Bangor is one of the fastestgrowing towns in Wales. Over the past few years the high street and town centre has been in a constant state of expansion and development. The latest stage will be the completion of the town's Menai Centre; a retail development that will provide a major extension to the existing high-street shopping environment.
The piling for the Menai Centre has been designed and delivered by Bachy Soletanche, with work on the first phase completed at the end of October. However, a combination of ground conditions, logistical considerations and physical constraints meant that the piling project was far from standard.
The Bachy team proposed a comprehensive piling redesign, not only changing the number and formation of piles, but also the technique. By switching from continuous flight auger (CFA) piles to the patented Screwsol system, the subcontractor says it significantly reduced working costs and inefciencies for main contractor Watkin Jones.
Bachy contract engineer Peter Lloyd says: 'The original plan was to provide a piling solution that only used CFA. But although we needed to construct more piles using Screwsol, the process has been developed to create a minimal amount of spoil and uses far less concrete than with CFA.' Because the Menai Centre project is in a city centre and on the side of a hill, the transportation of waste spoil with CFA would prove very costly and create a signi cant logistical issue.
Rig crews used a Bauer BG22 in August to build test piles that would verify the new Screwsol design.
Bachy claims that unlike other piling methods, the Screwsol system creates a unique pile that compacts the displaced soil around the auger, rather than creating large amounts of spoil above ground. The compacted soil around the pile enhances its load carrying capacity.
The firm says that although alternative minimum displacement solutions exist, the Screwsol technique is different in its spoil reduction. The secret ingredient is a thin, angled fin built into the lower flight, 75mm wider than the auger. This fin effectively creates two diameters for the pile bore: one standard diameter for the inner core, and one wider diameter for the width of the thread created by the fin.
Once the auger has reached depth it is extracted in a clockwise direction at 300mm per revolution, forming a continuous helical thread.
Along with the rest of the pile bore, the thread is lled immediately with concrete, which is fed under pressure through the hollow auger as it is raised - just as with CFA piling.
The thread then forms an integral extension to the pile shaft.
'Using the computerised controls of CFA rigs, we can regulate the rotation, concrete feed and extraction rates very precisely, ' says Lloyd.
'This means we are able to accurately monitor the auger rotation and extraction rates to ensure the thread is formed at the correct pitch.' The piles have a central diameter of 350mm with the wider thread stretching to 500mm. The gradient of the development is such that the piles vary greatly in depth, ranging from 7.5m to 19m.
The continued clockwise rotation of the auger, in conjunction with concrete pumped under pressure, ensures there is no soil contamination of the pile concrete. Bachy installed about 759 Screwsol piles under the £500,000 contract. As Screwsol piles are constructed at the same rate as CFA there was no significant impact on the works programme after the redesign.
But the sloping site and its relatively weak ground conditions also meant that Bachy needed to develop a bespoke piling schedule for the work. The site has been split into 10 separate stepped sections. To maintain ground stability, piles were built using an unusual programme.
'Normally on a project such as this, we would build a cluster of piles and then move on to the next cluster, ' says Lloyd.
'In Bangor this isn't possible because the ground conditions wouldn't allow for piles to be immediately constructed so close to one another, even taking into account the ground compaction of the Screwsol piles. We therefore had to create a bespoke programme to avoid compromising ground stability.
'We overcame the problem by constructing one pile at a time for each cluster, then returning to do the second in each cluster. It may sound very simple, but many of the best on-site innovations are, ' he adds.
Ground conditions were soft silt, overlying sands and gravel. Bachy suspects that the soft silt was likely to be the main cause behind needing to extend the distance between the most recent pile built and the next one.
Lloyd says that using Screwsol meant far less lorry movements on site and has reduced the impact on the surrounding area. 'We are generating far less traf c through the town centre than we would have using traditional CFA, ' he says.
Bachy is due back on site next month to install 40 CFA piles where the site ofces were located.