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Fully instrumented rigs are installing piles over two bogs as part of a major road scheme in Ireland

The N7 road is the main link between Dublin and Limerick, a distance of 200km identified in the Republic of Ireland's National Development Plan as needing urgent improvements.

This led Limerick County Council in partnership with the National Roads Authority (NRA), on behalf of North Tipperary County Council, to begin a major upgrade to a 38km long section between Nenagh and Limerick.

The scheme involves developing a new 28km motorway standard cross-section on a greenfield site, with various interchanges, underbridges and link roads. It will traverse two bogs – at Annaholty and Drominboy – and connect to a 10km section of the Nenagh Bypass. This will be widened to become a dual-carriageway.

Irish foundation contractor FK Lowry Piling is delivering the piling contract for client Limerick County Council. Main contractor is Bothar Hibernian N7, a joint venture between Coffey Construction, McNamara and Portuguese firm MotaEngil. Engineer for the works is RPS and Hyder Consulting is project manager.

The !1.5M (£1.02M) scheme is one of the largest piling projects Ireland has seen for years. At Annaholty bog alone, the piling contractor is installing 4050, 270mm2 precast reinforced concrete segmental piles to form foundations for the road and earth embankment.

Annaholty is a peat bog down to a maximum 12m, overlying sand. Piles are being driven to an average depth of 30m, with pile loads of between 49t and 80t, and pile spacings of 2.5m on a herringbone grid pattern.

Each pile is finished with a 600mm deep reinforced cap either 1m2 or 1.2m2. Each pile is a stand alone element of the works because pile

caps are independent of one another.

A geotextile layer spans the pile caps supporting the 3m to 8m high embankment to create a load transfer platform.

Such is the scale and programme of the works that the subcontractor has set up a pile casting yard on site, where 12 site workers are employed. This enables the team to fabricate much faster and more cost-effectively than transporting 120,000m of piles 354km from the contractor's yard at Lisburn, just outside Belfast.

Casting began at the end of May with an average 80 piles per day – about 800m per day and a total of 4000m per week. The maximum number it can produce on site is 960m per day, or 4800m per week.

The contractor began installing the first working piles at Annaholty in July using two Junttan PM20 hydraulic piling rigs, each driving up to 600m a day. The rigs are fully instrumented and produce a computerised read out for each individual pile installed, which can be analysed for quality control purposes.

"Although not in real time, the instruments provide a range of data including information on pile reference numbers, the total number of blows struck, the hammer drop height and the final set, as well as giving an indication of the ultimate load bearing capacity and the type of pile modules used," says FK Lowry managing director Mark Walsh. "They also provide important graphical information for rate of penetration, drop height, set, driving resistance and driving stresses."

Piling at Annaholty is expected to complete in December. Following this, piling work will begin at Drominboy Bog. Options for this second phase are still being discussed with the client due to different pile sizes and pile loads required. However, the contractor anticipates being on site in November to complete all works by February next year.

"Projects of this kind are challenging due to the difficult ground conditions, but we are specialists in delivering piling schemes over bogs, having completed a couple of jobs for the electricity board (ESB) in the Republic of Ireland," says Walsh.

"We were involved in piling projects in Sligo and Longford, where we had to install a number of precast segmental piles to support the bases for an electricity pylon, which involved tracking from one piling location to the other."

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