Bachy Soletanche gives Tesco great value
Bachy Soletanche Limited has recently completed a fast-track ground works project for leading supermarket chain, Tesco. The project required the construction of three contiguous piled walls for the redevelopment of a Tesco store in St Leonards near Hastings. Due to Bachy Soletanche’s innovative design, the leading geotechnical specialists completed the project two weeks ahead of schedule.
As Tesco has a prime location in St Leonards, the supermarket chain wanted to make improvements to the original store by carrying out a store upgrade from a ‘Tesco Superstore’ to a ‘Tesco Extra’, complete with a bus station and large car park. The best way to make these improvements was to knock the old store down and start again in a multi-million pound redevelopment. To assist in the construction of these improvements, Taylor Woodrow awarded Bachy Soletanche the £1.6M piling contract.
The overall aim for Tesco’s is that new store will be open by Christmas 2008. As the piling project starting at the beginning of the year – and is the first major stage of construction works – there has been no contingency for delays. Therefore any time saving solutions have been classed as a blessing. To this end, Bachy Soletanche came up with a productive solution for conducting the piling works and this design has kept the project on schedule.
As the site is on a slope, Bachy Soletanche’s role was to construct three contiguous piled walls. These stand alongside another foundation wall from the original store layout and cumulatively assist in levelling the site. Two of the smaller walls are to the northeast and northwest of the site while the longer wall runs through the middle to eventually act as the building’s under croft. The original wall stands between the two, shorter walls.
The contiguous wall method is a series of piles with intervening gaps which provide an effective retention solution. The standard procedure entails the use of the conventional, yet highly effective, Continuous Flight Auger (CFA) technique to construct the piles. However, it was found after a detailed soil investigation that the ground strata included clay, overlaying mudstone with bands of limestone at approximately 10m. CFA rigs find limestone difficult to cut through due to its inherent strength. Therefore, it was initially planned to use a combination of the CFA and Large Diameter Auger (LDA) techniques. This would provide the desired solution but perhaps not within the desired programme time so Bachy Soletanche had another trick up its sleeve.
Jonathan Spyvee, Engineer for Bachy Soletanche Limited explained:
“We decided upon a piling method that delivered a ‘best of both worlds’ solution known as cased CFA. Using the latest piling technology, the cased CFA technique has the productivity of CFA piling and the strength of LDA, due to pre-attached casing. As Bachy Soletanche has the powerful Bauer BG28/36, we knew we would be able to use this technique successfully on the Tesco project.
In conjunction with the cased CFA rig, we also used the conventional Soilmec SF 120 CFA rig in areas without the limestone bands. With this flexible approach, Bachy Soletanche was able to improve upon our target production and complete piling two weeks early.”
For the overall piling works Bachy Soletanche constructed 436 no. 750mm diameter piles, drilled to an average depth of 12m. The northeast section used 56 no. piles to construct a 70m wall and the northwest section used 94 no. piles to construct an 110m wall. Finally, the largest under croft wall used 286 no. piles to create a 340m wall.
To save precious time, the contiguous structures are un-propped cantilever walls meaning each pile is drilled a few extra metres into the ground – about 3m or 4m – to provide solid foundations. Consequently, the structures do not require extra support, such as ground slabs which are time consuming to install.
The original wall, included in the design, was previously constructed with a ground slab. This was strong enough to withstand the new store, however, work had to be conducted on one section of this wall to form part of the loading bay for deliveries and this part of the slab had to be removed. To pin back this section of unprotected wall, Bachy Soletanche installed 7 no. 14m temporary ground anchors. When the full structural works are complete, these will be removed.
With Bachy Soletanche completing the piling contract two weeks ahead of schedule, the excavation process is currently well underway. This is taking place to the north of the site, where the back of the store is located and is excavated to depths of around 4.3m, levelling the old store with the new. Contractors are also excavating in front of the main wall to form an under croft car park.