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River Liffey Service Tunnel

Large areas of the neighbouring city districts have been integrated in a modern distance heating network in conjunction with a scheduled extension of the Poolbeg power Station in the form of a waste incineration plant in the east of Dublin. The River Liffey, the city's natural artery, divides the north and south parts of the districts being provided. In order to connect the two part-networks to the north arid south of the river, Dublin City Council decided to build a cable tunnel for supply and disposal purposes. The location of the new tunnel was chosen with regard to an existing cable tunnel running parallel to the East Link Bridge. This tunnel was constructed to accommodate the main gas pipeline Poolbeg-Abbottstown (600 mm diameter), then filled with concrete and sealed towards the shaft. The former access shaft for the tunnel that was completed in 2000 was not backfilled so that the follow-up tunnel could also be driven from this pit. The contract was awarded on the basis of the JV's special proposal that the tunnel be driven by pipe jacking instead of the segmental method contained in the original tender. Prior to tunnelling operations the existing, flooded access shaft, which was roughly 18 m deep, had to be secured by external sleeve pipe injections, a shotcrete inner shell installed from a pontoon together with 12 rock anchors against uplift without the main gas pipeline exposed in the shaft being affected by settlement. The 20 m deep target shaft was set up by means of overlapping bore piles (900 mm diameter), which were integrated some 3 m deep into the rock zone. The tunnel was then driven by the pipe jacking method using a shield machine with fluid-supported face as a planned spatial curve. Single-section reinforced concrete pipes (precast parts) were applied for the lining, which possessed a 25 cm wall thickness, a length of 2.5 m and a minimum concrete strength class of C45/55 as well as being highly resistant against chemicals. Altogether three stretching Stations were used. The geological investigations revealed that a roughly 6 m high layer of fills was to be reckoned with, followed by an approx. 4 m thick sand/ gravel layer as well as an 8 to 10 m thick layer of boulder clay. Although occasional intrusive blocks were predicted, they were only rarely encountered. Instead the extremely high fine grain content of the resultant suspension caused a number of disturbances during separation. Groundwater was encountered between 2 and 4 m below sea level on account of the location on the river estuary. As a consequence all Installations were situated in the groundwater and had to be correspondingly sealed to cope with up to 1.5 bar water pressure.

 

  • Country: Ireland
  • Region: Dublin
  • Tunnel utilization: Utility
  • Type of utilization: Cable Tunnel (distance heating as well as various utility lines e.g. water, electricity and communications cables)
  • Client: Dublin City Council
  • Consulting Engineer: Atkins
  • Contractor: Ed. Züblin AG/Cleary & Doyle Contracting Ltd.
  • Main construction method: Trenchless
  • Type of excavation: Shield machine (SM)
  • No. of tubes: 1
  • Tunnel total length: 260 m
  • Cross-section: 2.50 m (internal diameter), 3.00 m (external diameter)
  • Contract Volume: approx. 8 mill. euros
  • Construction start/end: 2006 till 2008
  • Opening: 2009