Work is now under way on a £1.6 million project to replace bearings under the bridge deck of the Clyde viaduct.
The three span, 90-metre, steel structure stands on three sandstone piers above the River Clyde near Uddingston and carries traffic on the strategically important West Coast Mainline, as well as local services.
The bearings sit between the bridge deck and the piers and serve to control movement in the structure and absorb stresses generated by trains travelling above.
With the viaduct now more than 140 years old, and given the weight and volume of trains using it on a daily basis, the existing bearings have begun to show signs of deterioration.
Twelve sliding bearings in total, four on the east abutment and eight on the west pier need to be replaced. The new steel bearings are one metre long and half a metre wide and only about 10cms deep but weigh in excess of 140kg each. Because of their modern design, they are less bulky than those they are replacing and will be set on pre-cast slabs which will have a combined weight of around 1.5 tonnes each.
The team has been on site for three months undertaking preparatory work on the bridge before even touching the bearings themselves. As well as creating the scaffolding and hoist system, the team have strengthened and braced the bridge and prepared sections of the structure around the bearings in readiness for jacking-up for the work – with the first bearing set to be removed on Saturday 1 September. Work will be on going until December and delivered with no disruption to train service.
The weight involved means that a bespoke system of getting the bearings to and from the bridge then up onto the structure needed to be developed using a motorised all terrain pallet truck and a pulley and rail system to move the old bearings out and the new ones into position.
As well as the logistical challenges of working at height above a river, the Network Rail team and specialist contractor Taziker also have to work within the restricted space under the bridge deck to firstly insert hydraulic jacks to take the weight of the bridge and raising it a maximum of 5mm. The bolts will then be drilled out and the old bearings removed and replaced.
One bearing will be removed and replaced at a time over the next 12 weeks with three of the 12 only able to be replaced when the line is closed. The other nine will be removed and replaced while trains continue to run above.
Jeremy Spence, Network Rail’s programme manager, said: “Our structures are subject to a regular programme of inspection and routine minor maintenance which allows us to monitor their condition and react if any faults develop or the condition deteriorates to a point where we need to intervene.
“Having reached that point on the Clyde viaduct, the challenge from an engineering perspective was to come up with a way to replace the bearings while keeping the railway open.
“Every bridge we work on has its own set of challenges but replacing bearings on this type of structure within the constraints of the railway environment is unique; logistics, engineering, the physical size of the space we have to work in and the sheer weight of the bearings going in and coming out. Add to that the element of the unknown in how a century old bridge deck will react to being jacked and how easily, or not, the bearings will be removed, has certainly focused minds on this task”.
The scaffolding will be removed and the site compound cleared by December 2018.