Engineers save Lamington Viaduct from total collapse
LAMINGTON Viaduct was “in a total state of collapse” before being rescued from the waters of the River Clyde, reports the leader of the repair project.
Speaking exclusively to RAIL on January 22, Network Rail Route Delivery Director Stewart MacPherson revealed that “the bridge would no longer be here” had work to fix it not started promptly after defects were first reported on New Year’s Eve.
He added that engineers have now won the race against time to prevent the bridge’s complete collapse, despite the scale of damage proving far worse than initial assessments had indicated.
The problems at Lamington first surfaced on December 31 2015, when a passing train driver spotted a dip in the track above pier two (the piers are numbered one to three from the bridge’s southern end on the right-hand side). Track maintenance staff investigated, and all services were suspended once missing masonry was spotted just above the waterline.
“Storm Frank created a torrent of water, and flows were so large that water came up almost as high as the bridge deck,” said MacPherson.
“We immediately began placing rock armour around pier two, but at that stage we did not know the extent of the damage.”
Following heavy rainfall from Storm Frank on December 29 and 30, water levels in the River Clyde were recorded at 3.124 metres high on December 31, compared with a yearly average of 0.69m. It was only on day three (January 2) that divers were able to properly inspect the piers beneath the waterline, once floodwaters had receded to safe levels.
Their survey revealed the depth of scour to be 2m in places, along a width of 25m. Underwater stonework from the original parts of the piers (built in 1850) had fallen away, while the more resilient concrete areas (built as an extension in 1938) had survived but with severe scouring of the earthworks beneath.
“The whole footprint of the pier was heavily compromised, and the bridge was only bearing down on about 40% of the pier base,” added MacPherson.
“The whole pier was rotating and about to fall over, so we imposed a total exclusion zone beneath all the bridge spans until day 12, when the structure could be stabilised.”
With access to the viaduct’s west side (by going beneath the bridge) unavailable, a haul road was constructed over the top
of the adjacent embankment. This allowed engineers to begin constructing causeways across spans one and two on both sides of the viaduct, to reach pier two, and to start pouring 300 cubic metres of concrete into the voids.
At this stage Network Rail was still targeting a re-opening date of February 1. “Unfortunately, while we were doing this, there was a spike in the river flow from a tributary of the Clyde two miles upstream,” said MacPherson.
“700mm of water overtopped the causeways and, as we had blocked spans one and two, increased flow tried to get through spans three and four.”
Under increased hydraulic pressure, the leading edge of pier three fell away on January 10, although thankfully it was not part of the weight-bearing portion of the structure. Additional rock armour was placed around pier three to prevent further damage.
Relieving pressure on all three piers was now critical, and work began on January 12 to widen the width and angle of the river, distributing flow more evenly beneath the four spans. Higher flows caused by snowmelt briefly disrupted work on January 17, but by January 19 relief channels were dug in the causeway, allowing the partial re-opening of spans one and two.
MacPherson stressed that the damage sustained by pier three, although caused by damming parts of the river, was unavoidable - engineers had little alternative but to build the causeway to stop the whole structure from collapsing.
“Had we not done it, pier two would have collapsed and the bridge wouldn’t be here. Pier two would have moved further, leading to a catastrophic collapse. It really was a race against time.
“We are pleased with how we’ve tried to manage the flow of the River Clyde. If someone could show me a way of holding back the waters, then we could have gone in and fixed things in just five to six days. The work is not difficult, but it is difficult getting safe access.”
To re-open the bridge by the beginning of March, 130 rock anchors (steel cables) are being drilled to depths of eight metres around each pier. These will hold down concrete jackets that are being constructed to encase the damaged piers. The bridge will also need to be re-aligned, after being deflected by the movement of the piers.
“We will need to jack the bridge back up and put it into position,” added MacPherson. “Its steel bearings are designed to move slightly for temperature variations, but one in pier two has been moved 254mm in a horizontal direction, breaking it apart. Only then can the final part of the project get under way, and damage to track and overhead wires be addressed.”
Network Rail has committed 150 contractors, designers and engineers to ensure the bridge can open as soon as possible. Three consecutive eight-hour shifts are in operation per day. @paul_rail
For more updates on this story see www.railmagazine.com.