PERFORATED GARDEN hoses were used to solve a load transfer problem on BAA's new £50M multistorey car park at Heathrow Airport last month (GE August 2005).
The 1500 space facility for Terminal 3 is directly above the Heathrow Express station and tunnels.
Designers had to thread a number of 1,800mm diameter insitu concrete piles into the spaces around the tunnels. They then had to connect them with massive ground beams up to 39m long and up to 3m deep by 20m wide, cast on to made ground just above the tunnel crowns.
'The problem is loads transferred from the beams into the tunnels, which are only 300mm below, ' said BAA project leader David Rebello.
'While the dead load of the beams during construction can be accepted, the total loading when the project is complete would be far too high.' BAA and principal contractor Laing O'Rourke Construction came up with the perforated hose idea.
'Basically we cast the beams on two layers of blinding concrete separated by a layer of sand, ' Rebello explained.
'In the sand we laid perforated hoses. Then when the beam concrete has gained enough strength, the hoses are turned on. The idea is they will ' ush out the sand and leave the ground beams spanning between the pile caps without transferring any load to the fill around the tunnels.' The technique was tried out for the first time this week and 'worked well once we got the pump capacity right, ' Rebello said.