Peter Kenyon and Shani Wallis, TunnelTalk
- Dublin in Ireland has put the tunnel and civil engineering industry on standby in readiness to tender a design-build contract for construction of a 9km wastewater outfall tunnel under Dublin Bay.
Launch shaft location at Ringsend Treatment Plant
- The estimated €250 million (US$306 million) final phase of the Dublin Bay project comprises an increase in the capacity of the Ringsend Wastewater Treatment Plant and construction of a 5m i.d. gravity-fed outfall tunnel capable of handling discharge rates of up to 13.8m3/sec.
- The tunnel will run out from the treatment plant under Dublin Bay at a depth of up to 70m below the seabed. Ground conditions are expected to consist mainly of stable Calp Limestone of up to 170MPa in unconfined compressive strength, although the area also includes several fault lines.
- According to conceptual designs completed by project consultants CDM Smith and J B Barry & Partners, the project includes, along with the tunnel, an access shaft of 20m-25m diameter and some 60m deep with a 4m i.d. riser shaft of about 50m deep to discharge effluent at about 24m below sea level.
- The project has required one of the most comprehensive geological investigations of the bay. "There were no borehole drillings into bedrock in the Bay prior to this project," explained Anthony Kerr, Technical Director for the project with CDM Smith. "Our site investigation programme, of 22 deep boreholes and various geophysical surveys, has resulted in extensive revision of earlier assumptions about the bedrock and its interface with the upper, highly variable and soft sedimentary deposits. Interpretation of the data is still ongoing and studies confirmed that a major fault towards the riser end of the tunnel is actually further out into the Bay although the access shaft near the Ringsend treatment works on the Howths Peninsula does lie close to the Howths Fault." Early studies also indicate that the drive will face significant challenges due to heavy rock fracturing and high permeability ratings particularly at the access shaft site.
Fig 1. 9km undersea outfall tunnel alignment
- Constraints imposed on drilling boreholes in the shipping lanes into Dublin Port introduced further risk. While poor bedrock conditions have been identified by the onshore land-based boreholes, bedrock conditions 1km out to sea are classified as fair to good. What is not known is how far into the proposed alignment will rock conditions improve.
- An additional borehole, completed in June, explored a 2.5km-long section of the alignment that was excluded by shipping lane restrictions from the earlier drilling programme. Interpretation of the cores is currently progressing and in particular, a suspected fault somewhere along this part of the alignment is being investigated.
- Construction methods are not specified in the environmental and planning approval documents but the expectation is that a TBM will be used to excavate the proposed 5m i.d. tunnel and install a precast concrete segmental lining of about 400mm thick.
- "Various tunnel alignments were considered," said Kerr, "with all located in the limestone bedrock. The horizontal alignment will be fixed by the planning approval with the vertical alignment open to alteration by the successful design-build team. The proposed 9km length of the outfall is determined principally by water quality criteria at the point of discharge."
- Early studies indicate that much of the tunnel length through the stable limestone could be advanced without active face support. There is however the possibility of faults and fractures connecting the heading with the full hydrostatic head of the bay. "These present considerable safety concerns and will influence the choice of excavation method with a TBM operation including perhaps closed-face slurry or EPBM technology," said Kerr.
- CDM Smith and J B Barry & Partners completed the environmental impact assessment in early 2012 and Dublin City Council, the project owner, submitted a detailed application to the Irish planning authority in April. Once approval is granted, pre-qualification of design-build teams will begin. The tunnelling and shaft sinking contract is expected to attract design-build proposals in the range of €130-180 million (US$159-220 million).
Core of heavily fractured rock conditions at launch shaft site
- Despite the economic difficulties in Ireland, that caused cancellation last year of underground metro projects in Dublin, the outfall is a high profile project and likely to go ahead. As well as meeting future demand, the project is required to comply with the European Union Urban Wastewater Treatment Directive to remove the current treated water discharge from the sensitive Liffey River Estuary and take it to a point further offshore where strong currents will readily dissipate the treated effluent.
- The project is identified as "one of three or four strategic infrastructure projects," explained Kerr, and its funding is reported as being secure. Complying with EU regulations also maintains the momentum and keeps the project moving forward.
- There are no independent water companies in Ireland, and while there is talk of creating a national water authority all major capital works at present are funded directly by the central government.
- In a press statement released in June, Pat Cronin, Executive Manager of Dublin City Council said: "As owner of the project, we are proposing this extension to meet current and future requirements for treating our wastewater. These proposals will allow the Ringsend Plant to treat wastewater for a population equivalent of 2.1 million which is some 30% above current capacity."
- If the project stays on its current programme, a design-build contract for the project could be awarded by late 2013, with construction starting in early 2014, and the new outfall coming on-line in 2016.
Tunnel award seals Irish gas project - TunnelTalk, June 2012
Dublin drops €5.6 billion metro projects - TunnelTalk, December 2011
Coping with complicated conditions on the Dublin Port Tunnel - TunnelTalk, September 2002
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