A mate had to have test drilling done for his proposed new bungalow, 5 holes drilled down to the No2 Rhondda seam with one corner hole dropping into a 5 foot void.
He had to move the bungalow footprint over about 6 feet.
No drilling mud as the rig was air blast flushing but still very messy as a great plume of rock/coal dust was blown everywhere.
The usual system of drilling hardish rock doesn't involve mud. If you hit groundwater in mudstone/soft slate/shale it makes a mega mess. The best one I remember involved hitting flooded workings, the resulting geyser filled the blokes garden up with tomato soup. Knee deep.
I gather there has been similar amusement with attempting to do ground source holes in clay. I think someone's whole house got covered in 'white'.
It would be interesting to know how deep they drill under what circumstances for coal. I imagine it's a totally different ball game. I wonder if the CA are exclusively concerned with coal workings because of the extreme variance and unpredictibility of cover rocks.