Case Studies - Wharf Rd, Whaley Bridge, Derbyshire: CFA Piles

In early December we were asked to install 57no. 450mm diameter CFA piles on a site at Wharf Rd, Whaley Bridge, Derbyshire for a proposed development of flats and maisonettes.

Our contract value was approximately £28,000 and the overall programme time was expected to be 6-7 days. Although not an enormous contract the site itself posed its own problems to overcome even before work commenced. The machine used on the site was a Soilmec CM-48 piling rig.

Firstly, the access to the site was deemed too tight for us to deliver the piling rig. However, after several visits by rig operators and transport personnel it was decided that we could successfully deploy the piling rig onto the site as long as this was carried out very early in the morning when traffic would be much lighter.

The second problem was the close proximity of the working area to a railway embankment carrying a busy rail line. At the closest point the rig mast would be working approx 10 metres from the railway. Network Rail was involved on the site and was worried of the potential risk posed by the CFA rig toppling onto the railway line. The overall height of the rig at 26 metres towered above the railway embankment. A method statement to satisfy Network Rail had to be produced before any work could begin. The proposed method initially involved the construction of a suitable working platform for the piling rig.  Secondly, the decision was made that the piling rig would always operate with the tracks parallel to the railway line. The combination of these two precautions removed all of the avoidable risk of the rig toppling.

The ground conditions were highlighted by the investigation carried out by Sub Soil Surveys Ltd. Made ground was found in the top sections as a mixture of colliery waste.

From a depth of approximately 1.5m to 3m was found clayey Sand and Gravels. Below this lay stiff sandy Clay to a depth of 10m where the borehole was terminated.

Our responsibilities also included the design of the CFA pile. Our engineer provided a design based on a 450mm diameter pile to take a SWL of 450kN with a Factor of Safety of 2.5. The design length of 12.5 metres included a 4.8 metre steel cage consisting of 6T16 bars to the top of the pile to overcome any assumed surcharge from the railway embankment.

During the works we did encounter problems with obstructions in the top layer of the made ground. Large sandstone blocks, some as large as 2metres across, forced some of the piles to be installed slightly out of position.

Due to the problems with obstructions, three extra piles had to be installed. This combined with problems with concrete delivery to site resulted in the programme nudging into a ninth day.