Roger Bullivant's Quiet Hammer piling rigs are usually only used on busy city centre foundation jobs, where noise and vibration levels are restricted. So their use in Raglan near Abergavenny, south Wales at first seems unusual.
But working quietly is of prime concern as Bullivant is installing precast concrete piles and ground beams for two new wards just 50m from Llanarth Court Hospital's mental health rehabilitation and assessment centre.
'Our 3004Q rig has a 3t hammer and a special damping mechanism that is designed to suppress noise along with the associated perception of vibration, ' says Bullivant contracts manager Derek Murphy.
But keeping quiet is not the only challenge for Bullivant and contractor Kier Western.
Access to the hospital estate is via a Grade II listed gothic stone gatehouse dating back to 1863.
The rig only just fits through the 3.5m high and 3m wide opening with its mast laid flat. The building's wooden trusses also had to be carefully selected to ensure they could pass through.
The ground profile consists of silts and very sandy clays overlying gravels and dense cobbles. Piled foundations were necessary since the site is liable to flooding, says Murphy. But with restricted access, bringing in lorry loads of concrete and reinforcement steel for in situ piling would have caused too much disruption in the tight construction programme, he says.
Instead, 323 precast concrete piles between 11m and 17m long and 31 steel piles for a perimeter security fence were used, installed in just three weeks. The number of blows from the QHammer was set according to the required penetration.
Piles are joined as they descend, a reinforcing bar protruding from the top of one locking into the socket of the adjacent pile. Once at the desired level, they are cropped and capped.
Precast pile caps and ground beams were also used to speed up construction. Some 680m of ground beams was installed in a 24 hour period.