Drill Bits for soft rock

[GarDouth]

Drilling shot holes in the soft Jurassic limestone of the North York moors has proved problematic. The issue is that once you get about 400mm in, the rock is so soft and porous that the dust turns into a kind of paste which the drill bit can't expel. The sludge and suction in the hole prevents further drilling even with extra plunging.

Does anyone have any suggestions? A particular type of drill bit that might be better or some other way to get a 10mm hole deeper in this crappy rock.

 

[Cantclimbtom]

Question: once in the in the sludgy section - would a hammer drill (lots of twist, but pathetic hammer) outperform a rotary hammer drill (good hammer, but twist is secondary)? It sounds like twist and extraction would be more useful than pounding the sludge in the hole even finer. Dunno just wondering if using a much "worse" drill would help.

 

[Ian P]

Would using a wood “Auger Bit” clear the sludge. Obviously no good for drilling the rock !

 

[Leclused]

Changing to a hollow core drill from the moment you hit the problem perhaps

https://m.aliexpress.com/item/1005002276543984.html?spm=a2g0n.productlist.0.0.29a669f2AGobAS&browser_id=90c30ad44860499b950b8aa24c30a7a8&aff_trace_key=a64ee0b143a6469da4ec77dcd0fecaa1-1627846475383-04078-_pw5lIaQ&aff_platform=msite&m_page_id=001181a8eee564226f3da3623a66ce8188d7c1c224&gclid=&pdp_npi=2%40dis%21EUR%21%211.77%21%21%21%21%21%402101e9d316563966376914765ec6db%2112000027406370484%21sea&algo_pvid=bea8225d-510d-41e1-a0b4-3ac00aa33fde

 

[Mark]

We use big 32mm bits when drilling chalk for rock bolts and get the same problem (even though the drill has an air flush), when we get the paste problem we swap the air flush for water flush and that works a treat, also the drills we use are rotary only

So I suggest trying rotary only and plenty of water in the hole, (the water won't affect whatever you are using in the shot hole)

 

[GarDouth]

Thanks for the advice all. I was thinking about using more water actually so I might just try taking a bottle with fine tube to keep flushing out the hole.

 

[tim.rose2]

We have a similar problem in some of the rock on Portland (lower cherty series); drill in the wrong place and you get paste! Our digging is generally plug & feathers / hammer & chisel so we drill a lot of 14 mm holes 10 to 15 cm deep. We've found that if you find the paste plug & feathers don't work as the soft rock just absorbs them (and then they are a b*****d to remove). Capping, similar - they don't fire (we've generally found P&F much more effective). We can make limited progress in this stuff with the drill on rotary lock using a chisel bit but as soon as you find a nodule of chert or the hard limestone that approach becomes useless. Our solution, if we find the paste, is to drill a hole somewhere else. On Portland the paste producing rock is very localised so a few 10's of cm often finds something more solid. Once the surrounding solid rock is removed the 'paste rock' is easily knocked off with hammer and chisel. We generally find this stuff whilst enlarging narrow rifts so going up or down a foot or two to avoid the worst of it is usually an option. I'm not sure this helps much but it's worth considering that if you do manage to drill a hole in the 'paste rock' it might absorb any energy and have a less than desired effect.