DE
Eiffage Infra-Nordwest-Ingenieurtiefbau constructed the excavation pit for a new hotel building with the beautiful name "Magic Cube" in a central city centre location in Osnabrück. The work was often a logistical masterpiece in a very confined space.
The excavation pit is 11.5 metres deep and will house 2 basement floors for the sales area and building services.
Initially, the excavation pit was enclosed with 140 overlapping bored piles, which were drilled to a depth of 22.5 metres. Under the protection of a temporary dewatering system, the remains of an old tunnel that used to pass under Osnabrück's Neumarkt were demolished. A massive steel belt stabilises the excavation pit surround. The existing groundwater could not be lowered due to the existing buildings. The excavation pit was therefore flooded and a pontoon was lowered onto the water, from which the excavation pit was excavated. Threaded piles were drilled from the pontoon into the subsoil to secure the concrete base from floating later.
Divers created a plumb plan in the excavation pit and a pump was used to create the final subgrade and remove the remaining sludge from the water.
The solids were separated from the water with the addition of biodegradable flocculant. While the water flowed back into the excavation pit, the fines collected in a large geotextile tube.
The 1.2 metre thick underwater concrete base was then poured under the supervision of the divers. After the concrete had hardened, the excavation pit was drained and handed over to the shell builder for further work.
