GEOTECHNICAL CONTRACTOR Keller Ground Engineering was this month expected to start work to fill a huge cavern beneath a residential tower block in Norwich.
The 9m deep and 8m wide void, which covers 25% of the footprint of the Normandie Tower at Rouen Road, is thought to have been gouged out of the chalk bedrock by a leaking water main. Some 600,000 litres of water had leaked into the ground.
An investigation into falling water pressure in the block led to the discovery in the middle of February.The tower's 150 residents were immediately evacuated and are not expected to be able to return for two months.
Council archive drawings revealed that the flats were situated over tunnels which may date back to medieval times.
The 16-storey, 92-flat building, built by Wimpey in 1965-66, is founded on 165 piles up to 20m deep in the chalk.No movement had been reported, but the evacuation was prompted by concerns about stability due to the size of the cavern.
The piles, installed by contractor Franki, are between 600mm and 700mm in diameter and may have been under-reamed, although this has not been assumed in the investigation.The void round the piles and loss of restraint led to fears that they could buckle.
'Potentially they could be acting as slender struts rather than piles.Although there has been no movement this gave cause for concern, 'explained Norwich City Council director of technical and environmental services Andrew Cowburn.
But as skin friction is only provided by the lower 10m of the piles, loss of the weaker soil would not have affected its load carrying capacity.
Dynamic probing through the block's basement had to be used to determine the size of the hole after ground radar failed to reveal its full extent.'Because of the presence of piles, the results could not be interpolated with any confidence, 'said Cowburn.