LARGE EXCAVATIONS in the Netherlands are traditionally dug by installing temporary sheet piles, excavating between them and then building permanent concrete walls.
But to construct the 150m long, 15.6m wide and 15m deep TBM launch chamber for the Groene Hart tunnel on the country's new high speed rail line, contractor Bouygues-Koop chose a different approach.
The joint venture of French firm Bouygues Travaux Publics and Dutch firm Koop decided to use diaphragm walls installed under bentonite.
At 7km long the Groene Hart tunnel, north of the Hague, is the longest on the Netherlands' 100km high speed rail link. It passes up to 40m below the water table beneath environmentally sensitive reclaimed polders and drainage canals.
An enormous 14.87m diameter TBM will bore the single tube (European Foundations Spring 02). The launch chamber was excavated through 12m of relatively soft peat and clay over 25m to 30m of sand with a 2m clay layer below.
The base of the excavation was piled and topped with a slab of underwater concrete. The slab, which supports the track base, is at least 5m below the water table and must resist uplift from water pressure.
The next stage was to build a short length of roof slab over the excavation to support a portal building.
This 7.5m long, 850mm thick slab spans the entire width of the cut, 15m above the base slab.
SGB Formwork supplied shoring and formwork for construction of the roof slab as well as other scaffolding and support systems on the project.
Using its lightweight aluminium GASS frame system meant fewer components were needed than on conventional steel systems, halving erection time.
Once the roof was cast, formwork and shoring was dismantled to allow assembly of the TBM. After the machine had cleared the launch chamber, work began on casting a slab for the rail track, secondary walls inside the diaphragm walls and a separating wall between the tracks.
SGB then returned to site to build formwork for the 800mm thick cut and cover roof, 8.5m above track level, tying into the roof of the bored tunnel.
Two 22m long, 6.6m wide and 8.5m high mobile GASS structures were used either side of the central wall, working along the roof which was built in six sections. This saved time because static formwork would have had to be dismantled five times.