Objective
Conventional roof bolting - either wide or close-pattern - is ineffective when massive rockfalls occur leaving cavities greater than the height of the roadway.
Using this technique can convey a false sense of security, as massive rockfalls have proved recently, and there is therefore a temptation to use frames or arches at very close intervals which is exorbitantly expensive, in particular for the Lorraine iron ore mines, and has other disadvantages such as reducing the cross-sectional area, cutting down air velocity or hampering vehicle movement etc. The mines will thus have to fall back on roof bolting but henceforth will use bolts which are considerably longer than the height of the roadways. However, the equipment for drilling very long holes and setting long bolts or cables is not geared for the pace set by rapid rates of advance or the current dictates of productivity and profitability of highly-mechanised workings.
Not only does the technology for drilling the holes and setting long bolts (or cables) have to be developed but the load-bearing capacities, deformability and type of bonding must also be determined. Long bolts are intended to support very large masses of rock susceptible to falls which have the unfortunate characteristic of moving or swelling, causing the bolts passing through them to extend considerably in length.
The research programme is broken down into four sections:
Development of drilling equipment for very long holes (at least 6 metres) from roadways 2.5 or 3.5 metres high:
Hole diameter will be kept on a minimum to permit resin bonding which has the advantage of not requiring injection pumps and providing the immediate support required for safety.
Study of resin bonding on a test rig:
The aim of the study is to define:
- the most satiable resin
- the operating method to be deployed for each type of bolt (and particularly for different lengths) and for each setting unit characterised by:
-'rotation speed/torque resistance' profile,
- penetration speed.
Systematic rig tests will have to be carried out to explore methodically the various parameters and their effect on anchoring quality.
The factors given above are the most influential but there are certainly others (drilling diameter, type of bolting rod, presence of water, vibration of the rod etc.). The researchers will try to identify these and to determine what effect they have.
Development of a long bolting technique without using expensive sleeved elements.
Several techniques will be tested:
cables treated to allow a mixture of two resins
components, bolts with a flexible element (or cable), rigid bolts forced into curved guides etc.
Determining the bolting parameters:
This will cover:
- bolted height, i.e. the length of the bolts or cables,
- load-bearing capacity which is expressed in bolt or cable diameter and setting pattern density,
- type of anchoring (rigid or flexible),
deformability of the bolts expressed as yield capacity, which can be increased using external deformable components such as flexible plates, wooden blocks etc.
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Programme(s)
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Coordinator
60550 Verneuil-en-Halatte
France
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