Barrett Byrd Associates

Mini piles, anchors and augered foundations are all playing their part in the construction of apartment blocks on the site of a fire damaged warehouse, reports Mike Walter

Conversion of a former bonded warehouse into luxury apartments beside a Newcastle-upon-Tyne quayside and a new development on an adjacent plot are both making use of a host of specialist foundation systems.

The seven storey Grade II listed warehouse was ravaged by fire two years ago, leaving little structural support. A steel frame structure going up within the shell of the warehouse is being founded on 264 mini piles installed by Roger Bullivant Ltd (RBL). Construction of the new build development alongside the warehouse will follow completion of sectional flight augered piled foundations and installation of a series of ground anchors by RBL.

Property developer Mandale Commercial acquired both the warehouse and adjacent plot for the development of two apartment blocks last year. The warehouse will include 48 apartments, the new build block alongside will house 86 flats and retail space is to be provided throughout each ground floor.

Developing a pair of apartments on the site of a dilapidated warehouse is proving a considerable challenge, according to Mandale’s senior site manager Peter Remminton. “This is certainly one of the most complex jobs I have ever worked on,” he says. “The steel framework we are building within the old warehouse has to tie in to and secure the remaining brick and stone facade. The walls had buckled in the fire and it was too risky to put loadings on the old structure.

“Development of the second apartment block is also tricky because the site needs to be reduced in height and is bordered by a listed road that cannot be moved,” he adds.

Charred timber beams within the fire damaged warehouse were all that were holding up its facade, so a large scaffold tower was erected on one side of the warehouse to provide temporary support.

A new concrete floor had to be founded on piles at the base of the building to support the steel frame, but restricted headroom beneath the timber beams precluded use of a rig with a large mast. RBL chose a top driven mini rig to install 183 steel tubular piles on the ground floor to pick up point loads from the steel frame. Each pile measured 140mm in diameter and extended to a depth of 8m into the ground.

Completion of the tubular piles on the ground floor was followed by installation of 82 drilled and cased mini piles to support the internal steel frame at an elevated height. Each of these 220mm diameter piles were formed on top of the historic city wall, built in 1311, which passes along one side of the warehouse.

The new internal steel frame structure is being built one storey at a time to allow each concrete floor to be connected to the facade with dowel bars. Charred wooden beams immediately above the new floor are then removed to allow construction of the frame to continue.

Beside the new apartment development, a row of 126 sectional flight augured (SFA) piles have been installed by RBL to depths of between 10m and 16m to form a contiguous piled retaining wall. The wall is being built to support Hanover Street, a cobbled highway that runs along the rear of the site. When the ground is lowered from infront of the wall this will allow building of the apartments to commence at a reduced height.

Creating an SFA pile involves connecting together a series of two metre long sections of auger as the flight descends into the ground. At full depth, concrete is pumped through a central shaft and the rotation of the auger is reversed to form a column of concrete. A cage of eight reinforcing bars is then lowered by crane and pushed into each concrete pile to full depth to provide added strength.

Ground was excavated from infront of the wall in May 2007, to a depth of 5m, before 80 hollow stem anchors, each measuring 10.5m, were installed by RBL. These were installed between the vertical piles at 700mm intervals to support the wall during the construction phase. Each anchor is drilled into the ground at between 20 and 30 degrees and makes use of a sacrificial ballistic bit attached to a threaded steel bar.

Each ballistic bit is formed from tungsten carbide and is drilled into the embankment using a piling rig that delivers a ‘drifting load’ of repeated hammering action, aided by a flush of water. Once the anchor has reached full depth, two operatives insert a pair of 10m long threaded steel bars to give each anchor added strength.

Completion of the anchors will be followed by a further lowering of the ground level, by another 5m, to form a level platform on which to develop the second apartment block. Conversion of the old bonded warehouse and construction of the new apartment block are set to be finished by November.

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