The concept for the new facilities was based on excavating the site to provide the extra room required for the development. This required a contiguous pile retaining wall to support the surrounding ground and neighbouring roads. Van Elle, one of the UK's leading piling and ground engineering specialists were contracted to carry out all the piling works on this technically challenging site, as their expertise had been proven on similar projects.
The piling works, awarded by main contractor Quibell, worth over £400k involved a total of 298 No. Duplex and CFA piles.
In addition to the piling works, Van Elle's geotechnical team carried out a series of boreholes - as only limited information was available during the early tender stages. This gave a better picture of the actual ground conditions enabling Van Elle's engineers to make an informed choice of piling method.
The issues and the solutions
Not only is the school on a steep hillside surrounded by historic buildings, there is also a convent within 4m of the piling site and an adjacent main road. Lincoln itself sits on a rocky outcrop of Lincolnshire limestone with made ground above, making the ground conditions as challenging to work with as the site itself.
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Because of these issues the use of Duplex piles was the only economical and practical solution. The Duplex piling method allowed the piles to be temporary cased through the made ground and socketed a minimum of 5m below dredge level into the bedrock.
The site was live during term time and as well as restricted access to and on site, deliveries could not be accepted before 9.30am and after 3.30pm to ensure the safety of the students and parents. This meant rig choice and logistics were crucial. Rigs were selected based on their size, weight and manoeuvrability as well as their suitability for the job. Well planned logistics and pre-ordered deliveries meant that there were no delays waiting for materials to arrive.
Construction
188 No. Duplex piles were drilled to form the contiguous wall, 660mm diameter temporary casing was drilled 5m onto the rock then the rock was drilled out using rotary percussive and Down The Hole Hammer techniques. The bore was then tremied with concrete, reinforcement was installed and the temporary casing removed.
The site investigation showed potential shallow mine workings at 1-2m deep along the south and east sides of the site. Van Elle solved this problem by pumping in a weak concrete grout, allowing it to set then re-boring through the concrete to form the Duplex piles.
The contract was complete within the anticipated programme and budget, even with the several unexpected challenges which arose during construction.
The retaining wall has now been excavated out by main contractor Quibell, and the steel frame is also being erected. The project is due for completion in 2010 when the students will be able to enjoy their new facilities.
