Parapet collapse ‘may have been caused by weight of ballast’
The weight of extra ballast put under tracks may have caused the Aberdeenshire rail bridge parapet collapse, according to industry experts.
The theory came as Network Railsaidrepairstothe180-yearoldbridge–amilefromthefatal landslide collision site near Stonehaven – would take until February 22.
Thelinebetweenstonehaven and Montrose has been closed since January 15 when an 80ft long (24m) section of masonry fell from the bridge parapet or sidewall.
Industrycommentatorphilip Haigh, writing in RAIL magazine, said: “Over the years, the track engineer had dropped more and more ballast to keep the track level. It might be that this increased burden of ballast has increased pressure on parapets not designed for such a load. To this should be added the effect of drainage.
"If the increased burden absorbs water, it increases its overall mass and thus the force on the parapets. In cold weather,itexpands(ortriesto),which again increases the force pushing the parapets outwards.
"There are doubtless other factors which Network Rail’s engineers will be investigating.
"Myworryisthatifmyhypothesis is anything like right, then Network Rail might find itself with an awful lot of work to do to remove excess ballast from its masonry arch bridges.”
Rail engineer Gareth Dennis saidiftheballasthadincreased, it could have been a contributory factor.
He told The Scotsman: “Photos and inspection reports show the ballast level was far above the level of the structural masonry. Ballast (the stones that support the track and hold it in place) gets dropped when the track position is corrected.
"Over time, this means the track level rises, and on this bridge, with the track being on acurve,theballastlevelishigher on the outside of the track.
"After three or four passes of this type of maintenance, the ballast is supposed to be renewed and the track level reset to its original position.
"If this hasn't happened here, then that could certainly be a contributing factor.
“What is more, when the speed was increased along this stretch of track some decades ago, this would have likely beenachievedbyliftingonerail higherabovetheother(abitlike a road tilting through curves), whichwouldalsoresultinmore ballast being piled up against the parapet.”