what if this was after TSHTF?

[NorthernRaider]

Quite correct Dev, We have loads of those stone arch bridges in Co Durham , providing they are not under scoured or battered by fallen trees from upstream, which has happened to one in Teessdale they should last for centuries, but the roads leading up to them are far more vulnerable if not trafficked and maintained. The bridge in Durham just under the Cathedral wall is typical of the design and providing the drains remain clear and don't clog up they will go on for another 1000 years BUT if the drains do clog up and no one clears them each winter the water trapped on the deck area causes ice fractures and damages the mortar as well as drives the stones apart. But even though these bridges may stand eternal the weak point is the roads and paths around them.

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One of the Railway viaducts up here was built in the 1800s by the NE Railway, it served the railway for over a century, but after the Beaching cuts it was abandoned, Unfortunately no one bothered to keep the drain systems clear and within ten years the road surface was wrecked but worse thee water each winter had escaped the drains and worked the way into the brick arches themselves. By the time the lads who wanted to open up a Heritage Rail line got around to looking at the viaduct it was uneconomical to repair and it had to be demolished. Yet oddly enough another viaduct that was almost identical but its drains stayed clear was in such good condition that the council turned it into a roadway and its still in use.

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Dev what I think you MAY be missing is not the vulnerability of solid great stone, concrete or steel structures standing in magnificent isolation, but the roads and rail tracks that serve them, thats the weak point, roads and rail track beds are lined by fields, trees, buildings, foliage, they are often with drains, sewers, storm drains, cable ducting etc under them. they face a continuous onslaught from play overgrowth ( Think how destructive IVY is at ruining mortar and brickwork) flooding, leaf fall, land slip, erosion, etc etc etc The bridges may last a 1000 years but without maintenance and regular traffic the roads and trackbeds will disappear.