News
Trinity Walk Shopping Centre, Wakefield
Simplex Foundations completed piling on the £210m Trinity Walk Shopping Centre Wakefield development for Main Contractor Shepherd Construction at the end of November following a three month £1.5m contract which included installation of approximately 300 linear metres, rotary bored contiguous piled retaining wall, and 500 tubular steel driven foundation piles.
The new complex being built by Modus Properties, the Manchester developer, will provide 500,000 sq ft of shopping, including a 96,000 sq ft store for Debenhams; outlets for Next, River Island and H&M, together with 42 further retail and catering units on the upper floor.
The lower floor will contain a 92,000 sq ft Sainsbury’s store with a direct access into a 950 space car park. Trinity Walk will also include an indoor market hall; an art library facility and two public squares.
The 16.5 acre site in the centre of Wakefield was created through the demolition of an open air market and a multi-story car park, a demolished and back filled highway underpass and incorporated the re-routing of the Marsh Way dual carriageway ring road.
The known presence on site of a number of sub-soil obstacles which included the workings of an ancient stone quarry – the likely source of stone for Wakefield’s Cathedral – and a buried gas storage holder, created a number of significant problems for the Consultant Engineers, BWB, and the Piling Contractor who worked together throughout to resolve these issues. Continuous modifications to the contiguous wall design were made as installation work progressed.
During the initial desk study by BWB, information came to light in a late nineteenth century note lodged with the British Geological Survey which indicated the former quarry. So the consultant engineers were aware of the possibility. However since this was deep below the foundations of the pre existing building on the site, a full Site Investigation revealing the true extent and implications as regards to constant or fluctuating levels of the workings could not be established until piling works commenced.
The contiguous wall follows the profile of the proposed new basement structure and the quarry workings running across almost the entire path of it. As installation proceeded, it was found that a rock profile at 3.5m below ground would suddenly drop into a different one, frequently requiring a radically different pile length, and therefore bore depth, cage and concrete requirement.
The retaining wall installation utilised approximately 700, 600mm diameter bored piles to depths of between 6m and 9m, which were reinforced with 450mm diameter steel cages, manufactured by Simplex on site.
Because the contiguous wall is designed to provide vertical load bearing capacity in addition to horizontal load – aspects of the steel super structure will be founded on them – additional 600mm rotary bored pile groups were installed in conjunction with reinforced concrete tie beams to accommodate the shear loads imparted by the bracing system within the steel superstructure.
The new complex is to be constructed within a wedge shaped cut into the initially sloping site to provide a constant basement level to the whole – a five metre dig against the face of the contiguous wall. This called for the use of both traditional load bearing concrete pad foundations and piled foundations.
The sloping nature of the site prior to excavation called for foundation piles to be installed as many as eight different levels. Initially both CFA piles and driven pre-cast piles were considered for these, however, the existence of the buried gas holder led the designers towards a different solution.
Driven steel tubular pile scheme was devised to deal with the obstructions within the gas holder, where the requirement was to penetrate the base of the holder, and socket piles into the underlying competent strata.
The tubular piles were driven using Simplex’ Banut 700 rig, in groups of 3, 4 or more, to be capped in multiples as required.
Piling operations on site were scheduled for completion at the end of November, with the steelwork programme following hard on it’s heels.
It is intended that Trinity Walk will rejuvenate the city centre of Wakefield and establish a unique retail and leisure environment for the local community. It is expected to be open to the public in Spring 2010.