Posted on Apr 7, 2009 in General

Traditional top-down (blind sink) shaft sinking methods involve making a hole in the ground which is progressively deepened and lined until the required shaft depth is reached. This technique has been employed on six of the seven Emergency Access Shafts in the underground section of the Gautrain Project. It requires quite a large working area on surface to establish the required equipment and support facilities. In addition, the spoil from the excavation needs to be trucked away for disposal which does create some inconvenience for other road users.

If however, as in the case of E6, underground access is already available to the position of the bottom of the proposed shaft, a method known as raise boring may be employed. The method is commonly used in the mining industry both in South Africa and overseas.

At the E6 location, the raise borer rig has been set up on a concrete pad on the surface at the corner of Melville and Tweedale roads in Illovo. A small-diameter “pilot” hole has been drilled to intersect with the Gautrain tunnel which had already advanced beyond this point underground. Once the pilot hole has broken through into the tunnel below, the drill bit is removed and a reamer head is attached to the drill string which is then rotated under pressure against the rockface thereby slowly enlarging the pilot hole to the required shaft diameter (in this case 6.1m) as the reamer is drawn toward the surface.

The drill cuttings from the reamer head fall to the floor of the tunnel below and are removed via the tunnel. The finished raise has relatively smooth walls which will be lined with a sprayed concrete (gunite) lining.

The process is illustrated in the following graphic produced by Atlas Copco. The raise-boring rig used at shaft E6 is a Robbins 103R.

Raise-boring offers a host of time- and cost saving benefits over conventional development methods: faster advance rates, less disturbance of the rock structure, reduced labor costs and most of all much greater safety for the operating personnel.

Figure 1 : Raise bore machine on the surafce above shaft E6. Figure 2 : Reaming Head about to start upward excavation of shaft E6.
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