Cutting edge CFA piling techniques are being used to bore though hard boulders on an Irish redevelopment site. David Hayward reports from Kilkenny.
Tourists arriving by train at Ireland's popular southern city of Kilkenny have been greeted by a station platform seemingly cut in two by piling rigs. Temporary buffers and site hoardings separate the tracks from rows of hefty continuous fight auger piles, bored right across the station and its car park.
The work is part of an £84M mixed retail, commercial and residential development adjoining Kilkenny's MacDonagh station. Alighting passengers find themselves surrounded by a vast, bustling building site and from the footbridge they can see the deep excavations for underground car parks and piled retaining walls protecting the city's once notorious Victorian workhouse.
Nearby similar rows of piling appear to support an entire 160-year-old station shed which stands suspended as ground is dug away around it.
What the onlookers are also seeing, although they will not realise it, is innovative piling works at the cutting edge of CFA technology.
Ged Neary, geotechnical contractor Bachy Soletanche's general manager in Ireland, says: 'If the design had taken the piles any deeper, through the numerous hard cobbles and boulders we have met on virtually every bore, then we would have had to abandon our preferred CFA method in favour of traditional large diameter rotary piling.
'We have met cobbles and boulders on virtually every bore and are at the limit of CFA design and rig capability. But we took a well calculated risk and it has paid off.' As the last of 1,500 up to 17m long piles was completed earlier this month, Neary pointed to a nearby H³tte drill rig. It was on call to deal with troublesome boulders over 1m thick, but was never needed.
Bachy won the design and build contract from main contractor - and one of Ireland's largest - Michael McNamara Construction. The project heralds the company's planned expansion into the Irish piling market (see box).
The client, Chesterbridge Development, plans four levels of shops, of' ces and ' ats, plus a hotel, bowling alley and underground parking for 1,200 cars. To achieve all this on the congested, 5.7ha city centre site, every square metre of available land must be used.
Bachy Soletanche's prime role, over its five-month contract, has been to enclose the entire site with perimeter retaining wall up to 10m deep formed with 600mm and 900mm diameter piles, some installed only 200mm from surrounding property. And the list of neighbouring structures and their owners was quite a roll call.
The derelict, shallow founded rows of 1830s cottages bordering on one side are the remains of a historically important workhouse, once the overcrowded home of more 1200 men, women and children. It is now a listed building, as is the even more dilapidated Victorian brick and timber goods shed within the site itself.
Nearby is another listed building, Kilkenny's original train station of cast iron columns, now temporarily separated from more modern platforms by construction for a piled underpass.
Additionally, the development borders main line rail tracks and the station's goods yard, which is a nationwide distribution point for Ireland's most famous export, Guinness, arriving by road from the nearby Kilkenny brewery.
Tight against another site boundary is the home of local hurling team and long-time Irish national champions Kilkenny Cats. Its 35,000- seat stadium is squeezed between other sensitive structures such as the city council offices and electricity supply headquarters.
The presence of such in'uential neighbours helped trigger 25mm pile positioning tolerances for the exceptionally close retaining wall, and verticality of 1 in 150, combined with lateral movement limits as low as 5mm. Neary describes these as 'onerous but understandable and achievable'.
Following neighbours' requests, more than 70 piles contain monitoring inclinometers or sonic logging tubes to measure pile performance and integrity.
At tender stage for the design and construct geotechnical subcontract, the client and its consultant Arup were suggesting rotary bored piles of 600mm and 900mm diameter. But Bachy looked at the ground conditions - dense gravels peppered with hard cobbles and large boulders - and opted instead for the cost and time benefits of CFA piling.
With standard penetration tests in the soil giving readings in excess of 150, CFA rigs would normally favour softer ground. But the subcontractor reckoned that, by beefing up a standard Bauer BG18 CFA rig to ensure all the energy from its 18tm torque was transmitted right to the cutting head, the densest cobbles and boulders could be displaced or broken up.
The rig was fitted with a 200mm thick heavy duty auger stem and its four cutting teeth strengthened with carbide tips. Cutters were angled steeper for rock boring and the small H³tte drill rig put on call to deal with any particularly stubborn boulders.
On average, some two dozen, 200mm diameter cobbles, and several larger boulders were met during each pile bore. But the strengthened BG18 coped with the lot, peaking at 22 piles per day.
Bachy claims that opting for CFA over rotary piles has saved the project about 15% in overall piling costs, and even more in time given the limited space on the congested site.
Back at tender stage, about 40% of the piles - mainly those bordering Irish Rail land and track - had already been conceptually designed by Arup, as planning consent for railside operations had to be initiated long before the geotechnical contract was let.
Arup opted for conventional bored piles, with their 15m long rebar cages detailed as two sections spliced together. With the required T25 and T32 bars normally available in 12m stock lengths, spliced cages were considered a logical approach.
Bachy, however, faced with many of its 1,500 piles longer than 12m, sourced a local supplier able to prefabricate cages up to 17m in a single length - equal to the deepest pile length needed.
High quality, factory style preassembly of all the cages, plus avoiding the need for splicing and welding on the congested site, has speeded up the piling operation by about 20%, engineers estimate.
Lifting and installing the more robust unspliced cages also minimises safety risks, Bachy claims, particularly when working alongside a railway with strict limits on lifting radii and timings.
It was also uncertain at tender stage which sections of perimeter retaining wall would be propped internally and which could be anchored back to adjoining land.
But the flexible design has allowed for either option and, as the wall is gradually exposed by up to 10m deep digs, both methods are being employed.
The H³tte drill rig is now tying back sections of retaining wall pile cap with up to 20m long ground anchors, some later to be stressed to over 750kN.
Near the centre of the site is the 80m long Victorian train goods shed. More than 200 piles have been installed tight around the rectangular structure, positioned to within 200mm of its brick walls.
Topped by a 1.2m deep ground beam, the piles now support the seemingly elevated shed as varying depth excavation all around exposes the pile shafts.
This differential excavation, with one side of the shed's support piles exposed 3m deeper than the other, could cause the building to sway. So rows of tie bars, laid across the shed and fixed to the perimeter ground beam, are being stressed to make the structure more rigid.
This 1840s train shed, smokestained by decades of puffing billy shunters, will be restored and reincarnated into the development as upmarket boutiques.
The Kilkenny contract is an indication of Bachy Soletanche's longer term plans to expand into Ireland's multi-million euro heavy piling market.
The whole island, north and south of the border, is enjoying a construction boom, both in road, rail and water industry projects and extensive private development.
Dublin is seen as approaching development saturation, but a dozen towns, including Cork, Limerick and Kilkenny, are keen to improve their attractions with new retail and commercial developments accessed by improved transport links. The total annual geotechnical market is estimated at more than f50M (£34M) though it is the heavier, up to 2m diameter, piling market for bridge foundations, underground water tanks and sewer shafts, plus secant or diaphragm walling for basements, that attracts Bachy.
'We have no desire to compete with local companies that dominate the smaller piled property sector, ' says Bachy Soletanche general manager in Ireland Ged Neary. 'But we see an opportunity to grow within Ireland contractordesigned heavy foundations sector and are aiming at a 20% share of that expanding market within a couple of years. We have now established an independent office near Dublin, able to directly serve Irish customers old and new.' The Kilkenny project's main contractor, Michael McNamara Construction, has just set up a new civil engineering division speci'cally targeted at this larger construction sector.