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The Modern Antiquarian Forum » Silbury Hill » Silbury's structural integrity |
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Littlestone 5386 posts |
Aug 14, 2007, 21:06
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English Heritage's Week 13 Update is now available here at http://www.english-heritage.org.uk/server/show/nav.17511
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Pilgrim 597 posts |
Aug 15, 2007, 02:34
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Littlestone wrote: English Heritage's Week 13 Update is now available here at http://www.english-heritage.org.uk/server/show/nav.17511 Hi Littlestone, Hmmmm.... From: “An assessment of the conservation risks and possible responses arising from antiquarian and archaeological investigations deep into the Hill” by Fachtna McAvoy, February 2005: http://www.english-heritage.org.uk/upload/pdf/Risk_assessment052005.pdf 5.3.1 The observations detailed above allow for a very, very approximate prediction of future collapse rates. If the collapse follows a log-rate process, as shown in Chart 1 below, then a period of 1000 years will be needed for a collapse of 0.5 x cavity diameter. Even if the collapse rate is linear (Chart 2), 300 years are required for a collapse of 1.0 x D. So, that “very, very approximate” above puts the figure anywhere between the two outliers; 1.The log-rate process of 1,000 years which predicts a rate of migration that gets slower with time: and 2. The linear-rate process (the worst-case scenario) which calculates out to 300 years. That’s a mean of 650 years to achieve a collapse of 1.0 x of the Tunnel Diameter, apparently. This is nowhere near the Update 13 prediction of possible visible damage in: “ a few decades time” and of “at least 1,000 cubic metres” So we’ve come down from 300 - 1000 years to a “few decades” for damage to appear? In two years of accumulating knowledge? Despite trumpeting Tomography as the be-all-and-end-all? In 2002, on behalf of the Silbury Hill Project Board, Professor Richard Chandler, - Professor of Geotechnical Engineering at Imperial College London – reported that: …..“ It appears from the recent investigations …. that the general body of the Hill is otherwise stable” http://www.english-heritage.org.uk/upload/pdf/silbury_hill_commentary_report.pdf …you’ll forgive me for being a little unsure as to who to believe in all this. Peace Pilgrim X
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moss 2897 posts |
Edited Aug 18, 2007, 11:48
Aug 17, 2007, 11:45
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nigelswift wrote: , “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. That’s interesting, when I was wandering round Silbury a week or so ago just after the flooding, first thing I noticed was how dry the ground was. Apparently 3 days before the water had stood over a metre round the mound., it struck me though at the time that any water entering the mound would have come from the top as the Atkinson tunnel is much higher than the moated area, so are any of the the turves that are being examined from this latest EH update .... http://www.english-heritage.org.uk/upload/pdf/q7.pdf contaminated as well? It also must surely have followed that the new chalk infill lying in the low ground of the compound also would have been contaminated with seed and spore from the floodwaters?
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Littlestone 5386 posts |
Aug 21, 2007, 20:23
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No updates on the English Heritage Silbury Hill Update page this week. Hear tell that the whole site has fallen silent and the visitor centre is no longer open. Wonder what's going on?
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Pilgrim 597 posts |
Aug 22, 2007, 09:28
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Littlestone wrote: No updates on the English Heritage Silbury Hill Update page this week. Hear tell that the whole site has fallen silent and the visitor centre is no longer open. Wonder what's going on? Update 14 is here: http://www.english-heritage.org.uk/upload/pdf/Silbury_Hill_web_update_14.pdf Peace Pilgrim X
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Pilgrim 597 posts |
Edited Aug 23, 2007, 06:47
Aug 23, 2007, 06:45
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Pilgrim wrote: I’m a bit concerned about Figure 2. There is an awful lot of ironmongery in that shot. Corrugated iron, Atkinson's supports etc. Comparing it with other tunnel shots like those in Update 10 and 9 seems to indicate that its taken in the narrow part of the tunnel. If that is Atkinson's ‘59’ on the leaning support, then this isn't far off from where the roadstone fill ended (‘57’). Above the ‘59’ the small numerals on the yellow patch on the ring are identical in style to one being marked by an archaeologist in Update 9. The caption in Update 14 says: Figure 2 shows the old Atkinson tunnel arches where they have moved and settled as a direct result of the mass of loose chalk pushing these arches. These are being removed progressively and securely. Given that this leaning arch bears a small yellow panel and a number which is identical to others used in this conservation project, I would surmise that the arch was marked some time in the last few weeks and has moved since. The caption in the Update is ambiguous; it gives no indication of the timing of the movement of the loose mass of chalk which surrounds the leaning arch. I can only perceive that this is a recent event, and I’m concerned that for this arch to be displaced in this manner (and by this much) would need a distinct lateral force. I would therefore question the statement in Update 14: Otherwise, the geotechnical conditions within the hill remain unchanged, with the 3 separate voids all remaining stable and the remainder of the tunnel standing without any observed movements or structural changes. I think that the last two arches beyond the fluorescent are different in curvature to the ones directly above it, and they may well be the beginning of the widened section before the central chamber. I think the two pieces of wood on the spoil are noggins; similar noggins can be seen between the rings in the first photograph in Update 10. They are being presumably used to maintain the relationship between the ring supports. Again, I presume that these noggins were moved by the lateral force of the loose chalk? I'd like to know this: From where exactly did this loose chalk originate? When was this damage caused? And How Much of this 'loose chalk' is there? Peace Pilgrim X
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Pilgrim 597 posts |
Aug 30, 2007, 02:15
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Littlestone wrote: No updates on the English Heritage Silbury Hill Update page this week. Hear tell that the whole site has fallen silent and the visitor centre is no longer open. Wonder what's going on? Update 15, anyone? Or were you having too much Bank Holiday sun and fun? Peace Pilgrim X
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nigelswift 8112 posts |
Aug 30, 2007, 10:04
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Well let us hope the wait is worth it and the next update clarifies some of the questions that seem to remain. For instance, Moss’s observation must have a simple yes/no answer – “It also must surely have followed that the new chalk infill lying in the low ground of the compound also would have been contaminated with seed and spore from the floodwaters?” And your query is worth attention – I can only perceive that this is a recent event, and I’m concerned that for this arch to be displaced in this manner (and by this much) would need a distinct lateral force. I would therefore question the statement in Update 14: “Otherwise, the geotechnical conditions within the hill remain unchanged, with the 3 separate voids all remaining stable and the remainder of the tunnel standing without any observed movements or structural changes“. I must say your puzzlement on that particular point corresponds with my own. I read that If the collapse follows a log-rate process, as shown in Chart 1 below, then a period of 1000 years will be needed for a collapse of 0.5 x cavity diameter. but then I look at that bent steelwork and can’t help thinking it got like that in the past few years due to an enormous lateral force applied to it, perhaps lasting only a second or two. Not log-rate progress at all was it?
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Littlestone 5386 posts |
Aug 31, 2007, 18:59
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Update 15 is now up.
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Pilgrim 597 posts |
Edited Sep 01, 2007, 10:53
Sep 01, 2007, 10:50
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From Update 15: This material was selected specifically for its inert properties and capabilities to penetrate and consolidate rubble chalk such as the material created by the continuing growth of the Merewether/Atkinson tunnel voids. So the voids continue to grow, do they? By how much? At what rate? You’ve gone in, having told us that you're doing the best thing for the Hill (at a cost of £600, 000 to the British taxpayer), having given up on the grouting option because – in your opinion – “there was little support for the idea of remotely filling the voids by grouting.” Funny that. Who did you ask? And who did you listen to? Oh; hang on…. English Heritage’s Investigations into the Collapse at the Top of the Shaft and the Stability of Silbury Hill Rob Harding, English Heritage, Revised February, 2005 The results of this work were presented to a seminar held at Devizes in September, 2004 attended by invitees from the wider archaeological community and interested local bodies. So you asked the “wider archaeological community” and “invited local bodies” (presumably invited by you) what should be done, and funnily enough… At the seminar, a range of views were expressed as to how English Heritage should proceed. Three key points could be drawn from the contributions made. Firstly, there was little support for the idea of remotely filling the voids by grouting. Secondly, there was however considerable support for the idea of reexcavating the tunnels and refilling them and thirdly, if such an intervention was to take place, it should, on this occasion, be accompanied by the highest level of archaeological recording and investigation. Surprise, surprise – they gave “considerable support” to going back in, and little support to grouting. So everything you have done has not been in the best interests of the Hill, after all. And now – after seven years – we are injecting foaming resin into the rubble and roof in what looks like a race against time to get this job done before any more water arrives: If this injection work were not being down however, it has been estimated that progress in the tunnel could be less than half of that actually achieved. So we are using foaming resin in order to meet a deadline, then? Not because it’s in the best interest of the Hill? Just one more thing from Update 15…… Once the injection work has been completed, the tunnelling work resumes, with the stabilised rubble being solidified and competent to allow the roof and sides of the tunnel to be excavated without the serious risk of collapse and potential injury to the tunnelling personnel. Tunnelling personnel? What happened to the archaeologists? Isn’t this a conservation project? Is there any internal archaeology going on? Or is it – as I fear - a rescue mission? Peace Pilgrim X
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