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Onsite modifications kept piling on track during works close to the public at a site in the centre of Edinburgh. Damon Schünmann reports.

When a job falls behind it is often down to the onsite team to crank things up again. At the University of Edinburgh's Potterrow site, engineers have done it by designing a system that allows a down the hole hammer (DTHH) to be used in hard rock, very close to public pathways.

The piling is for a £37.5M informatics (computing information technology) building on a site underlain by mudstone and siltstone. The building's basement is surrounded by a contiguous pile wall. Some of these piles are load-bearing with no need for any across the basement due to the wall founding in the mudstone which gets stronger with depth.

In the diffiult ground, the problem of how to keep piling on track proved to be the mother of invention.

The onsite down the hole hammer setup, acquired as part of the Pennine takeover by the scheme's piling subcontractor Stent Foundations, could make short work of the hard material.

But because rock is ejected from the hole during boring, as well as the dust it creates, the rig could not be used on the site's edge, which is close to a student thoroughfare.

This would have meant using far slower rotary bore rigs.

'We've always had backup to use conventional rigs, but it would have meant flooding the site with them, ' says Stent senior contracts engineer Lindsay Archibald. 'Some of the rocks found by the site investigation were 60MPa and the varying strengths would have meant a great risk of wear to tools and plant.' The clouds of dust have been grounded by using 23l/s of water in the pneumatics that power the hammer. Water can be seen jetting at high pressure from the drill rod.

Shielding against flying debris was not so simple. Stent produced a system it is improving as the job progresses, and it was in its fourth incarnation at the time of GE's visit.

'This is the first time Stent has used a large diameter DTTH rig.

We normally use conventional techniques but the ground is so variable in Edinburgh, ' Archibald says. 'We're putting in about 300 contiguous wall piles at 150mm spacing requiring 5m to 7m of rock removal. These piles are going in at 9m to 10.5m depth.

'So we have a metal shroud over the hole as well as a deflection concertina around the drill rod to catch any material that might escape from the shroud. The system is now at a point where we are happy to work on the edge of the site next to the public. We would really like to develop the concertina further to make it more robust as it does get damaged.' Crews were able to tweak the design of the shroud and concertina while testing the DTTH rig in hard rock areas away from the site's edges, where there were no public safety issues.

'We used the job's first two weeks to get the DTHH set up how we wanted it and got a bit behind programme, ' Archibald says. 'But once we got it right we caught up because it's so productive.' The next advance Stent would like is to develop is a concertina that rig crews could easily lift to allow drill rod inspection.

Before getting the system to its current state, Stent was using a conventional Junttan 1830 rotary boring rig. Although powerful, it was only managing about three piles a day. Now, using both rig types in combination, the crews are putting in seven.

The process sees the Junttan doing a 3m temporary cased prebore until refusal, generally in areas where there is underlying sandstone in addition to the mudstone and siltstone. Site workers then follow up with the THW 4019D, DTTH to remove the harder materials.

However, Archibald believes the site's confines and relatively small number of contiguous piles is restricting the combination's potential. 'You're limited by the DTTH but on a larger site with the right support [of preboring rigs working in advance of it] we could do more, perhaps as many as 20.' The 600mm wall piles will form a basement for the new building and are integrity tested using crosshole sonic echo tubes in 25% of them. This is because the close 150mm spacing means that where piles occasionally touch, standard integrity testing would be distorted.

A fault running across the project is thought to be influencing groundwater at the site.

Stent design engineer Brian Mooney says: 'I think it's running east-west at the north end where the ground is mostly sandstone.' Archibald adds: 'It was known there was a fault and it's interesting because where we are working in the southern basement area, we are down to 4m and it's dry. But further north where the fault is it's quite wet.' He adds: 'We're also putting in rotary bored bearing piles [elsewhere on the site] 4.5m to 7.3m, depending on the rocks we find.' The company is installing 133 of these with 600mm or 750mm diameters.

The current phase of piling, under a £900,000 contract awarded by client Balfour Beatty Construction, was due to finish at the end of last month. The project is scheduled to complete next year.

Pollock town centre redevelopment

Another of Stent's Scottish contracts is in the Glaswegian suburb of Pollock, which is getting a £300M overhaul.

Retail Property Holdings is building the 93,000m2 Silverburn complex that will include 95 shops and 13 restaurants on the site of the old town centre.

Subcontractor Stent, which completed 1000 piles for a multi-storey car park on the site last summer, won the £1.2M contract from client Bovis Lend Lease.

When GE visited in February it was busy installing 3700, 270mm square driven precast piles over an area about 30,000m 2, at depths of 9m to 12m.

Contracts engineer Chris Winters says: 'The ground conditions vary but in general are lime stabilised soft parts down to 3m. Below that are bands of silts and clays over sands and gravels then we are into the clays again.' Piles commonly found in the lower clay layer, but in one area a shallow rock shelf 1m to 16m below ground level means the shortest of the current batch can only get down to about 3m.

Winters explains that as anything less than this depth does not really constitute a pile, another subcontractor will be brought in to fill shallower areas with mass concrete.

The 700kN piles are designed to 1.5 times working load and Stent is static testing to this and dynamically testing to twice working load. It was due to finish this phase of piling by the beginning of this month and hopes to win the contract for a second phase.

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