Bullivant Quiet Hammer, entered by Roger Bullivant
Roman engineers may not have had a great understanding of the principles of soil mechanics but they knew how to found structures on soft ground. They sharpened tree trunks, reinforced the points with metal shoes and hammered them in using a heavy weight suspended from a timber A-frame.
Many of these 2,000 year old timber piles still survive - as does the basic technology. Driven piles, be they timber, steel or concrete, are still the preferred solution to many geotechnical challenges. Advantages include speed, minimum equipment on site and, when working on contaminated brownfield sites, low pollution risk as no material is brought to the surface.
But there is one major environmental drawback - noise - which increasingly those living and working close to construction sites are unwilling to tolerate. Bullivant's new Quiet Hammer offers a significant reduction in noise levels, which the judges believe to be a major benefit to contractors and the public alike.
Conventional modern pile driving rigs differ only in detail from those used by the Romans. A large heavy lump of metal is repeatedly lifted and dropped on to the top of the pile until it achieves the specified degree of resistance. This is an inherently noisy process. Previous attempts to produce a quieter piling hammer have generally involved surrounding a standard hammer with a noise reducing jacket, but this approach was rejected by Bullivant, as plant design manager David Spriggs explains.
'We were aiming for a noise reduction of at least 6dBA - that's 75% quieter than normal - and the only way was to start with a clean sheet of paper.'
Apart from being quieter, the new design had to be easily reproducible in a wide range of sizes and capable of being fitted to conventional piling rigs without major modifications. Analysis of a conventional hammer identified the main sources of noise as - obviously - the impact of the hammer on the anvil cushion, plus the clatter and rattle of the moving parts, and resonance or ringing. Bullivant's first move was to eliminate all metal to metal contact.
This was achieved by the extensive use of phenolic sliding bearings and urethane cushions. The final touch was an all-enveloping enclosure, and when the new tool was tested against a conventional equivalent the sound reduction turned out to be way better than expectations. Measured on the logarithmic dBA scale, and depending on ground conditions and pile type, the sound output of the new hammer was between 11 and 12 dBA lower than the standard tool - a reduction ofmore than 90%.
In fact, the new hammer was so quiet that hitherto overlooked sources of noise became noticeable. 'It was mainly the sound produced by the wireropes supporting the hammer slapping against the rig mast immediately after impact,' Spriggs reports.
'This is down to the reduction in tension in the wires as the pile recovers slightly after the impact and the hammer bounces back. We got round this by incorporating a sliding carriage to support the ropes at midspan and by 'pre-tensioning' the wires hydraulically at impact to keep them as taut as possible.'