Publication Abstract

Proceedings - Fourth International Conference - Piling and Deep Foundations, 1991, Stresa, Italy, (DFI)

17 Establishing an Island and a Dry Excavation in the Sea Bottom for a Tunnel Crossing
M. Porsvig; Danish Geotechnical Institute, Denmark L. Lovgren; MT Group, Denmark

For the Great Belt Link - Eastern Railway Tunnel - a large excavation has been made in the middle of the 17 kn Great Belt of Denmark. At 6m water depth, close to the existing island Sproque, 2 km of sand bunds surround the 70,000 m^2 excavation for 400m Cut and Cover of the twin tunnel. The dry excavation, deepest cut reaches level - 27m, has been performed in glacial till which is overlaying highly permeable layers of glacial melt water sand and fissured paleocene marl. The pore pressures in the permeable soils were at any time to be kept at a level 2m below any point of the excavation. This has been done by retaining the sea water with a cut-off steel sheet pile wall in the sand bunds and by a temporary dewatering system of 81 Nos. 12 inches pumping wells for lowering the groundwater. The dewatering system has been established from top of sand bunds and from sea bottom into the permeable marl. Extensive monitoring of piezometric heads, as well as slope stability and soil strength, has been performed during and after excavation. A special risk analysis has been carried out in order to identify high risk events due to a fast rising water table.

 article #457; publication #37 (IC-1991-IT)