Parked up for the summer
Bachy Soletanche Limited is midway through a multi-million pound bearing pile and secant wall groundwork project at the retail and mixed use development, Parkway in Newbury. The specialist geotechnical contractor is utilising the CFA (continuous flight auger) and cased CFA piling techniques to eventually form a two storey underground basement/car park for the retail development on behalf of Costain and its client Standard Life Investments.
As the retail industry continues to struggle in the current economic climate, Bachy Soletanche, Costain and the client’s engineers, Cundall, have worked hard over the past 12 months to deliver the leanest of solutions. Several geotechnical solutions were worked up before piling commenced and just as many considered during piling to address the array of services, site programme and other physical constraints on the urban site. As a result, the wall has an irregular shape which suits the bored pile solution finally chosen.
Piling for the basement wall was carried out and required 391 900mm diameter CFA unreinforced female piles up to 13m deep. Cased CFA male piles, 387 and 880mm in diameter, were drilled to lengths ranging between 13.5m and 18m deep. The male piles were reinforced with heavy cages up to 16.5m deep. The relatively rare cased CFA piles were installed with a high torque rig, required to cut into the female piles with high productivity and to the high level of verticality tolerance specified. A further 374 450mm diameter CFA bearing piles have also been installed up to 18.5m in depth.
The ground conditions are four metres of made ground overlaying peat and four metres of sandy gravel overlaying chalk. The ground has a high water table, which has always been an issue from the outset, and as such, CFA was the preferred piling solution along with a slight raising of the piling platform. High water inflows into the basement are anticipated during the proposed excavation so Costain has commissioned W.J. to dewater the site during the excavation works. To resist the water uplift, 586 450mm diameter CFA tension piles will be installed on the base slab up to 24.5m deep.
The first phase of the piling project completed around the end of June and the second phase restarts in August this year. During the gap, Bachy Soletanche and Costain will investigate their options to reduce the amount of temporary propping required to support the basement wall. Still to be installed are 109 CFA bearing piles up to 16.5m deep and some restricted access mini-piles.
Barrie Osborn, Contracts Manager for Bachy Soletanche, said:
“This project has proved that a good site team can deliver a challengingly high quality project, on budget and on programme.”