What is Rock Bolting in Tunnels?

Hello 1935Ford.

I worked for Utah Mining Co. in the late 1960's.

Most rock has faults or cracks, which are not obvious until the parent rock has been "opened up" or removed, then because the "pressure" has been removed or decreased, what is left behind often expands, and cracks further.

Rockbolts are used to prevent possible rock falling in the present and future, from a rock face which has been blasted or drilled out of the parent rock, for a tunnel or face above a road or railway.

They are often used in conjunction with large square steel plates placed over high tensile industrial netting, to slow or prevent smaller pieces of rock falling, causing damage or injury.

Rockbolts were also used as anchors for items such as machinery, access ladders, metal arched roofing etc, to be "bolted securely to a rock surface".

The rockbolts we used then were high tensile deformed bars, threaded externally at both ends, and with an internal hole through the centre lengthways. This hole was also threaded at each end.

A hole was drilled, some 2 inches in diameter, many metres into the parent rock, (using pneumatic rotary drills), rockbolts were screwed together using the external threads and special pipe type couplings.

The inner end was expanded by a wedge inserted at the far end of the rockbolt, (with a split already machined to accept the wedge), which was struck with a large hammer on the outside end to expand the wedge into the inner end of the rockbolt, preventing easy withdrawal.

At this time, a 6 inch x 6 inch 1/4 inch thick steel plate "washer" was placed over the end, and a nut then placed over the exposed thread, (Netting underneath if required) and the initial tightening was done.

Care had to be taken, so that further loose rock did not spall (break and fly off) off the rock face at this time.

Later high pressure/high cement grout was pumped down the centre of the rockbolt, the grout returning back outside the rockbolt, to fill the voids between the rockbolt outside diameter, and the drilled 2 inch hole.

The grout was very thick, almost solid to look at, and required to be pumped at a high pressure, and the grout pump removed.

Because of fissures in the parent rock, sometimes water would permeate through, from the mountain or hill above, then drip or flow from the centre hole in the rockbolt after the grouting was completed.

A transparent plastic tube was often connected as a"drain pipe' to the inner hole of the rockbolt, to move water away from the rockbolt surroundings. This drain tube was generally cut off and removed later.

So....later, after the grout hardened off, in a few days, the nut would be tightened to specified torque, the rockbolt being now firmly anchored by the initial wedge, then the cured grout.

It cases where small rock particles continued to "spall" off the face, the rock face/netting and rockbolts were sprayed with "Gunite" a proprietary process of grout with very little water, but pumped at extremely high pressure (In the manner of "Spray-painting" - but extra protective clothing etc, was required by the contractor),to coat the surface several inches thick - that "Gunite" set almost immediately.

Reply back here, if you want more info.....