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COLLEGE BASH

Steel sheet and precast concrete piles have been installed at the site of a new split level, state-of-the art dining hall at Trent College, Nottinghamshire.
The imposing entrance to Trent College in Long Eaton houses a foundation stone dating from 1868, when Francis Wright, influential businessman and High Sheriff of Nottinghamshire, together with many notable men of Derbyshire, Staffordshire and Nottinghamshire established a new school for boys.

One hundred and 40 years later, foundations of a different kind have been laid as part of a capital investment programme at the college. The latest stage of this is a new hall and meeting area for students. The cost is somewhat higher than the £300 originally paid to build the entire school.

Dew Piling won a £130,000 contract to design and install the piles for main contractor GF Tomlinson.

The new building is being constructed on two levels on a sloping site with a 3m height difference. The sheet piles have been used to create a retaining wall between the two levels and to carry axial load, while 139 precast concrete bearing piles will carry the load of the building.

One stipulation of Dew's contract was that there could be no temporary propping or strutting of the sheet piles as this would restrict the contractor's construction activities, so the piles have been designed as cantilevers.

"A consideration for the college was ensuring all the piling work would be completed by the end of 2007 to prevent disruption to examinations taking place in January," explains Dew's contract manager Tom Fox.

Dew began the sheet piling in early November using its ABI TM14/17 leader rig fitted with a 925kN variable movement high frequency hammer, with a 50t all terrain crane offloading the piles working alongside the rig.

A total of 155 VL605 sheet piles were installed, with an average length of 8m, to form a cofferdam with three straight sides and a curved wall running from one corner to provide a ramp between the two levels.

"We drove the piles through made ground, sands and gravels, to found in the underlying sandstone/mudstone," says Fox.

"The rig was powerful enough to install the piles to the correct depth to produce the required cantilever solution without the need for any back driving."

The sheet piling was completed in a week, then Dew temporarily cleared the site so the contractor could continue with bulk excavation at the lower level. Dew returned with its PM20 driven rig fitted with a hydraulic 5t accelerated hammer, and started work at the higher level. As the founding level of the sandstone/mudstone varied, Dew probed the ground to establish the lengths of pile required across the site, with the result that 8m and 9m long, 250mm x 250mm piles were driven.

Piles in some parts of the site encountered more resistance in the upper layers. This meant there was a risk of damage during installation. But adjusting the hammer drop height and dwell time between blows overcame the problem with little loss to production.

When the upper level was complete, the PM20 was de-rigged and moved to the lower level to install 6m long precast concrete piles capable of carrying axial loads of up to 425kN.

"We were able to remove all our equipment before Christmas, fulfilling our undertaking to complete all the piling works by the end of term," says Fox.

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