Rock bolt Installation.
A full column resin bolt unit consists simply of a two-part polyester resin-catalyst capsule and a steel bar with a plate and nut. The installation of the unit is carried out as follows.
Drill hole to correct diameter and length for bolt being used. Over-drilling will waste resin and reduce the performance of the bolt.
The drill should be marked at the correct depth using tape or some other method(Figure 22). The Ideal hole length should be 50-60 mm shorter than the bold.
With rotary drilling, energy is transmitted to a cutter via a steel drill, which rotates
and presses the drill bit to the cutting face. The cutting edge exerts a pressure against the rock and chippings are broken loose.
Drill bit performance is dependent on correct roof bolting machine settings and the correct operation of the bolter. Improper settings contribute more to poor bit performance and excessive wear than any other factor.
The rotation speed and thrust settings must be set for the strata conditions on site. In a soft roof reverse Is true: reduce thrust and increase rotation speed.
Drilling alignment is the single most crucial aspect of drilling. After collaring the hole it is vital that alignment with the drill pod is checked and adjusted as required.
- Maintains the drilling forces/energy travelling in a straight line;
- Improves drilling speeds/times;
- Enables strata support to work as designed
- (and the resulting misalignment of holes) compromises the design strength of strata support;
- Can lead to difficulty in installing bolts;
- Accelerates wear on drilling components;
- Directly leads to breakages of drill steels.
It is essential that the correct hole diameter is drilled. An optimal diametric difference between bolt and hole is between 4 and 10mm. Larder diameters can be used but this can result in:
- Longer resin cure times which must be allowed for before the bold can be pretensioned; in this car often the resin described is too slow;
- Longer resin cure times due to intermixing of the slow into fast components (e.g. in the TooSpeedie® Capsule);
- Less efficient mixing of the two components in the capsule, poor shredding of capsule film, and possible ‘finger gloving’;
- A reduction in load transfer (pull out strength);
- A reduction in encapsulation length (e.g a 29mm diameter hole results in a forty per cent reduction in encapsulation compared to a 27mm bit).
If diameters outside the recommended range are considered, then pull tests must be performed to verify that required load strength are achieved.
As many bits, in particular strata, will in fact drill a larger hole diameter then the actual bit diameter. It is recommended that the internal hole diameter is measured to within 0.1 mm from top to bottom with a borehole micrometer.
Possible reasons for larger diameter holes are:
- Incorrect or out-of-specification drill bit;
- Change in strata;
- Starter bit;
- Incorrect feed/spin settings;
- Too much thrust;
- Too much play in the drill string
The hole should then be drilled to the correct length.
The bolt should bottom in the hole leaving sufficient thread protruding to accommodate the nut, plate, domed ball washer etc. The ideal hole length should be 50-60mm shorter than the bolt. Often a mark or tape can be attached to the drill steel to act as a guide for the hole depth. It is a good idea to measure the length of the hole from time to time with a tape measure. Correct flushing procedures must be used.
If the holy is too deep (overdrilled) then:
- Part of the resin capsule will not be penetrated or mixed by the bolt causing a reduction in strength;
- There may be insufficient thread left on the bolt to tighten the nut, so tension is recued and load carrying ability;
- There may be a reduction in encapsulation length;
Possible Reasons for overdrilled holes are:
- Incorrect length drill steel;
- Change in bolt length but drill steel not changed or left in panel;
- Rig pushed in too far;
- Marker moving or being worn away.
If the hole is too short (underdrilled) then:
- The designed bolt horizon may not be reached with potential partings left unsupported;
- Little or no thread remains to tighten the bolt;
- There is the possibility of resin splashing operators;
- The bolt may protrude too far into the excavation causing injury to personnel.
Possible causes of underdrilled holes are:
- Incorrect length drill steel
- A change in bolt length but not the drill steel, or the old bolts/steels may not have been removed from the panel;
- A change in roof height so the machines is unable to drill as deep;
- Insufficient depth drilled.
Install the prescribed number of capsules of the correct diameter and length to fully grout the bolt in borehole (Figure 25). Table 1 gives an indication of how many capsules are required for a given bond length. Check that the use by date is on the box label and has no expired. If a tensioned system is being used, the order of insertion is important: the faster resin capsule/portion must be inserted first. (Never insert upside down.) At this point any accessory required can be selected (e.g. plastic hate, installer etc.). Insert the capsule and push it gently all the way to the top of the hole using the bolt or insertion device if available.
Connect the bolt to the spinning device such as a dolly/spanner. Forcibly spin the bolt into the hole at the same time rotating for the manufacturer’s recommended time at high rpm throughout the entire of the capsule (Figure 26). When the back of the hole is reached a further 2-4 seconds’ spinning will suffice to ensure complete mixing. This results in shredding of the capsules and mixing the resin and catalyst. Note the spin time will vary with temperature and must always be less than the resin gel time for operational temperature. It is essential the bolt I pushed and spun to the back of the hole before mixing is completed.
The manufacturer’s recommended spin time must not be exceeded as this will result in damage to the setting resin, leading to an ineffective and dangerous bolt.
Hold bolt in place without movement until resin has gelled sufficiently to hold the bolt in the hole. Retract the spinning device.
Attach washer and nut, if not attached during installation. (It is possible to install a complete unit in on operation.) In most modern bolting systems the plate and washer are added prior to installation.
Pretesioned systems require the bolt to be tightened to a predetermined level. This is usually done by means of torque-limiting device which allows the nut to tighten up the bold at a predetermined load. These devices include shear pins, chemdrive, nuts, nuts with crimped inserts and modified thread sections.
Once the resin has reached a sufficient strength, usually determined by the manufacturer’s spin and hold time, the nut is spun causing it to break the torque-limiting device and move up the thread, imparting the tension into the bold via threaded section.
The hold time is the approximate time allowed after completion of the spin time before bolt tensioning is attempted. The times listed on the box are an indication of only and may vary with temperature, mining conditions, equipment, hold:bolt annulus, age and storage conditions of resin capsules. Each mine site and area should be evaluated to determine optimum installation parameters.
Source: The Orica/Minova guide to resin-grouted rockbolts manual.