Slieve Bignian tunnel

The Department's description

Exterior description and setting

The tunnel conveying the headwaters of the Annalong River into the Silent Valley reservoir emerges from the west side of Slieve Bignian through a segmental opening edged with a stepped vee-jointed concrete blocks. The rock above the opening is also faced with vee-jointed blocks in regular courses, above which is a section of shuttered concrete bearing the date ‘1952’. The wall is topped with ashlar granite blocks. The intake to the tunnel is in the Annalong Valley - see HB16/01/041.

The water flows along a concrete-lined open channel into a circular concrete pipe underneath the road and bank which feeds into the reservoir. The parapets on either side of the road are of roughly dressed granite blocks in regular courses with quarry faced copings. The east parapet bears a bronze plaque commemorating the tunnel’s opening in 1952. The end of the pipe is embellished with a horseshoe shaped trimming of vee-jointed cement blocks. The bank is similarly faced. The water discharges onto a shallow apron pitched with large granite blocks.

Historical information

Originally the Belfast Water Commissioners envisaged that there would be a reservoir in the upper reaches of the Annalong Valley. However, after encountering major geological problems with the construction of the Silent Valley dam, this idea was abandoned in favour of a tunnel through Slieve Bignian, which would divert the Annalong River into the Silent Valley.

Excavation of the tunnel commenced in April 1948. The contractors were Messrs A.M. Carmichael of Edinburgh, and the consulting engineers Messrs Binnie, Deacon and Gourley of Westminster. Work was carried out simultaneously from both ends, and the two headings met in December 1950. The tunnel is 3.4 km (2 1/8 miles) long with an overall fall of 5.5m (18ft). It is 2.3m (7ft 6in) wide by 2.45m (8ft) high and the walls are lined with concrete to a height of 1.25m (4ft). Viscount Brookeborough, Prime Minister of N. Ireland, opened it on 28 August 1952.

Primary sources:
1. Plaque on bridge at tunnel exit, and also on 1950 screening chamber below Silent Valley dam.
2. Original drawing of the tunnel and weir in DOE Water Service, Westland House, Old Westland Road, Belfast (cat. No. 2F1).

Secondary sources:
1. J.A. Robbie, ‘The Slieve Binnian tunnel: an aqueduct in the Mourne Mountains, Co Down’, in Bulletin Geological Survey of Great Britain, vol.8, pp.1-20. (1955).

Criteria for listing: architectural interest

Not listed

Criteria for listing: historic interest

Not listed
Not listed
Not listed

Evaluation

The tunnel was a novel solution to the problems encountered with tapping the waters of the Annalong River and is the longest tunnel in N. Ireland. It is of national interest in being part of the Mourne Scheme to supply Belfast with water. It also adds interest to the landscape where it emerges. PHB have asked that consideration be given for inclusion on Schedule of Historic Monuments by Protecting Historic Monuments.