Dam excavation shaft and trench shuttering, Silent Valley Reservoir Scheme, Head Road, Kilkeel, Newry, Co Down, BT34 4PU

Dam excavation shaft and trench shuttering, Silent Valley Reservoir Scheme

This is an exhibition feature demonstrating the construction methods used for the Silent Valley dam, comprising cast-metal shafting and shuttering set into a terrace overlooking the discharge channel.

The structure consists of two circular shafts positioned 10.5 metres (34 feet 6 inches) apart, extending to a depth of 2.3 metres (7 feet 6 inches). These are separated by two parallel walls of segmental cast-metal plates spaced approximately 1.8 metres (6 feet) apart. Each shaft is formed from a series of rings, each 3.8 metres (12 feet 6 inches) in external diameter and 0.46 metres (18 inches) deep, with internal flanges bolted together. Each ring comprises ten cast-iron segments, also bolted to one another on internal flanges. The segments are embossed with "RP&S" (Pearson & Son, the contractor). Timber baulks run between the segmental walls, and a post and wire fence surrounds the structure.

Though not strictly in situ and incomplete, these items are considered to form an integral part of the dam grouping because of their close spatial proximity to the dam and their historical and technical interest.

The shafts were excavated between 1927 and 1929 by S. Pearson & Son (London) as part of exploratory work to establish bedrock for the dam foundation. The original plans by the Belfast City and District Water Commissioners had anticipated bedrock at approximately 50 feet below ground level based on corings, but these had struck glacial erratics. When contractors found it impossible to reach true bedrock at this level and encountered water ingress, the matter went to arbitration and work continued on an exploratory basis. Various dewatering methods were attempted before these vertical shafts, filled with compressed air, provided the solution. The shafts served two purposes: to establish bedrock into which a cut-off trench (to prevent water seepage under the dam) could be excavated, and to dewater the area around the intended embankment so that overburden could be removed and the cut-off trench excavated. At one point bedrock was found to be 212 feet below ground level. Segments were added to the bottom of each ring as excavation progressed, with seepage minimised by compressed air maintained inside the shafts via an air lock and decompression chamber at the top. Once bedrock was reached and groundwater pumped out, the overburden was removed. Excavation of a 6 to 7 foot wide trench in the rock then began. As this progressed, segmental castings were bolted together between the shafts along the trench walls, with timber baulks maintaining uniform spacing. Bedrock was excavated to depths exceeding 50 feet, and the shuttered trench was filled with concrete between 1929 and 1930. The upper sections of the shafts were subsequently removed prior to formation of the earthen dam above the trench; the bottom sections were retained for use as shuttering for the concrete trench.