Lisnamuck Bridge Replacement, Tobermore

Tobermore, N Ireland

Demolition of the existing 3 span continuous reinforced concrete beam and slab deck of the existing Lisnamuck Bridge; the construction of a new twin span fully integral prestressed beam and slab superstructure on reinforced concrete abutments and central pier; followed by the reconstruction of the B42 Lisnamuck Road over the Moyola River.

The Lisnamuck Bridge in Tobermore carries the B42 Lisnamuck Road over the Moyola River. FP McCann won the £700,000 Roads Service contract to demolish the existing 1931 three span reinforced concrete bridge, and replace it with a new two span structure comprising prestressed, precast concrete beams bearing on new abutments and a new central pier in the river. The Moyola River flows from the Sperrin mountain range to Lough Neagh and is renowned for good sport and leisure fishing of trout, eels and salmon.

The contract included a number of environmental constraints and considerations, and consultations. The river was initially diverted away from the proposed location of the new central pier using an array of FP McCann precast concrete pipes, and a causeway of clean imported rock fill from our quarry at Knockloughrim. This initial works was carried out within an environmental window outside the fish spawning season.

The river is prone to flash floods with levels regularly rising by 1.5m in just a few hours and peaking very quickly after rain on the surrounding hills. It was a condition of the Rivers Agency consents that the causeway was designed to permit over topping prior to any flooding the surrounding lands. After demolition of the original 1931 bridge the construction of the new central pier commenced with excavation to bed rock. An impermeable bund was constructed on the causeway and excavation to approximately 2m below river bed level was achieved using pumping, dewatering with supplementary sheet piling.

Water quality and turbidity, upstream and downstream of the works was independently monitored throughout the contract to ensure the operations did not affect the vital environmental river balance. The 22 precast concrete beams were erected using a 500t mobile crane.

The 4 edge beams which were cast with an integral parapet upstand weighed over 38t. The 18 intermediate beams were approximately 18t. The beams were temporarily supported around the central pier until the deck and a central reinforced concrete diaphragm could be poured. The 145 cubic metre concrete pour was supplied from our Knockloughrim batching plants in under 4 hours. The new bridge was completed on time and within clients budget.