Friday 20 December 2013

Venting some heat

The Boy, after educating himself, is now a railway engineer - signalling - working in the West of England. He'd have a different job if it wasn't for structural engineers like George Stephenson, who built the Leeds and Manchester railway between 1937 and 1841.  We've met him before at the birth of railways.  At that time Manchester (in red-rose Lancashire) and Leeds (in white-rose Yorkshire) were leading the industrial revolution in Britain as Britain led the world.  The Wars of the Roses are still fought on the footie field in historically correct shirts (red for Lpool and Man U, white for Leeds).  These two great cities are on different sides of the Pennines, a range of hills that runs up the spine of North-central England and that makes communications a little difficult.

Stephenson followed the route of the existing Rochdale canal but at the highest point he chose to drive a Summit Tunnel through 3km of the hill-top - you can't run trains like canal barges up through a stair-case pf locks.  It was in its day, the longest tunnel ever driven, a massive engineering feat: dug by pick and shovel through a geological slice of coal, shale and sandstone which are all comparatively soft.  To keep the tunnel on track a series of shafts were surveyed and sunk vertically from the ground above
You can still see the chimney tops of these shafts today because they were left open to vent the steam from the many passing trains - it was designed as a two-way tunnel, so busy was the route anticipated to be.  Shale and sandstone are easy to dig but also tend to collapse and to prevent this Stephenson lined the inside of the tunnel with 6 courses of bricks laid with "Roman" mortar that would tolerate the weeping damp that was also foreseen.  6 onion-layer courses of bricks over 3km amounts to 23 million bricks laid at a maximum rate of 60,000 a day.  The bricklayers were working by candle light for 9/- to 11/- (that's shillings @ 20 to the £) a day. That's cheap, as were their very lives in those days before Health & Safety - 41 men died, almost exactly one a month on average, in the construction of the tunnel. Which is a little ironic in that Stephenson is quoted as saying "I stake my reputation and my head that the tunnel will never fail so as to injure any human life".  But they did good work those craftsmen, even if some said that the brickwork was rather over-engineered.

The bricks were certainly put through a stress-test (short youtube) on 20th December 1984 when a train loaded with a million liters of petrol derailed in the tunnel and ignited. The train-drivers were able to decouple the first three tanker-cars and drive them out of the tunnel, and the fire-brigade were on the scene from Manchester and West Yorkshire very quickly but they had to deal with something like 500 tons of burning fuel in an awkwardly confined space.  They tried to hose down the fire until they saw the heat of the flames spalling off the outer layer of bricks. As the temperature rose, the safety valves on successive trucks opened and more fuel was discharged.
The firemen then retreated and tried to starve the fire of air by pumping foam down the ventilation shafts. This worked to a certain extent: the hot vaporized petrol whoomphed at 150km/h up other shafts where it ignited on striking the fresh cold air of hilltop.  Flames 50m high ejecting burning detritus and white-hot melting brick must have made the hillside look like Mordor as it set fire to swathes of the vegetation. It was conflagration far greater than Los Alfaques (only 20 tons of propylene) but perhaps less explosive than Sailor Hat and nobody was killed. The bricks melted off the lining of the shafts into heaps on the rail-bed.  BUT, the structural integrity of the tunnel was preserved and eight months later the railway was again open for business. Hats off for engineers.

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