Gatwick Airport (GAL) operates the busiest single runway in the world and is the second largest airport in the UK.
However the existing aircraft stands along Pier 5 at the North Terminal cannot accept larger aircraft such as the Airbus 320 adjacent to one another. Pier 5 had to be re-configured in order that the larger planes could be accommodated, including the huge Airbus A380 (the world’s largest commercial passenger aircraft).
It was essential for GAL to provide the right number and size of aircraft stands to support their airline partners’ operations. As a result, the £75 million Pier 5 Reconfiguration Project was executed to increase the capacity at the airport’s existing Pier 5. Part of the reconfiguring necessitated the trenchless construction of a new foul drainage network beneath the aircraft stands. However, the pipework diameter was small (300mm) and the ground conditions included weak rock and non-displaceable weald clay
Terra Solutions assessed the difficult ground conditions and Guided Auger Boring was ruled out because the ground was non-displaceable and the SPT n-values were given as 50 – 100. As such a relatively new technique (a first for Europe) was decided upon. This was the Eliminator boring technique, a hybrid between tunnel boring and guided auger boring for small diameters. The new methodology negates the need for a pilot bore and can deal with weak rock up to 34.5 MPa in strength. It is known as Front steer augerboring.
In total 270 metres of 300mm were installed using the Eliminator front-steer augerboring technique over 5 No. tunnel spans. The machine was launched from 2.4m Ø shafts, over 6 metres in depth. The ground conditions were more adverse than expected, and the tunnel head had to deal with n-values greater than 180 as well as unexpected ground water which, when mixed with the hard weald clay, created high rotational torque. Furthermore, an unexpected strike with a disused fuel line, mid-way through a drive, led to Carbon Monoxide formation in the launch shaft. This necessitated City & Guilds Level 3 confined space training, and the operatives had to wear full Breathing Apparatus with direct airline in order to complete the drive. The Eliminator dealt with the fuel pipe in the manner in which it was named.
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