Wednesday, 9 September 2015

9/10 September 1940: The London Underground on this day 75 years ago

Third day/night of the London Blitz.
"During daylight of 9th September, enemy action was almost entirely lacking, but from darkness to dawn on 10th September heavy attacks were made, and largely confined to London. The main attack appears to have developed on the City, and the areas near the river, and apparently one of the main objectives was the destruction of rail communications to the South."[1]
On the morning of 10 September it was realised that a potentially catastrophic danger to part of the Underground network had occured during the night.

In 1914, the original section of what is now the Charing Cross branch of the Northern line was extended to a new terminus at the then Charing Cross station (now Embankment) on the north bank of the Thames. Rather than reversing, trains were turned by means of a wide clockwise loop tunnel, part of which passed under the River. A single platform for both alighting and boarding was provided on the final part of the loop before the tunnel straightened out again on a northern alignment. A ventilation shaft also linked the loop to the sub-surface District line tunnels above.

When in 1926 the line was further extended to Waterloo, on the south bank of the Thames, it was by means of a new southbound tunnel bisecting the loop. A new southbound platform was also added, with the original one on the loop becoming the northbound platform (this is why the platforms are the distance apart that they are, and why one is curved and other straight). The new trackwork was not connected to the loop, but the tunnel was retained for ventilation purposes by means of the shaft to the District line, and so not completely sealed off, although watertight bulkheads were installed on the river side of the ventilation shaft, and where the other end of the loop met the new southbound tunnel, numbered 13 & 14 respectively.

During the air raid on the evening of Monday 9 September 1940 - possibly around 23:25[2] - the schooner Seven Seas, moored on the north-east side of Hungerford Bridge, was rocked by the detonation of a 50kg High Explosive (HE) bomb that landed in the River. The crew reported that, "the water appeared to spout up for some 30 seconds after the explosion," caused by air escaping from the ruptured loop. It was later discovered that water was seeping through bulkheads 13 & 14, further indicating that the disused tunnel had been breached.

Soundings were taken of the river bed on Wednesday 18 September, and it was determined that the bomb had exploded almost directly over part of the loop, about 60 feet (18 metres) from where it joined the southbound running tunnel, and causing a crater 5 feet (1.5 metres) high around its edge, and 15 feet (4.5 metres) deep in the centre. As the crown of the tunnel was only some 12 feet (3.5 metres) below the river bed, this meant that the bottom of the crater was actually inside it, partially filled with clay and other debris.

During another air raid on the evening of the 18th, a "stick" of bombs straddled the river, and the crew of the Seven Seas - along with some members of the Auxiliary Fire Brigade who were on board at the time - again reported their vessel being rocked by the impact of a bomb in the river. Efforts to locate either this bomb or any crater in the river bed it might have created, however, were fruitless.

On the morning of Friday 20 September, divers examined the crater caused by the first explosion, and found some 12-15 feet (3.5-4.5 metres) of the tunnel had been fractured. A considerable quantity of ballast and mud was lying inside, to a level of around 3-4 feet (1 metre) above the original track level, and the loop between the bulkheads was completely flooded. The extent of the damage was clearly worse than expected, as it was apparent that another bomb landing in the vicinty could cause a "water hammer" effect through the flooded tunnel that would easily breach the bulkheads, flooding the southbound running tunnel, and also the District line via the ventilation shaft.

It was decided to use divers to build a wall of concrete bags inside the breach in the tunnel, after which the crater would be partially filled with ballast, and then capped with a layer of clay (see original plan below). At the same time, a disused passageway connecting the northern end of northbound tunnel to the other side of the loop was to waterproofed, as was bulkhead 14, and finally a new bulkhead would be erected between bulkhead 13 and the ventilation shaft.

While this work was ongoing, at 09:00 on Tuesday 8 October, the crew of the Seven Seas reported a third bomb landing in the river, just to the west of the northbound running tunnel. A maintenance party making a routine inspection inside this tunnel corroborated this impact, but while there was flaking of the cement pointing of 25 rings of tunnel segments, it was initially doubted that the bomb had actually exploded. It was only later when divers recovered the tail fins of a 50kg HE bomb from the river bed near the tunnel that it was decided that it had may have detonated, although no crater could be found.

Work on the loop tunnel continued, with the plugging of the main breach being finished by Wedneday 6 November, the waterproofing of the disused passageway on the following day, and the construction of the new bulkhead some time after Tuesday 12 November.[3][4]

[1] Ministry of Home Security, Key Points Intelligence Directorate: Reports and Papers, Daily Reports - September 1940: Damage Appreciation 09-10/09/40, page 1 [Kew: National Archives, reference HO 201/2]
[2]This was the time an HE bomb pierced a Charing Cross mainline station platform, exploding on the road below, and putting tracks and signalling out of action.
[3] Ministry of Transport and successors, Railway Divisions: Correspondence and Papers, Air Raid Damage - Underground Railways, 1940-1941, page 1 [Kew: National Archives, reference MT 6/2759]
[4] Ministry of Transport and successors, Railway Divisions: Correspondence and Papers, Air Raid Damage - Underground Railways, 1941-1942, page 1 [Kew: National Archives, reference MT 6/2766]


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