The Storm Water outfall Tunnel (SWOT), part of Heathrow’s Terminal 5 project (T5), UK, broke through at the end of August, marking the completion of a successful drive.
The 2.91m diameter, 4km long tunnel, which forms part of the drainage system for T5, was completed in just five and a half months. “The speed at which the team has achieved this tunnel drive has been extremely impressive,” production leader for tunnelling works, Ken Henderson said.
The tunnel was constructed by the Morgan/Vinci JV with a full-face Lovat TBM, using pre-cast fibre reinforced segmental wedgeblock lining. Mott MacDonald handled design engineering on the project.
Averages advance rates of 50m/day were achieved during boring, with maximum rates of 72m/day – continuous production was the aim of the project team. “We chose a narrow gauge rail system, which meant four trains running in and out of the tunnel every 15 minutes delivering the segments and taking out the spoil,” Henderson said.
A High Angle Conveyor System (HAC) was used for removing spoil from the pit-bottom – a first for London Clay. “The safety benefits are enormous and we will be reusing this conveyor system when work begins on the Piccadilly Line extension later this year,” Henderson said.
SWOT’s alignment runs beneath sections of the southern runway and taxiways, the southern fuel farm, and two intermediate shafts. It will transfer surface water from T5, stands and paved areas to the Clockhouse Lane pit, south of the airport.
For T5 tunnelling analysis, see T&TI, September 2003, p18.
