On 21 February, the route of the 9.5km long revised length tunnel for the Belfast Sewer Project in Northern Ireland was announced in advance of the March tender for the works (T&TI, March 2004, p7).

As part of Belfast’s upgrade to its sewerage network and bid to improve the quality of the Lagan and Blackstaff rivers, the shortened tunnel, from the previously mooted 10.5km, of up to 4m in diameter will be driven at a depth of up to 30m. The tunnel will run from the northwest to the southeast of the city, with two feeder branches to the River Lagan.

The tunnel route is largely in clay strata with some incursion into the sandstone bedrock where dolerite dykes exist. A geophysical survey was carried out as part of the planning and design to locate the extent of the dykes. High water inflows are not expected.

The design includes 530m of 1.5m diameter pipejack, with a further 1,250m of 1.8m diameter pipejack. Bored tunnel lengths are 3,805m of 2.1m diameter tunnel plus 3,800m at 4m diameter. The shafts vary from 35m diameter for the terminal pumping station, to between 12m and 1.6m for shafts along the route. Of the 27 shafts included in the scheme, the typical diameter is 7.5m.

Katharine Bryan, chief executive of Northern Ireland’s Water Service, announced the route saying, “structural repairs to 500 sewers will commence in April 2005 and construction work on the tunnel will commence in early 2006. Following completion of the tunnel further strengthening of the network will involve the upgrading of 19km of sewer.”

The prequalified contractors for the tunnel will be confirmed later this month, with invitation to tender for the works following shortly. Tender award for the three year contract is planned for October. Water Service has not held any discussions with TBM manufacturers and told T&TI the selection of machinery lay with the tenderers.

In late November last year, a US$38M contract for the sewer repairs was awarded to the John Graham (Dromore) Ltd. and Dawson WAM/McAllister JV. The contract will be implemented over a five year period.

The project will result in the closure of a significant number of combined sewer overflows and give a reduction in the pollutant load on the River Lagan and its tributaries of 85% compared to present levels.



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