There is 214M (£9M) of purely temporary works being done to help build the tunnel, before it's all taken out again, explains contractor Dew Piling director Alec Courts.
Courts is talking about his company's contract to install crane stagings, similar in appearance to piers at the seaside, steel combi-wall cofferdams and sheet pile wing walls driven into river bunds.
All of these will make it easier for site workers building the new road tunnel under the River Shannon in Limerick.
This is part of phase two of Ireland's National Road Authority's Limerick Southern Ring Road scheme that aims to ease the city's congestion and divert some 40,000 vehicles along the N7 road. As well as the 900m long tunnel, nearly 10km of new dual carriageway will be built.
The 180t capacity stagings - also known as temporary jetties - are designed to provide a stable platform above the river bank's soft ground. This is for heavy plant and equipment needed to build the tunnels as well as providing safe working access over the tidal zone of the River Shannon. These are being put up in advance of work starting on the cofferdams that will provide a space for tunnel sections to be built and then act as tunnel portals.
In addition to the jetties, temporary sheet pile wing walls will be used to support and extend the protective river bund during construction.
Courts says the whole project is using up a lot of steel. 'Supply of steel is the most critical thing to our business, ' he says, which is why the company sourced second-hand steel tube piles for the job, typically the kind used in oil pipe structures. Each steel tube is 1.42m in diameter and the supplier had them adapted so that they could be fixed to adjacent piles.
Work began on the four temporary stagings in September 2006. Each is about 9.5m wide and will run along the length of where the cofferdams will be built. On the north bank, at the town of Coonagh, the stagings will be 121m long and to the south, near Bunlicky, they will be 132m long.
Pre-fabricated steel frame sections form the structure for the jetties. These are welded into place on steel tube pile foundations installed at up to 20m depths before a wooden decking is laid on top for the crane platforms.
Rig operators are installing the same 1.42m diameter modified steel tube piles to form the combi-wall, each separated by two Hoesche L605 600mm sheet piles. The tubes have steel clutches welded along their lengths making it possible to slot together the sheet piles on both sides. 'Essentially the structure is a huge box and all the piles do is hold it open while the tunnel is being built, ' says Courts.
The availability of steel has not been the only challenge for the project team. A variable ground profile posed further problems for rig operators installing the large diameter piles at up to 27m depths. 'The whole point of building the jetty is because we know the ground here is really poor, ' says Dew general foreman Mick Little.
About 17m of very soft alluvial silt clays lie above a 1m band of slightly firmer clays, on 500mm of weathered limestone, before more competent limestone appears. All piles are founded on competent rock.
But the narrow band of weathered limestone is not enough to support some of the cofferdam steel tubes. The designer came up with a plan to install 4m long, 508mm diameter steel dowels grouted into 600mm rock sockets extending 2.25m below the toe of the steel tubes.
Site workers use a percussive down the hole hammer to create the rock socket. Concrete is then used to secure the dowels in the centre of each steel tube.
'It's quite a complicated solution, but there was no way around it because we are reaching competent rock so quickly and the tubes are providing most of the support to the structure, ' says Courts.
A 160t Liebherr crane and 150t Hitachi Sumitomo crane are doing most of the work to install the piles. Each crane picks up a hammer weight (2.5t for the sheet piles and 9t for the tube piles) that clamps on to the pile.
These use an eccentric rotation motion to vibrate the pile to the limestone rock head.
Once piles are in place, two Casagrande rigs - a B250 and a B150 - use a digging bucket or an auger to excavate material from inside the tubes. 'The digging bucket rotates inside the tube and as it goes down it slices softer alluvial material into the bucket like a cheese slicer, ' says Courts. Rig operators switch to an auger - similar to a continuous flight auger - when digging out firmer clays from inside the piles.
Sheet piles founded in weathered limestone provide a sufficient water seal. Combined with the tube piles, they act as a water-resistant structure.
Piling work is due to be completed by August when rig crews will finish installing 132 of the tubular piles up to 23m deep on the south side and 116 up to 27m deep on the north side.
The whole scheme is due to be completed by mid-2010.