Zig-zag path for drillers

For more than 24 hours rescuers in teams of three, working rotating shifts of six hours, chipped a 1m diameter tunnel beneath a cavity next to the trapped miners.

The problem is that Brant Webb and Todd Russell are confined in a steel cage, protected above by a huge rock slab holding tonnes more rock at bay. Beside them is a small cavity about 1m square, opening on to the gate of the cage.

Immediately below them is an uncertain depth of compacted rubble used to floor the tunnel, and beneath that again rock too hard for jackhammers to penetrate.

So for the past day, the rescuers have been drilling 52 bores, packing them with a low-power explosive called PCF and blasting a honeycomb of cone-shaped holes, allowing them to break the rock apart beneath the cavity.

Work frequently paused to allow safety inspections before new blasting began.

Yesterday afternoon miners reached a point they believed was directly below the cavity, and surveyors were deciding the best methods of attacking the shaft upwards.

This will require driving exploratory rods through the rock and rubble to determine how far rescuers will have to tunnel - expected to be between 1m and 2m, but possibly more - and to ensure they will emerge in the cavity beside the cage.

If the rods cannot be seen by Webb and Russell, they will have to dig further horizontally and try again.

When they do begin the shaft they will use drills, rock-splitters and other equipment to excavate in a series of sloping steps, each of about half the size of the tunnel. At each step the walls will be squared, shored in hardwood and a ceiling made of plywood.

This will allow them to attack the remaining half, working upwards in a zig-zag pattern until they reach the rubble floor.

"Brant and Todd understand that it will still take some time to get them out," mine manager Matthew Gill said. "They wanted the work to be safe."