Drilling/blasting in broken ground
Drilling/blasting in broken ground
(OP)
Has anyone had experience in open-pit production drilling/blasting of previously broken ground?
We are working down to expose old, pillar-extracted coal seam workings. The sandstone overburden for about 15metres above the seam is severely fractured due to highly variable subsidence, and has significant voids. Our objective is to fill any major open voids to enable trucks & excavator to work on benches above. However we are having trouble drilling blastholes (100mm holes) due to lack of air return clogging holes with cuttings, and also broken ground jamming drill and holes collapsing. We can probably use bottom plugs/stemming & liners to contain the explosive - it is mainly a drilling rather than blasting problem.
Any advice would be appreciated.
We are working down to expose old, pillar-extracted coal seam workings. The sandstone overburden for about 15metres above the seam is severely fractured due to highly variable subsidence, and has significant voids. Our objective is to fill any major open voids to enable trucks & excavator to work on benches above. However we are having trouble drilling blastholes (100mm holes) due to lack of air return clogging holes with cuttings, and also broken ground jamming drill and holes collapsing. We can probably use bottom plugs/stemming & liners to contain the explosive - it is mainly a drilling rather than blasting problem.
Any advice would be appreciated.





RE: Drilling/blasting in broken ground
pour a fine dust into the blast hole/s and let the water flow in the drill so it turns the dust into mud. Move the drill rod up & down and this will allow the mud to act like a grout, filling the voids. If you still have voids down, say, 2-3 rod lengths, then turn off the air on the drill, and try and use the chippings in the same manner as the mud. This process is slow and may take quite a while to complete each hole.
RE: Drilling/blasting in broken ground
RE: Drilling/blasting in broken ground
RE: Drilling/blasting in broken ground
RE: Drilling/blasting in broken ground
RE: Drilling/blasting in broken ground
Doug Hambley,Ph.D., P.E., P.G.
Practical Environmental Consultants, Inc.
Schaumburg, IL
RE: Drilling/blasting in broken ground
greg Pearsall,I have been in the drilling industry for some time and have come across similar probs in the Geotec and blast hole industry.Drilling in alluvials presents simialar probs as you loose your air into the strata and the hole collapses in on your rods.One of the method that I use is a casing advance using PVC casing so you can case the suspect ground as you drill.There are many other cost effective methods you can employ, all though some of them are time consuming.
Regards
Greg
RE: Drilling/blasting in broken ground
RE: Drilling/blasting in broken ground
RE: Drilling/blasting in broken ground
If you are using T51 rods, it may be worth reviewing whether the rig can be altered to use EL68 rods, the increased diameter of the rod will reduce the annulus between the rod and the hole walls, thereby increasing the ability to clear cuttings from the hole.
Can the compressor be upgraded or modified to give better performance?
Have you tried drilling with retractable bits? I have used them when drilling in areas where the holes fall in behind the bit.
Another option available is to split the benches and drill and blast in 3 x 5 metre or 2 x 7.5 metre becnhes etc. This will help if the voids are typically at a similar depth, eg within the bottom 6 metres. Also drilling a shallower hole will immediately improve compressor performance.
An option to consider is also the use of a down hole hammer rig, I have had positive experience with this type of rig drilling in highly variable, slumping ground with numerous cavities.
Regards
Sean Burston
Project Engineer
Leighton Contractors