Drilling/blasting in broken ground 6

[macajm]

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.

 

[swaggy]

Macajm,
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.

 

[phillyfrank]

I have done blasting in subsided and broken ground in the anthracite coal fields of Pennsylvania, USA, and drilling was not much of a problem. We drilled to the planned depth and there was almost always a large void at the bottom. We used random lumber and scrap paper and cloth to build up out of the void, then used sleeves to control the ANFO in the hole. We found shorter satellite holes very helpful in breaking a sandstone cap. We had to check the wind and evacuate downwind because we always generated a large NOx cloud due to the lack of real confinement. Hope this helps.

 

[macajm]

Thanks for your comments "phillyfrank". We have been able to get some productive fragmentation from blasts in this area once we get holes down. However the big problem remains that the overburden drill loses too much air into the broken/caved ground above the old workings (not just into the open "cavernous" voids). This loss of return air leads in turn to jamming rods in holes etc. So it is really a drilling problem first and foremost.

 

What type of dill rig are you using ?

 

[macajm]

re "Mineur" - we are using an Ingersoll Rand LM600C rockdrill and drilling 100mm & 114mm holes, aiming to drill about 15 metres.

 

[Hambley]

In other words, the problem is the holes collapsing on the drill rods so they get stuck! An answer to this problem is to use what we called auger steel at Falconbridge when I worked there in the early 70s -- weld spiral beads on normal drill rods so that you have a helical outer surface like a screw. When these stick, you can get them out by reversing the rotation (where that's possible) or rotating slowly while pulling the rods.

Doug Hambley,Ph.D., P.E., P.G.
Practical Environmental Consultants, Inc.
Schaumburg, IL

 

Hi my name is
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

 

[PBroad]

As your problems are mainly drilling have you tried

http://www.drillersclub.com.

This is an excellent problem solver site run by Atlas Copco, but has all types of users?

 

[creyes1]

I would suggest you drill the boreholes with "overburden drilling" which is a method that you insert an outer casing tube with a ring bit of cemented carbide at the lower end. This casing tube encloses an inner drill string of standard drill steel with a cross o bottom bit. When you reach the required depth break the casing, remove the string and insert a pvc or card board casing to load the explosive in. Then remove the steel casing. With OD drilling you won't have collapsing holes.

 

[SeanBurston]

Further to Doug Hambley's recommendation, I would strongly urge anyone to perform a risk assessment prior to welding anything onto a drill rod. There was a fatality at a Western Australian mine during the 1990's when a boilermaker was either welding or cutting some old T51 rods. The rods had become blocked while drilling and had been discarded on that basis. The blockage was caused by an explosive product, most likely pciked up while drilling the butt of a drill hole from the bench above. When the boiklermaker applied heat the explosive product detonated, resulting in a falatlity.

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