ADAPTED MINIPILING rigs working in as little as 1.9m of headroom are installing more than 350 low noise, low vibration, bottom driven piles for a retail scheme in the heart of Derby.
Subcontractor Branlow is installing the 273mm and 323mm diameter piles for loads up to 600kN for new and upgraded column foundations in the Eagle Shopping Centre redevelopment.
Work began in September 2005 with a final phase of about 30 piles planned for this month. Project consultant is MPN UK and the client is Westfield Shoppingtowns.
Piles are being installed from within a basement car park 3m down.
The presence of the River Derwent nearby results in groundwater at 5m depth. Conditions are made ground over alluvium over sand with Mercia Mudstone below.
Engineers considered an augered solution a high risk technically as well as significantly more costly.
Branlow contracts director Derek Hildebrandt said: 'Trying to install and control the quality of an augered pile in the poor, wet ground conditions would have been extremely difficult, slow and expensive. If you lose a pile it can have serious consequences.
'Apart from the headroom issue the site was straightforward for our bottom driven piling system. An augered solution would have seriously messed up the car park area.' Rig crews, working nightshifts to minimise disruption, have used up to four machines equipped with 1.8m masts and exhaust filters.
Installed lengths vary between 4m and 7m dependent upon bedrock level. Trial results ruled out the need for load tests but integrity testing of each pile is ongoing.