Warrington in Cheshire will soon have a new bus station, car park, offi es and shops courtesy of the £100M Golden Square development.
Like most town centre sites, there is not much room to manoeuvre.
Although the site is about 200m square, the Midland Way dual carriageway runs along one edge, complicating the project for main contractor Bovis Lend Lease.
The road runs on an embankment which will be cut away to make room for the bus station. The original plan was to support the embankment with a retaining wall of 750mm diameter contiguous bored piles. This would have meant either building a berm to bear a CFA rig or closing half of the dual carriageway.
Bovis engineering manager Phil Lockwood says the original design was first changed to a secant wall and then back to contiguous piles, this time 1.8m diameter. Aside from the logistics of installing such huge piles, temporary earthworks for any of the proposals would have been substantial and costly.
In the end Bovis chose another solution to reduce site preparation works and minimise disruption. It awarded the £500,000 piling contract to Japanese company Giken, which makes innovative 'silent' driven piling systems. The rigs produce about 68dB - ordinary speech is 60dB.
While the equipment has been used in Japan for some time (European Foundations spring 2002), this is the fi rst time a Tubular Super Crush piler has been used in Europe.
The system uses a rig that travels along the tops of completed piles and grips the last three. This gives a reaction force against which it can press new piles into the ground. A pile runner follows, supplying fresh piles to the rig via a small crane.
Giken Europe project manager Alan Shepherd says: 'Midland Way is built on an embankment where an old part of the town was demolished, so the rig needed to press piles through the made ground of the embankment. This was comprised of cobbles and old street structures going into stiff boulder clay and then into sandstone.' Lockwood adds: 'There aren't a lot of piling rigs that will go into very stiff materials, let alone material with an SPT N value of 200 that we're going into here.' Because of this, the Super Crush rig at the Warrington site used an auger 300400mm below the foot of the pile to get through the tough ground.
Two rigs were used to install the piles. The first, a Tubular Super Crush Piler SCP260, pressed in 62, 900mm diameter tubes to depths of 12.8m to 16m. The second was a standard Silent Piler UP150 installing more than 80, 10m to 16m U-piles in areas where the retained height was lower.
Giken claims it takes about two hours to install one of the 900mm tubular piles. Sheet piles take 10 minutes. Shepherd says the sheet piles are installed in very stiff boulder clay but the rig can compress up to 150t, enough to destroy the pile, so the operator is trained to use the minimum force required.
He says although the process does not produce much spoil, 'there are of course some arisings because we are not compacting, but they are very little compared to a standard secant wall.' Much of the spoil is put into the tubular piles once finished.
An advantage of the system is confi rmation of the site investigation.
Shepherd says: 'As we are installing the pile we can record the information a geotechnical engineer needs.
We have an observable record taken from reaction and extraction force and torque which can confirm the site investigation with every pile.' The arrival in the UK of the Tubular Super Crush Piler SCP260 is in advance of a more powerful rig that gyrates piles into the ground.
Shepherd says that in a test 'the gyro piler went through a 1m deep reinforced concrete block in eight minutes'.
Piling at Golden Square began in early December and finished in early January.