The steel bucket drops at only 9m/s in the deep dark shaft at Sedrun when it is carrying workmen, so it takes about 90 seconds to reach the small cavern at the bottom of a 800m vertical drop. A fine spray of water from the pump lines upwards drifts downwards, though the shaft itself is virtually dry.
When we go up again the shift leader uses a system of bell signals to alert the winchman at the top. No-one is allowed to talk to him while he is concentrating, to avoid accidents
Materials will be dropped at 12m/sec when work begins shortly enlarging the small chamber which has already been widened out from the base of the shaft in the Gotthard Massif. A drill rig and trucks have already been moved down.
A service tunnel and connector tunnels through to the adjacent line of the Gotthard base tunnel will be excavated as preparation for whichever main contractor takes on the difficult ground of the Tavetsch intermediate (see main story). He will use the chambers as his underground base for the main drives through 6.7km of cracked and broken rock.
While all that is happening, the “temporary” winding gear will be removed and a much bigger winch set installed in a special side cavern at the shaft top; this serves for the next few years to haul up and down a two-level 50-tonne capacity elevator which will bring spoil trucks to the surface for emptying. That will move at 16m/sec unless shift workers are also on board in a personnel car.
A small spoil removal railway and a bridge over a local valley already exist in the main valley, where the skiing village of Sedrun sits high over the main Gotthard route. Spoil will be re-used and some will fill part of a local valley.
Preparation work has been under way for three years here since even before the main project go-ahead was given. The first job was driving a 1km tunnel into the mountain from which the shaft now drops. A ventilator tunnel to the other side of the mountain was also made.
A specialist contractor group, Arbeitsgemeinschaft Schacht Sedrun, has spent the last two years forming the shaft inside the tunnel. Members included Shaft Sinkers, a firm from South Africa with experience in deep shaft work for the gold mines. To them 800m is routine. The other firms for the SFr85M contract were all Swiss, and included Zschokke Locher, Murer, and Marti Tunnelbau.
The consortium used a special five-level work platform hung on eight cables for the task. This allowed materials to be carried and two levels of concreting work to be carried out while drilling and mucking out proceeded below.
“Drilling a shaft seems like just making a tunnel vertical,” says Jakob Blickenstorfer, AlpTransit‘s project engineer, “but it is not.”
Most crucial, he says, is the need to avoid water. “There is nowhere for it to go, and even a small amount will accumulate at the bottom. Unless you make a sump, it is hard to pump out and anyway, tiny amounts can get into boreholes and threaten the connections of the explosive.”
The hard granite of the shaft was therefore drilled ahead with a downward fan of grout holes 42m long and grouted until tight. The shorter drill and blast sequence was then followed until near the end and a new grout fan made. It worked, he says, and no more than 1litre/s inflow can be detected in the whole shaft.
The concrete lining is in 6m jumps with a 300mm gap between each where bare rock remains to allow future drainage. “There is no point resisting pressure at these depths,” says Mr Bickenstorfer.
Drain pipes will fit in the gaps later before a watertight permanent lining is installed eventually. “That will have a very smooth surface to allow air flow,” he says.
The shaft will serve for ventilation and access to an emergency station below, in the chamber now being made for the construction base.
Related Files
Sedrun multi-function station
Shaft-sinking rig
