Demolition of Mossband Viaduct, Carlisle

Since the mid 1990’s, the main trunk road on the west coast of England for drivers heading to Scotland has been the M6 motorway. However, the motorway ends at Junction 44 near Carlisle, and a 9km stretch of the A74 dual highway continues before reaching the A74(M) just inside the border at Guards Mill near Gretna Green. In 2005, work started on a contract known as the M6 Extension to uprate the old A74 and provide drivers with a continuous 640km of motorway stretching from Dover to Scotland.

The old A74 was built in the 1970’s and while the roadways could be widened and realigned, the old concrete viaduct that carried the road over the West Coast Main Line railway had a history of corrosion in its post-tensioned tendons and could not be incorporated into the new scheme. It was redundant and demolition was the only answer.

The design of the old viaduct consisted of a number of 45.00m long x 10.30m wide deck sections, each weighing approximately 1000t. With an actual span of only 33.52m and constructed with suspended span bearings, clearly a novel approach was needed to demolish this aging structure, particularly because it was located both adjacent to and crossing over a live railway line. The main contractor, Carillion, decided to use a pair of Self Propelled Modular Transporters, (SPMT’s), to lift the deck sections and transport them approximately 20m sideways where they would be positioned onto temporary supports. From here, the SPMT’s would be removed and the sections would subsequently be lowered to the ground hydraulically before demolition.

However, from a temporary construction point of view, the bridge decks were not designed to be supported on the 19.0m span effectively dictated by the SPMT’s and an additional and substantial amount of external post-tensioning was required on the top deck to keep the structure stable during handling. To meet this requirement, the main contractor designed and specified 84 type 12 x 47mm Ø St 950/1050 tendons, each 36.0m long, installed on six of the bridge decks. New concrete restraint blocks 0.6m deep x 1.5m wide x 5.5m long were cast on top of the existing deck and the tendons passed through ducts at 300mm centres and 300mm above the existing deck.

Tendon Installation
Special timbers cut to the right depth were laid across the deck to both support the bars and aid their assembly prior to stressing. Despite their 170kg weight, the 12.0m long bars were easily slid into position and fed through the ducts. However, to avoid rotating the heavy bars during coupling, the technique adopted was to fully thread a coupler onto one bar, align the other bar using a simple guide to maintain the correct pitch and then screw the coupler back over the two bars. Set screws in the coupler were used to ensure it was retained in its correct position and additionally the bars were pre-marked to show the correct coupler engagement and give DSI’s technicians an accurate and permanent visual reminder that the joint had been assembled correctly.

Tendon Stressing
Stressing was carried out in a specified sequence of 50%, 75% and 100% of working load before all the bars were locked-off typically between 570kN - 713kN, depending on the span. In order to keep the load in the deck balanced at all times, 4 x 1100kN stressing jacks were utilized in conjunction with a 4 way hydraulic manifold and a petrol powered hydraulic pump unit. The deck was surveyed continuously at the restraint blocks during the stressing operation and an increase in height of 2mm was recorded – exactly as predicted.

Tendon Destressing
The individual bridge decks were subsequently lifted by the SMPT’s and transported and stored on a specially constructed concrete apron adjacent to the railway line. Following lowering of the decks to ground level, DSI’s technicians were recalled to site to destress the DYWIDAG Bars in reverse order using the same equipment and technique as was used during the stressing operations. From here, the redundant bridge decks were demolished using a variety of techniques and removed from site.

DSI UK Technical Department had a long involvement with Carillion from the on-set and because of the specialist nature of the work, they were invited to install, stress and subsequently destress these tendons, all meeting the very tight and demanding programme.