WORK HAS BEGUN on the second phase of an enormous ground improvement project for a new aircraft factory in Hamburg, northern Germany.
French contractor Menard Soiltraitement, part of the Freyssinet Group, is using vertical drains and its proprietary Vacuum atmospheric consolidation method to induce settlement of just over 1. 3M. m 2of highly compressible reclaimed river clays and muds for the extension to the EADS factory. This is being improved to adapt it for production and transport of equipment for final assembly of the new Airbus A380 airliner.
The first phase of ground treatment started in September 2001 and was finished the following month (GE December 2001). This involved treatment of a 120,000m 2platform for the first factory buildings, which will be constructed next year. The second phase began in February and is due to be completed in August.
The work is being carried out for clients EADS and the City of Hamburg and the contract manager is REGE A380.
The treatment area, on the banks of the River Elbe, is surrounded by a perimeter sheet pile curtain on which an unloading platform will be built. This 100m wide, 6m high dyke will be founded on sand columns.
For the first phase, Moebius Group, working with Menard as main contractor, placed a 2. 5m thick layer of hydraulic fill to act as a working platform. Menard then drove vertical drains down to 15m, to reach bedrock. A total of 6M. m of drains were installed, in 500,000 locations on a 0. 5m by 0. 5m grid. This very close grid enabled fast consolidation and the absolute residual settlement criterion of less than 300mm to be met.
The vacuum method was used to prevent soil failure close to the 6m high dyke while fill was placed. The system, for preloading and consolidating soft and very soft saturated impervious soil, involves installing vertical and horizontal vacuum pipes under an airtight impervious membrane and sucking out the air below it. This induces air pressure in the soil and speeds up consolidation. On the first phase, 1m of consolidation was achieved just three weeks after the drains had been installed.
The second phase covers a much larger area: 1. 2M. m 2. Treatment involves installation of 13M drains on a larger grid than the first phase, since the settlement time is longer - the final buildings will go up in 2008.
Menard is using 18 specially modified machines to install the sand drains. The rigs had to satisfy two criteria: a low specific bearing pressure (the ratio between the machine's weight and power) and a maximum allowable pressure of 0. 7bar on the soil. This was achieved by widening the tracks and by modifying hydraulic flow rates.
Operations are computer controlled to ensure the masts remain vertical and data is recorded for quality assurance.
Up to 150,000m of vertical drains are installed every day at between 12,000 and 15,000 locations.
The Vacuum process will also be used in this phase in areas (totalling 200,000m 2) where buildings or car parks with very high loads will be built. Menard Soiltraitement says the Vacuum technique is the only way of guaranteeing an absolute residual settlement of less than 100mm as specified by the client.