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Ground problems

UNDERGROUND CONSTRUCTION

The shaft was constructed through 26m of London Clay, 18m of Lambeth Group gravels, clay, sand and pockets of water, and a few metres of Thanet Sand.

Water in the ground below London Clay had to be dewatered to prevent flooding the shaft during excavation.

Contractor WJ Groundwater installed pumping wells to dewater the Lambeth and Thanet layers five months before excavation.

'Our biggest fear would have been a power cut and all the dewatering pumps turning off, ' admits Skanska project director Matt Cova.

'It takes just two hours for the water level to recover 2m. If this happened while [digging] in the Lambeth beds, the shaft would have flooded. So we always had a standby generator on site, just in case, ' he adds.

Skanska's geotechnical engineer Andy McNamara adds: 'The danger was that the dewatering wells missed an area of locked in water [in the Lambeth Group]. The only solution was to dig trial holes to check for water'. In the event of hitting water, secondary well points would be installed through the shaft lining.

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