A narrative on the sheet piling at Halo
Posted on June 8, 2020
Following on from publishing information about the sheet piling and we wanted to provide a narrative on the sheet piling at Halo.
In terms of the available technology, there are three methods of sheet piling:
- The pressing in system that is quiet at about 70dBa at 10m,
- The next option is a vibratory system that obviously has some vibration issues but typically has noise emissions of 80 to 85dBa at 10m
- Finally the last option is to percussively install with impact hammers. The hydraulic drop hammers are the best in this range with emissions typically 90 to 95dBa at 10m, then there are air hammers at 100 to 105dBa at 10m with the last option being diesel drop hammers with noise levels of 110 to 120dBa.
Unfortunately option one, which would have been the quietest option, is not a viable option for the site as the site consists of made ground over soft alluvial ground, which in turn is over mudstone. This system relies on reacting off 3 previously installed piles to push in the next pile. With a 16 to 18m long sheet pile the installation pressures would be 80 to 90 tonnes and the reaction off the alluvial ground would only be 20 to 30 tonnes. As such the sheet pile being pressed would not be installed and the 3 reaction piles would be pulled out of the ground.
Therefore the next best option is to adopt a vibratory system which is suitable to install through the upper ground down to the mudstone layer but will not penetrate the mudstone by anymore that 100mm to 200mm. That is not sufficient to achieve the axial load capacity, therefore we are required to push the final set of the pile by percussively driving, to achieve a 500mm to 1m penetration into the mudstone.
By using the combined system installing the majority of the sheet pile with the better vibratory system and limiting the noisiest installation system to a minimum. There is a shroud available on the market to help reduce the noise produced by the percussion hammer and are actively trying to source one of these.
In terms of why the noise levels are being generated, it appears to be for two main reasons, and is an unusual and unfortunate set of circumstances:
- Where work has started there appears to be an area of up to 5m of concrete backfill that has had pieces up to 500mm in size. The problem with this is that the sheet piles are hitting these large pieces and the vibration is being bounced back up the sheet pile causing it to oscillate and emit these louder noise levels. On hitting these pieces there is also an added effect of pushing the piles out of plumb that increases the clutch friction that can only be overcome by increasing the power of the vibratory hammer to install the pile that then increases the noise outputs.
- The site is surrounded by high level buildings that creates an echo box for the sound to be exacerbated.
That being said, the data appears to be improving as work moves to area of the site where piling is into more virgin ground, and with the shroud, should look to reduce the issues being experienced.
Apologises for the long, technical information, but we just wanted to make sure that everyone has all the information that may prove useful.