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nigelswift

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Silbury's structural integrity

Aug 01, 2007, 07:56

Some lay reactions to Update 11 http://www.english-heritage.org.uk/server/show/nav.17511

In the first half of the tunnel the voiding is said to be no more than 0.5 metres above Atkinson’s steel support arches. However, in two sections this voiding is said to be directly associated with the visible surface slumping uphill from the entrance, which is clearly a lot more than 0.5 metres higher, suggesting (to me at least) that the height of the voiding is only a small part of the story and that the damage and loss of integrity extends in a column above it right to the surface. Hardly a unique observation but it has further significance, see later.

Further along the tunnel and within the central chamber there is a very large amount of fallen material and above it “larger” voids and a “significant collapse zone”. How big? It matters, and the public is entitled to know. In fact it’s just about the crux of what the lay public DO want to know about their hill, urgently. So why not tell them? An official estimate or even an official guess would do.

One thing that makes me think “big” might mean “very big” is that I’ve heard, rightly or wrongly, that the Atkinson rings far in (of which no account or photographs have been provided) are badly distorted. To me, that suggests massive forces. Confirmation that I have been misinformed would be very helpful.

Whatever the answer to that is, an additional official opinion is owed, arising from the fact that the 0.5 metre voids have caused surface disruption far above them. Is there, like there is with the 0.5 metre voids, also a column of damage and loss of integrity extending above these “larger” voids, and if so for how far?

“There is no evidence of water cascading through the hill”. Well, not sure anyone ever thought there was (although actually, it might be better news if there was). Instead, “it is more a change of the overall saturation state of the whole mass of chalk”. This sounds worrying, to a lay person anyway. Is this a regular occurrence or unique? If the latter, is there a chance that prolonged saturation of chalk and clay causes irreversible chemical and mechanical changes? And if the latter, we still haven’t heard precisely where this water is thought to have come from and when. There have been tunnels in Silbury for 230 years and worse rainfall in that time. Is the structural integrity of the hill in August 2007 somehow more precarious and the situation more urgent than it has been previously? If so, why? The account so far provided simply doesn’t answer these perfectly natural questions.

Topic Outline:

Silbury's structural integrity – nigelswift, Aug 01, 2007, 07:56


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