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Silbury Hill
the view from inside
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I was one of the people fortunate enough to be invited on one of the EH tours into the excavations last weekend (boo, hiss, traitor etc) and it was one of the most amazing and honoured experiences of my life. It was also quite shocking to see the extent of damage from the inside and I can definately confirm that EH were right in opening up the tunnel to repair this. The problem lies at the centre, where Atkinson's workings join Northumberland's shaft. At this point the workings were not so much a tunnel as a canyon. Yes Atkinson's supporting rings are still in place, but the problem was they are not supporting anything for a good distance from the centre.
As you know from the EH updates the roadstone backfill only went in so far, leaving a void behind. Unfortunately from this point the underside of the hill was not chalk, but a chalk, clay mixture which has subsided and is what caused the dig to slow. This, and the surrounding sides was sodden when we were in there, showing the extent of water penetration. Because Skanska did not want to introduce to much extra modern intrusion to the dig they were using the exisiting Atkinson rings as shuttering supports, with shuttering plywood, steel sheets etc over the top to protect the excavators. At the centre, where the clay 'collapse' had been, there was of course a void above these roof supports, which already had a good mound of loose chalk piled on it, which presumably must have happened from the time the sheets had been put across to when we went in (2 weeks max?). It is my guess that it is this area that has become unstable and the collapse is spreading upwards, like taking the mould away from an unstable jelly.
As to apportioning blame, I feel that is a pretty pointless exercise. Without doubt the root cause of this is the water seeping down through the hill causing failure and collapse of material around an exisiting large void. Were EH right to open the tunnel, in my view most definately so? Who could have predicited the horrendous wet weather we have had recently? But plenty of people (including some on here) did mute the posibility so should contingencies have been made? Hindsight is a wonderful thing. I hope it is not too late to put up 'the brolly' and to backfill as safely as possible.
We were allowed to take photographs in the hill but EH did request that we would not publish them on the interweb so I hope the above will give you a good image.

the fool


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Posted by fool on the hill
27th July 2007ce
20:49

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Re: Silbury about to implode? (Littlestone)

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