FOUNDATIONS-WISE, some- thing rather different is going at a Bovis Homes site in Shoreham on the West Sussex coast, where contractor Roger Bullivant is driving concrete precast piles for a development of houses and flats. Firstly no one appears to be driving the rig, the second thing is there are birds singing. It is a very quiet site.
Bovis was attracted to Bullivant's new quiet hammer system because work is taking place adjacent to an already completed and occupied section of the development. The last thing Bovis wanted was complaints from its new home owners.
So how quiet is quiet? During comparative testing at Bullivant's base at Drakelow with an equivalent unsilenced hammer the new totally enclosed version proved to be between 11dBA and 12dBA quieter than the standard one, depending on ground conditions and pile type. A 3dBA drop is equivalent to about halving the sound level.
Key to the quiet hammer's low noise is a combination of special features, including total enclosure of all the moving parts, the elimination of all metal to metal contact provided by the extensive use of phenolic sliding bearings and urethane cushions at the ends of the ram's travel and on the drop weight's lift cylinder mountings.
The only unexpected problem with the hammer since its launch is the noise created by ropes flapping against the rig's mast. This is always present in normal driven pile operations, but is drowned out by the noise of the hammer. Bullivant's mechanical engineers are currently looking for ways to eliminate this.
The quiet hammer is mounted on an in-house designed and built piling rig, based on a 20t JCB JS200 hydraulic excavator, which has had its normal excavating arm and bucket replaced with an 11.75m tall and adjustable raking mast. The self-contained, crawler-based piling rig, complete with the quiet hammer, can be ready for work within only a few minutes of arriving on site on a low loader.
The hydraulically-operated free falling drop hammer is powered by the host JCB machine's hydraulic system and is fully adjustable by the remote radio control unit barely larger than a computer game control console with a radio transmitter the size of mobile phone. The operator is free to move to the best position, unburdened as most 'remote operated' rigs are, by a large stationary control unit connected to the rig by a multitude of cables.
A gentle flick of the controls gives a range of operations from a comparatively light tap for initially toeing in the piles up to maximum impact for driving to completion or set. To compensate for different piles and ground conditions blow rate can be infinitely adjusted up to 60 blows/min at maximum 750mm stroke or up a maximum 110 blows/min at 300mm stroke. Tests at Drakelow indicate that the new and efficient shrouded hammer is capable of transmitting up to 70% of its maximum potential impact energy into a pile.
According to Bullivant's Southern area manager Steve Walker, there have been no teething problems with the system and no loss in pile production.
The 5.5t hammer is soon to be supplemented by a 3t version, intended for relatively low capacity piles needed for domestic piling jobs. At the larger end of the scale Walker believes quiet driven piles may soon be competing with continuous flight auger piles in urban environments.
Noise and vibration go hand in hand, the argument goes, so take out the noise and perception of vibration decreases. Actual vibration levels may make driven piles acceptable in many inner city situations. Using the 5.5t hammer with 350mm section piles, it would be realistic to achieve piles of 100t capacity, says Walker.
Back in Shoreham, the latest phase of the development is against the harbour wall, with most of the 200mm and 250mm square precast piles penetrating through backfill onto gravel and then into chalk, typically at around 6m to 8m.
Piles have design loads of between 150kN and 500kN. Given the backfill and the potential for obstruction, piles are in this case being pre-bored.