Drilling in bad ground is not a driller’s most favourite pastime as he runs the risk of loosing his DTH tooling. Bad ground may be made up of broken or fissured rock, clays, sands or damp heavy conditions where dust can cling to the wall of the hole. Naturally it is important to keep the borehole open during the drilling process.
High air pressure helps to keep the hole clean and sometimes a “Back Reamer” can assist to break up loose rocks within the hole or help to ream away any built-up caked dust on the wall of the hole. Mincon Hammers have variable choke systems that allow increased airflows through the Hammer to assist in purging the hole.
Foams and water may be used in construction and water well drilling to purge the hole clean. Foams are particularly helpful where the air pressures and volumes are low.
Where there is water in the borehole it is preferable to have a reasonable flow rather than just a trickle. Insufficient water can cause dust to form on the wall of the borehole and to set like cement.
Some blast hole rigs use water-vapour suppression systems to quell the dust, but the damp conditions can encourage build-up on the wall of the hole and this can be a real problem. The “Back Reamer” may be the best option to keep the hole open and production at its best.
Not all drillers pay enough attention to chip size. Good chip size (the size of your pinkie finger nail) will give top penetration rates with less dust to stick to the wall of the hole and of course the larger chips will help scour the hole clean.
Drill Bit configuration, carbide shape, rotation speeds and feeds all have an effect on chip size as well as performance and it is the driller’s job to ensure that attention is paid to all these factors to produce the best results.
In Blast hole applications (where casings are not normally used) there is the danger of loose rocks falling behind the Hammer & Bit, usually at the commencement of the hole. So it is really important to ensure that any fragmented rocks from the top of the hole do not follow the Hammer & Bit all the way down to the bottom, as it will then be difficult to extract the tooling.
At the start of the hole, lifting the Hammer & Bit clear of the mouth after a few feet, will allow any loose rocks to fall into the hole to be broken into chips on re-entry. This practice should be continued until the wall of the hole is stable. It may be necessary to collar the hole at the top, with a little clay to stabilise it before drilling on. “Do not use hands or feet to clear the tops of holes or for collaring”
In some instances the use of a large diameter “Starter Tube” can prevent rocks falling from the wall of the hole behind the Hammer, whilst drilling through the first few metres of fractured top rock. When the Hammer & Starter Tube have been drilled-in, the tooling can be lifted clear of the hole to allow any loose material to fall free from the wall, to the bottom of the hole, to be re-drilled and the risk of jamming will be reduced
The fear of becoming stuck can cause panic with some drillers and at the first sign of the loss of dust, their reaction is to hastily and immediately withdraw the tooling. Sometimes this can be the worst thing that can happen, as any heavy dust within the hole may fall behind the Hammer to become compacted against the wall of the hole and any air-flow will be cut off, making extraction very difficult.
At the first sign of reduced dusting it is advisable to lift the Hammer & Bit off the rock, just sufficient to allow the Hammer to move into the flushing mode and no more, usually 50 – 70mm (2” to 3”). This should help reduce the risk of compaction and keep the airway open.
There are no fixed parameters to work to or recommendations to make for getting out of bad holes. Most times it is advisable to ream back slowly with the air turned on at half-cock to clear any blockage.
However when this fails, other things need to be tried, and I have known tooling to be recovered from dust clogged holes, simply by leaving the rotation on, but without the air and without making any effort to pull the tooling back. The movement caused by the rotating drill string can be sufficient to cause the dust to filter back to the bottom of the hole and leave the tooling free to be extracted.
One manager once almost fired me when I got stuck in heavy ground.
I had found it totally impossible to withdraw from a badly fissured hole but I still had a good airflow and reckoned that I could safely drill down. So I pulled back as far as the blockage would allow and fitted an in-line reamer adaptor with live air bleed holes and I drilled on, much to the aggravation of the drilling boss. He threatened to fire me if I lost all my tooling.
As it happened it did not come to that, because as I drilled on, with my fingers crossed, I was able to ream the blockage from the top and at the same time keep the hole clear below with the dust exhausting through the joints in the rock. Eventually there was a welcome blast of dust and the hole was clear to pull the tooling back.
Some drillers favour putting liquids into the hole to assist extraction. This may be useful as a last resort and water with foams may help in dust or clay laden holes.
Diesel should never be used: It is a contaminant and injurious to health (particularly in vaporised form). The presence of diesel within a percussive Hammer can cause it to “fire” through compression combustion – just like a diesel engine. When this happens severe damage can be caused to components. Don’t do it.