It is sometimes necessary to drill longer hols underground either for access or as part of a development stope or shaft.
There can also be a need to sample these holes so RC hammer drilling is sometimes used. The problem with drilling underground with a DTH hammer is that you need a lot of high pressure compressed air. When you compress air you generate a lot of heat and this causes a lot of problems underground.
Of course you also have to drive the compressor and if an electric powered compressor is used then it will require an electric motor of 300–400kW. Starting and operating such a large electric motor underground is difficult in most mines.
The other alternative is a diesel powered compressor and this produces more heat as well as exhaust fumes and the fuel is a fire hazard so diesel powered compressors are not very useful underground unless there is excellent ventilation and a good fuel supply which limits them to decline operations rather than shaft serviced mines. So the common way to solve this is to use the mines own low pressure compressed air supply and to then use a smaller booster to raise the pressure of the air to say 30–25 bar. In theory this works well there is less electric power involved, less heat produced and the equipment is more compact.
However there is a very common problem that occurs. A mine usually has compressors on the surface which сan be a great distance from the drill rig and booster. The mine compressors operate at 7–8 bar max pressure and the pressure down the mine will be less due to friction losses in the pipework and also due to leaks. We suggest that a test be done at the site where the booster is to be used to determine exactly how much compressed air is ACTUALLY available for use by the booster. Without this sort of test it is very common for boosters to be damaged and for the drillers to get poor performance from their hammers. The other approach which works well if the mine is not too deep and there is a reasonable amount of drilling required in a local area underground is to drill a hole from the surface to break into the underground workings and then to put high pressure compressors on the surface and to deliver the aid directly down to the drilling rig. This has been done successfully using either a hammer hole or a diamond drill hole. And either old steel pipe can be placed and cemented into the hole or heavy PVC can be used just be sure to fill the PVC with water and to pressurise it before cementing or you may squash the pipe. This approach remove, heat, fire hazards, ventilation and fuel issues from the underground workings and makes the mine personnel much happier to have a rig drilling in their workplace. In either case it is very important to use water to completely remove any dust from the material coming out of the drill hole to avoid serious health issues for the drill crews. If you have any questions please feel free to ask and we can supply a meter and instructions for its use if required. I cannot stress enough how important it is to test the mine air supply at the drill site as failure to do this will almost certainly result in failure to be able to drill and the driller will lose his contract and the mine owner will be upset at having wasted money and resources. With proper testing any compressed air supply problems can be rectified and a successful drilling program will be the result.