To make the initial cutting of the concrete plug, a manhole was created by drilling 24 overlapping boreholes through the 2.5 m thick concrete plug.

Some small diameter boreholes were drilled to allow a wire-saw to be used. This allowed large blocks of the concrete plug (2.5 x 0.45 x 0.88m) to be removed. These blocks could then be removed to allow entrance to GMT cavern.

The initial cutting of concrete plug

Cutting of concrete plug

The first excavation of backfill

Removal of the plug sections after sawing

The cores of the concrete plug

Removal of the sawn concrete plug