Contractors have used an innovative technique to install a pipe under the dual carriageway while keeping traffic flowing freely on the A55 in Gwynedd.
The team from Jones Bros Civil Engineering used the pipe jacking method to tunnel a larger floodwater drain from north to south near Abergwyngregyn.
The £1m Welsh Government funded project, which has taken a total of 22 weeks to complete, included earthworks, fencing, 1,600m of open trenching lined with concrete canvas and the upgraded culvert.
The pipejacking method was chosen by the contractor and YGC (Ymgynghoriaeth Gwynedd Consultancy) after analysing alternative options and weighing up their respective risks.
Jones Bros’ regional manager for North Wales and North West England, Hefin Lloyd-Davies, explained: “Surface water currently flows under the A55 in an existing 300mm culvert, from south to north towards the sea.
“The capacity of the existing culvert needed to be increased. Originally, we planned to drill a 600mm hole under the A55 to install a new culvert. We investigated and found a high number of large boulders, which can prevent drilling.
“Instead, we changed the method completely from drilling to a hand excavation method, which gave us the best chance to eliminate the risk from the boulders.
“We brought in tunnelling specialists to carry out pipe jacking. We upsized to a 1200mm pipe, which also helps overcome any problems of encountering large boulders.
“We built a launch pit at one side of the highway and a receiving pit on the other side.”
Hefin added: “The pipe jack method allows a front shield to be taken up to the face of the excavation. The operatives then work from within the round 1200mm steel shield to excavate the diameter required and install the pipework.
“As the operatives progress, the 1200mm steel shield is pushed forward using hydraulic rams.
“Each time a section of two metres is excavated, special pipe jacking concrete pipes are installed behind the cutting shield.
“It is the first time I have used this pipe-jacking technique in highway construction. It is a rare solution that was just right for the issues encountered on this contract.”
YGC’s project manager, Dave Meller, said:
“The A55 is a vital strategic transport route in North Wales and the Welsh Government want to minimise disruption to road users. There is a complete embargo on daytime non-emergency works on the route through the summer, and considerable pressure to avoid lengthy lane closures at all times.
“We therefore sought a suitable trenchless construction method for installation of this larger pipe, which itself serves to reduce the risk of this section of A55 being closed due to flooding.
“The road surface was monitored regularly throughout the tunnelling process to detect any settlement, and no significant movement has been measured.
“This pipe jacking technique offers opportunities for future work on this and other key routes to avoid any delays to the travelling public.”
Founded in the 1950s and employing almost 350 people, Jones Bros has grown rapidly in the last decade. It is currently working on contracts in various sectors including the construction of waste management facilities, highways, flood and marine defence and renewable energy projects around the UK.