Rock bolts to support & stabilize the unstable rock strata in mining & tunnel excavations

Rock bolts have been used since many years, to support and stabilize the unstable rock strata in mining and tunnel excavations. They transfer load from the unstable rock exterior to the more stable rock interior.

Rockbolt Installation
A typical rock bolts consists of a plain or threaded steel rod with mechanical or chemical anchor at one end and a face plate & nut on the other. The threads are usually rolled and not cut, to avoid damages during handling and installation. The rock bolt is tensioned after affixing, by tightening the nut on the face plate. The anchor may be further strengthened using a chemical or cement grout.

Many accidents have occurred, some fatal, due to falling rocks from mountain slopes, tunnels and mines. Some rock strata are inherently weak and tend to fail under slightest disturbance or shock. More often, the rock strata get loosened due to excavations and blasting, which cause fractures in rock, leading to failure.

S Krishnakumar
When a tunnel excavation is carried out, a de-pressured zone is created right above the tunnel opening due to redistribution of stress in the rock strata. Under the influence of gravitational force, the rock strata in the de-pressured zone tend to sag and loosen causing bed separation. The loosened rocks then fall off causing failure.

The rock bolts work by knitting the weak or unstable rock strata sufficiently together before they can move enough to loosen and cause failure. The bolting system is somewhat similar to clamping a number of thin weak layers to a one strong layer forming a composite beam. The binding effect is achieved through frictional forces created by the physical interlocking along the anchor and rock interface.

The type of bolting system depends on the type of rock strata, whether it is hard rock, medium or soft rock and the surrounding environment. Typically, a mechanically anchored rock bolt would tend to loosen in soft rock and in the event of mechanical disturbances such as blasting. The rock bolts can be broadly classified in to following categories, Slot & wedge type; Standard anchor; Expansion shell type; Resin grout; Cement grout; Combination type; Swellex bolts the other special types are Roof truss & Cable sling.

Rock bolts are presently used extensively throughout the world in mining, slope stabilization, underground tunnels & hydroelectric power projects.

Onward Chemicals is India's leading distributor of specialty raw materials for the road and infrastructure industry, headquartered in Mumbai with branch offices in Chennai, New Delhi and Houston, USA. We have recently started manufacturing thermoplastic road marking paints, mainly for exports. We offer Fibrex™ Glued Steel Fibers, Polypropylene Structural Fibers andFX Rockbolts for the infrastructure sector.

NBMCW September 2012

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