An ‘unconventional’ drilling method is expected to bore the escape hole for the 33 miners trapped in a San Jose copper mine in northern Chile, according to an industry expert.
Dr Andrew Wetherelt, a senior lecturer in mining engineering at the University of Exeter, said Chile’s state-owned Codelco mining company is planning on using a massive Strata 950 ‘raise boring’ drill to carve a hole 66cm wide to reach the miners trapped under solid rock 700m below the surface.
This hole will be drilled after the rescue team has completed drilling a smaller pilot hole.
In traditional raise boring operations, Wetherelt said mining companies use a drilling technique known as ‘up-reaming’ where machine cutters are fitted to the drill sticking out the bottom of the pilot hole. The massive cutters are raised up as they hack their way through thick rock.
Wetherelt said this is not possible to do in the case of the mine in Copiapo, Chile because the collapsed mine offers no access to the bottom.
Instead a technique known as ‘down boring’ will need to be deployed, he said, In this case, the operators will drill an oversized pilot hole and install a cutting head at the surface. The operators will guide the drill downward and rock clippings will flush down the pilot hole to the bottom where the trapped miners will likely be expected to sweep them away.
Wetherelt said this technique is less common than up-reaming, because there is a risk of plugging up the hole with rock clippings. The miners will play an essential role in making sure this doesn’t happen.
There is also the challenge of keeping the drill and its cutting head stable as it pushes its way through layers of abrasive rock. Wetherelt said rotating stabilising wheels clamped onto the drill will help ensure it does not deviate from its path. Otherwise the awesome piece of drilling machinery may just, as Wetherelt puts it, ‘bend like a hairpin’.
The 5 August collapse of the mine’s main access shaft happened about 400m below the surface. This was above where the miners were working at the time. The group took refuge in a gallery approximately 688m below in an area that stretches about 2km long, which should leave enough space for the likely restless men to walk around.
The mine, which runs under a mineral rich mountain in northern Chile’s Atacama Desert, incorporates a zigzagging main access ramp–large enough to drive a car down– and vertical shaft tunnels that are meant to be used for escape when the other means of egress has failed.
Without a 3D map of the mine, Wetherelt said it is tough to make certain judgments on how much safety was taken into consideration in its design. However, it is clear the catastrophic underground failure three weeks ago blocked off both means of escape for the miners.
Wetherelt said, ‘Clearly in this instance there is a problem that the catastrophic failure has also damaged the shaft,’ he said. ‘The failure has gone across the ramp so in doing that the ramp is out of condition but the fact is the shaft is also in close proximity to this failure.
‘We (the UK) have legislation that there should be two means of egress and they shouldn’t be next door to one another in three dimensional geometric space.’
Wetherelt said he is unsure what legislation dictates about mine design in Chile, but if there are others like the one in Copiapo up and running ‘that’s clearly not good’.
Meanwhile the trapped miners in Chile have been receiving food, water and oxygen supplies. Today it was announced the miners have been told for the first time the rescue operation may take until Christmas.
Wetherelt said there is a chance it might not take that long. ‘They’re not being pessimistic but I don’t think they want to get anybody’s hopes up to say they can do it in a week.’