excavation.

With this information, the engineer will decide on the best diameter, length, and type of rock bolt to be used. They will also calculate the optimal spacing and decide if any additional reinforcements such as spray concrete, lattice girders, or I beam girders will be used. Once the engineer carefully composes the plan, work can begin to install the ground support system.

Rock Climbing Bolts

Rock climbing bolts are in a far different category than the rock bolt used for stabilizing rock. These bolts and hangers are placed in rock and are only used to keep climbers safe. These bolts and hangers are inserted into rock at popular climbing destinations and left for other climbers to use at later times.

A climber will use quickdraws to attach to the hangers, so that they will have added security when climbing. Rock climbing bolts come in diameters ranging from 3/8 of an inch to 1/2 an inch and lengths ranging from 2 inches and 3 1/2 inches. It is important to understand the type of rock that the bolts are being placed into, as this affects the type of bolt needed. As an example, rock that is softer needs longer and thicker bolts.

When using bolts and hangers when climbing, it is important to make sure that they are in good shape before relying on them to hold all of your weight. Some materials last longer than others. Steel is the most common material used in climbing bolts.

However, there are several different types of steel, including carbon steel and stainless steel. Titanium is another common material.  It is very strong but is also new material for these bolts and hangers.

Carbon steel is the weakest and will rust over time, if you see rust, it’s likely that the bolt is no longer safe to use. Sometimes carbon steel is coated with zinc, which will protect against rust for a short time, but not forever. Stainless steel is stronger, but it’s hard to know if the bolt is still good. It can be rotted and rusted on the inside, but show no signs of this problem on the outside.

Over time, any material will corrode as it is exposed to oxygen. Most bolts and hangers will last between 10 and 15 years, however, in extreme conditions, they can fail in as little as a few months.  It is extremely important to check all rocks and hangers along a climbing route before assuming that they are ok to use.

How to Install Rock Climbing Bolts

The first step is planning the route, look for areas with good holds that are not too far apart.  Next, find solid rock by hammering and listening for areas that are not chipping or hollow sounding. Once you find a good spot, find a good, flat place where they bolt or hanger will sit nicely, drill the hole. After the hole is drilled, clean the hole out with air to make sure it is clean. After this, hammer the bolt into the hole and then tighten it with a torque wrench.

FAQs

How do rock bolts work?

Once rock bolts are inserted, they ‘knit’ a rock mass together. This keeps the rock from moving and loosening enough that it can fail and fall apart. Sometimes, wire mesh, beams, or sprayed concrete are used in conjunction with rock bolts to keep rock stable.

How do you install rock bolts? 

It usually takes less than one minute to install a rock bolt. The first step is to drill a hole. This can be done by using a stoper, a jack drill, a roof bolting jumbo, or other type of drill suitable for the job. After the hole is drilled, the drilling steel is replaced with a driver tool.

If using a plate, this is now placed on the tube, which is then driven in with the drill or drifter. If using a grout-anchored rock bolt, the grout is placed in the hole before inserting the rock anchor.

How do you anchor a rock? 

The first step in anchoring rock is to drill a hole that is the correct size for the rock anchor being used. After the hole is drilled, grout is injected into the hole if you are using grout. Once the grout is in place, the threaded sheathed rock anchor is inserted, using a bearing plate if applicable. Last, the anchor needs to be tensioned appropriately.

What are rock anchors used for? 

Rock anchors are used for a variety of purposes. They are used during excavation and tunneling to stabilize rock under the ground. Rock anchors are also used for rock climbing or to stabilize cliff faces.

They can also be used for reinforcing concrete dams and bridge abutments as well as stabilizing road cuts and steep slopes.  Rock anchors can also be used in construction projects to stabilize an excavation site as well as a part of retaining walls, to resist movement to a foundation, or to anchor a structure to rock.

What are the differences between rock dowels and rock bolts? 

Rock dowels are inserted into pre-drilled holes, just as rock bolts are. However, these steel rods cannot be tensioned. Dowels are grouted into place and are passive reinforcing elements. They need some displacement in the ground to be activated.

When these displacements happen along the joints, the dowels become subject to shear and tensile stresses. Rock bolts can be tensioned and evenly distribute the load to the surrounding rock immediately.

What is the difference between a rock bolt and a rock anchor? 

Rock anchors, or tiebacks, use a compressive or uplift force to stabilize a rock mass above ground or underground. Rock anchors are made of high tensile strength bars and are either untensioned or pre-tensioned by anchoring at the end of the borehole. If left untensioned, this becomes a passive type rock reinforcement and only develops tension when there are deformations.

If tensioned, the device will support the rock immediately. In contrast, rock bolts are steel rods that are grouted in the rock or use friction for tension. Once the anchor bolt is installed, it is tensioned and the force of compression is applied to the surrounding ground. The induced axial force makes the shear capacity of the mass increase.

How does rock reinforcement function?

Rock reinforcements are placed in rock structures to improve stability and increase load-carrying capacity. This reinforcement uses the inherent properties of the rock to improve support. Rock regularly undergoes widespread fracturing. As a result, individual rocks fall out and this eventually leads to the failure of the slope.

When rock reinforcement is used, these discontinuities can be connected, which helps the rock to act as a single unit. Bolting connects the mass and alters the stresses felt throughout the entirety of the rock.

What is the purpose of cement grout in rock bolt installation? 

Grout is used in the rock bolting process to effectively bond the rock to the bolt. This safely and securely bonds all zones of rock that is located within the vicinity of the bolt. Selecting the right grout for your needs is important to ensure a firm bond.

How do you screw into stone? 

First, you need a proper drill for the job. A hammer drill for a smaller job, or a stoper, a jack drill, a roof bolting jumbo for larger jobs should do the trick. Choose a drill bit that is slightly smaller in diameter than the bolt being used. If the hole is too big, the bolt will not adequately anchor into the stone.

After the correct equipment is chosen, drill the hole. Be sure to wear protective equipment, especially goggles when drilling into stone. After the pilot hole is drilled, blow air into the hole to make sure it is free from debris. Last, insert the bolt, anchor, screw or rod into the pilot hole to the correct tension.

How are shear forces transmitted across a reinforced concrete crack?

When a crack occurs in concrete or rock, the crack will widen when shear displacement occurs. This can cause the rock or concrete to crumble and break apart. By reinforcing the crack with rock bolts, rock anchors, or dowels, will keep the crack from getting wider and the chosen reinforcement will take on the axial stress instead of the crack.

Can you drill a hole in the rock? 

Yes, with the correct equipment, a hole can be drilled into the rock. A regular drill will not work, but a hammer drill, stopper, jack drill, a roof bolting jumbo will drill a hole in the rock.

How much weight can a climbing bolt hold? 

The answer to this question is not simple, as a climbing bolt not only needs to hold your weight as it dangles but needs to be able to handle the force of a fall. For this purpose, climbing gear is generally rated by kN.  For simplicity’s sake, 1kN is the static load that would be found from hanging 224 pounds on it.

Keep in mind, it’s not only a matter of how much the bolt can handle but also the rest of your gear, including carabiners and webbing, as well as the strength of the rock and condition of the bolt. That being said, as long as everything is in good working order, most climbing bolts are good for a minimum of 15kN to 20kN. Even with a fall, most climbers should not inflict a force of more than 8kN on a climbing bolt.

References:

Britannica: Rock bolt

ScienceDirect: Rock bolt

Dextra Group: Self-Drilling Rock Bolt

Sandvik Mining and Rock Technology: The MDX Bolt

Split Set: How Split Set® Rock Bolts and Friction Bolts Work

The Constructor: What is Rock Reinforcement?

Hunker: How to Screw Into Stone

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