The Great Pyramid of Cheops could be rebuilt five times over with the quantity of spoil produced from the Gotthard tunnels, the AlpTransit client likes to point out, to indicate the magnitude of its disposal task. Total volume of spoil was 13.3 million cubic metres, which is about 25Mt.
It was the client’s task to remove this, rather than leaving it to the main tunnel contractors. Like the tunnel water processing, it was considered too important and sensitive an issue from the beginning to be otherwise.
“You know of ‘A’ and ‘C’ tasks?” says Adrian Wildbolz, who supervised the northern tunnel drives for AlpTransit. “The ‘A’ jobs get high priority. If you give disposal to the main contractor it is a secondary issue for him. If you award it as a special separate contract, it is the only focus for that contractor, his ‘A’ job.”
Disposal had to be an “A” task because it was a sensitive element in the original political go-ahead for the Gotthard. Minimum environmental and community impact was decreed both because of general growing environmental consciousness and because of the special circumstances of the mountains, with already limited space on narrow valley floors, even for local roads.
A careful combination of rock recycling and landscaping work was used to full effect to minimise the need for disposal. Tunnellers were also instructed to recycle rock to the maximum for concrete needs. Old extraction pits from other projects were located that could be refilled and landscaped.
All of this had to be done without the use of trucks, says Charly Simmen, a project manager with AlpTransit with special responsibility for the disposal. “Only rail, conveyors or boats were possible.”
To achieve this required tight logistical control with most spoil handled only at the main portals, particularly Bodio, which took the greatest amount, some 11Mt. The portal at Erstfeld and tunnel exit at Amsteg handled another 8Mt between them. At the Sedrun shafts another 4Mt came out and 2Mt at Faido.
A first portion was sent back into the tunnels as aggregate in the concrete for linings and shotcrete, some 22 per cent of the excavated volume, he says. This was doubly beneficial, almost eliminating the need for quarried and imported aggregates.
But to use the excavated material for concrete demanded significant processing of the rock and even some new technology.
To begin with the chips had to be broken down from the (up to) 400mm-long pieces that the TBM cutter wheels flake away from the bore face. An initial crushing had to be done by the contractors inside the tunnel; contracts specified no more than 150mm size pieces to be sent to the portal.
There they were handed over to a specialist contractor running a sequence of crushers to produce the various sizes and grades of aggregate needed for the mixes, from sand to large stones. A laboratory constantly analysed the rock quality.
There was still a problem with the rock chips. Although mostly from hard sound rock, they were too generally too angular for good concrete. The acute angles can suffer stresses which reduce strength.
“We pursued a four year research programme with Zurich Technical University to find a way round this,” recounts Simmen. The end point was a new machine, christened the “Hurricane,” which used a particular arrangement of conical chambers to tumble the crushed stone fragments sufficiently to round them off.
Aggregate plants with the ‘hurricanes’ were sited at Amsteg in the north, at the worksite outside the Sedrun shaft tunnel and at the Bodio portal. Plant designs were by specialist consultant Ernst Basler in Zurich.
“To save space the spoil from the later Erstfeld contracts was also crushed at Amsteg, sent by train before returning for use in concrete,” says Simmen. “We built temporary rail tracks along the valley for that, next to the existing rail line.”
Each of the tunnel access points had its own concrete batching plants. But that still left a lot of material. Various sites were identified for fill and landscaping, the largest at Biasca near the south portal but in the next valley along. An extraction pit was made when the Gotthard motorway was built in the 1980s and was an ideal site, also restoring the valley profile.
To get there without long truck hauls, a short tunnel was made during early preparation, just big enough to house a conveyor and carry the material some 3km through the hillside to the old workings.
Smaller disposal areas were found in the valley at Faido for the early stage of the multi-function station excavation, though later material also went to Bodio. At Sedrun “we were able to use a small side valley near the village, creating a building platform for housing development later,” says Simmen. One area had to stay untouched due to important flora and fauna.
In the north about 2.5Mt of the material was dumped by barge into the top end of the Uri lake, which connects into Lake Lucerne. The apparently vandalous act was actually a carefully-tailored restoration of old gravel workings that had removed natural river aggregates from the Reuss for nearly a century, leaving 50m deep channels and underwater voids.
“That was causing slips and erosion of the upper lake, which led to flooding and dangers to the road,” says Simmen. Now embankments and small lake islands have restored it as a pleasant nature reserve.
These disposal sites took 24 per cent of the material. Much of the remaining spoil, more than half, was sent by train separate contractor ‘Oeko-Trans’ to produce aggregates for use by third parties. A lot has gone to the Zurich area as fill material for embankments on rail and road development projects. Some material has also gone into the brick making industry.
There is a stubborn 1 per cent left which is contaminated from tunnel oil spills and the like, which is sent to landfill sites.
Finally, on each site, separate contracts were also let for treatment of the drainage water from the tunnels. Tight limits were set on disposal into the rivers at only 1.5 degrees above their ambient level. Since the rivers are never much beyond 10 degrees, this was a significant task.
Much of the spoil from Bodio passed through a 3km tunnel by conveyor to a disposal site in the Buzza di Biasca, re-filling an area depleted for aggregate during earlier motorway Construction Uri Lake restoration using spoil disposal to fill in old gravel extraction lake bed depressions At Amsteg material was crushed for the Erstfeld site as well, returning there by railway to provide concrete aggregate Spoil conveyor from the Erstfeld portal to storage area. All photos courtesy of AlpTransit Gotthard
