Conwy Tubular Railway Bridge (Underbridge No. 110), which carries the North Wales coast railway line across the River Conwy between Llandudno Junction and the town of Conwy, was built by Robert Stephenson in 1848.
The superstructure is formed from two wrought iron tubular box girders which carry the railway lines, with masonry castellated abutments that compliment the adjacent Conwy castle. An assessment in 1899-1900 showed that the tubes were theoretically overstressed under loadings from the heaviest locomotives running at the time, and this entailed the construction of additional piers at each end of the tubes at points of the span.
Kaymac Marine & Civil Engineering Ltd were commissioned by Birse Rail to carry out works to remove two existing inspection gantries from the Conwy tubular railway bridge. The gantries were deemed redundant by Network Rail and required removal from both tubes. Conway Tubular Bridge is a listed structure set within a SSSI area.
Due to access restrictions for plant and equipment on to the top of the tubular spans, it was decided early in the project to utilise floating plant to systematically dismantle the inspection gantries. Therefore a modular pontoon system was mobilised to site consisting of 16 Linkflote units, which when assembled, resulted in a work barge with a deck area of 210m², complete with 2 no. 12m long spud legs and winches. Due to the high rate of tidal flow below the bridge structure, the support vessel ‘Sabrina’, a converted landing craft equipped with 600hp engines, was utilised to mobilise the work barge into position below the structure.
To allow access from the work barge to the gantries, 2 no. Genie Z34/22 articulated boom MEWP’s were mobilised from the shore to the work barge on board ‘Sabrina’. Each MEWP had the hydraulic oil replaced with a vegetable based replacement prior to mobilisation. The MEWP’s were then used to allow IPAF trained personnel to reach the structures and rig 4 6.5t air hoists at both sides of each tube.
Each air hoist weighed in at 170kg, and was lifted up the outside face of each vertical section of gantry using chain hoists. With the air hoists in position and securely rigged, the lower underslung sections of the gantries were flame cut from the vertical elements working from the MEWP’s. Once separated, the underslung sections were lowered onto the deck of the work barge on the air hoists for disposal.
Before removing the side sections of the gantries, the trolleys on which they were originally moved across the tubes were secured to their rails with cam type strops providing 14t of down force. This was to prevent any overturning moment of the trolleys during the dismantling process.