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Jumping the Gun Unexploded ordnance could have presented problems for the redevelopment of Gunwharf Quays, one of the UK's most historic naval sites, but deep foundations and working over water proved

Foundations; Gunwharf Quays

Admiral Nelson's ships were loaded from Gunwharf Quays in Portsmouth Harbour before the Battle of Trafalgar, and in the Second World War the area was a top secret centre for German mine intelligence.

Through the hundreds of years of naval occupancy, Gunwharf Quays has been inaccessible to the public, but in 1995 the naval base was closed and a development brief prepared by Portsmouth City Council. In response, developer Berkeley Group is now investing ú100M ($160M) in a mixed use development.

While privately funded, the Gunwharf development is an integral part of the Portsmouth Harbour Renaissance project which has received a ú40M Millennium Commission grant. The project includes a 6km 'Renaissance trail' around the harbour, a waterbus operation and a 168m tall Millennium tower at Gunwharf. The site will also host major international yachting events including two round the world races.

The 13ha Gunwharf site is essentially in two parts, with the commercial and leisure development to the north and residential to the south, separated by a manmade lagoon - the remnants of an infilled mill pond.

Main construction work began in January, although some demolition and groundworks started in August 1998. Berkeley is acting as construction manager, with main consultant Mouchel responsible for structures, transportation and environmental matters for the leisure and commercial development and for geotechnical investigations for the whole site. Consultant for the residential scheme is Gifford.

Ground conditions vary considerably across the site, with two deep incised channels filled with peat and alluvium bounding a ridge of gravel and the underlying Bracklesham Clay.

Geotechnically, the south side is relatively simple and structures will be supported on continuous flight auger piles founded in the Bracklesham Beds, which underlies the alluvium and a terrace gravel.

The northern end however, includes a massive underground car park beneath the commercial development and a marine deck spanning a tidal mudflat, and it is here that the main foundation challenges arise.

'One of the biggest issues was safe construction of the car park,' says Mouchel's principal geotechnical engineer Andrew Bowden. Numerous historic foundations and four generations of sea walls were known to be present, potentially hampering work. Fortunately, historical buildings were well documented and the majority of the foundations were accurately located before work began. These were dug out or worked around.

Because the site was heavily bombed during the Second World War, there was a chance of finding unexploded bombs. Again, records were very accurate but as a precaution, an explosives expert was present during work and geophysics was used to check the ground before excavating. No modern ordnance was found, says Bowden, 'just some cannon balls'.

For the car park, foundation contractor Westpile installed a secant pile wall around the 250m by 100m excavation using its cased flight auger pile system. The method was chosen over diaphragm walling because of the anticipated obstructions. 'With a secant wall you have more flexibility,' explains Bowden. The cased system produces a more rigid wall and ensures the auger stays on line, essential in this difficult ground, he adds.

Over 1,100, 750mm diameter and 18m long piles make up the wall. Male piles of seawater resistant C35 concrete are reinforced with cages made up of nine T25 bars, while the female piles are softer, formed using C15 concrete, also resistant to seawater. Inclinometers in some of the piles have to date typically recorded movements of less than 10mm, well within tolerance, says Bowden.

The excavation had to be watertight, and to reach a suitable cut-off, piles were taken through the made ground and alluvium keying an average of 3m into the stiff clays of the Bracklesham Beds.

Bowden says a major concern was that water in some of the sand layers of the Bracklesham Beds would enter the excavation from below, so pressure relief wells were installed into these layers. They have done their job, he says, and although there was some leakage into the excavation, this was due to poor sealing of the wall, which has been remedied by grouting. A further 1,345, 400mm and 500mm diameter CFA piles are being installed to support the car park and the commercial development.

To the north, between the car park excavation and Portsmouth Harbour station, civil engineering contractor Christiani & Nielsen has nearly finished piling for the massive marine deck that spans the tidal mudflat.

Nearly 370, from 610mm to 830mm diameter steel tubular piles were driven for the deck, which will support part of the commercial buildings and will also act as a servicing area. Piles are up to 40m long and founded between 1.2m and 1.6m into the Bracklesham Beds or the lower part of the Whitecliff sands beneath. Above the mudflat level, piles are filled with concrete to ensure they maintain their integrity if the steel is corroded by the seawater.

For most of the piling, Christiani & Nielsen used its Traveller system. Piling is carried out from two platforms that slide on temporary beams spanning already installed piles. After piles are installed, the crane lifts the beams from behind and places them in front, allowing the platform to move forward using hydraulic jacks.

One of the main challenges was pile driving within 5m of the harbour station, which is on a pier structure. This is supported by screw piles founded in the gravel and was also badly damaged by Second World War bombing. Mouchel was concerned that driving through the gravel could cause movement of the station but extensive monitoring showed vibrations from piling were less than those caused by trains.

Mouchel's deck comprises 318 precast reinforced concrete beams up to 1,330mm thick spanning between the heads of the piles. Due to the complexity of the deck's shape, 162 insitu reinforced concrete beams are also being used. Some 1,000, 250mm thick precast reinforced concrete planks are then placed with a final 150mm reinforced concrete layer cast insitu.

Construction is being carried out in phases, with the commercial development completed in November 2000. The residential sector will complete in early 2002.

Read this if

you want to know more about installing secant walls in difficult ground

you are interested in developing brownfield land

you are interested in piling close to sensitive structures

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