Squid Tempest
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Re: Silbury's structural integrity
Aug 08, 2007, 10:15
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That seems a pretty comprehensive report, to my inexperienced eyes...
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Littlestone
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Re: Silbury's structural integrity
Aug 08, 2007, 11:15
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Yes, the most detailed report so far, and one wonders why such reports could not have been issued since Week 1.
It's still too soon to feel confident that all is 'under control' (as the report endeavours to portray) but at least it seems that, to quote from the report, "The current work on the shaft cap is very limited in its scope, as the main work cannot and will not commence until the tunnels beneath the central portion of the hill have been fully excavated, supported and backfilled."
Further down in the report it is stated that, "The central collapse volume is 125 cubic metres, which as a volumetric ratio is 0.05% of the volume of the hill. Thus, from a structural perspective, this void constitute a very minor volume compared to the whole structure, and both Skanska and independent geotechnical experts have concluded that there is absolutely no risk to the overall structure of Silbury Hill, in terms of major collapse or rumoured 'implosion'." Perhaps English Heritage would publish the findings of these 'independent geotechnical experts' with their next engineering report so that the public can make up their own minds whether or not there is 'absolutely no risk' to the overall structure of Silbury.
There are other issues within the EH engineering report that require further scrutiny, not least the question of 'unusually wet conditions' within Silbury. Where does current thinking place the point(s) of entry of water into the structure and what measures are being taken to counter the problem?
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nigelswift
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Re: Silbury's structural integrity
Aug 09, 2007, 07:38
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Well I'm very glad some real details have come out and particularly that at last, at last, the size of the known voids has been released for the very first time.
As has been said though its not the full story of the degree to which the hill is in trouble as for every cubic metre of voiding there is the danger of additional collapse in weakened layers above it (since it has always been accepted that the voids can't be fully filled and the fill will slump anyway). An estimate of the total likely loss, and over what time period, and whether it will reach the surface, is what most people would wish to know and IMO its something they are owed urgently. Just the working estimate, nothing more.
I don't think comparing the size of the voids with the size of the hill as a means to illustrate how small they are is valid. After all, that could have been done with regard to the anticipated voids if the hill had been left alone to settle naturally. That damage also would be small compared with the whole hill.
As I understand it, before the project started, at risk in the inner mound was just 38 cubic metres, and in the rest was 1715 cubic metres, both over as very very long period. Bearing in mind the inner mound was said to be the most precious and delicate and sensitive to water I wonder what effect a central collapse of a saturated 125 cubic metres has had upon it.
I also wonder about the amount of damage that is yet to come as a direct result of the work - will it be greater since new supports are to be inserted? And will it be greater because of the saturated condition of the chalk and clay? Yes to both, surely? But I doubt a clear answer can be known since from the start it was said "No prediction can be made for the amount of unintentional damage that might occur (through collapse caused by work disturbance)”. That always struck us as indicating a leap in the dark. I can hardly think it isn't even more so now.
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Littlestone
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Re: Silbury's structural integrity
Aug 09, 2007, 10:55
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I don't think comparing the size of the voids with the size of the hill as a means to illustrate how small they are is valid.
Quite, this is something of a red herring. Even minor subsidence in a structure can result in major damage. A few cracks in the foundations of Durham Cathedral would be seen as minuscule compared to the total mass of the building - left unchecked they could well be enough to bring down half the structure.
Bearing in mind the inner mound was said to be the most precious and delicate and sensitive to water I wonder what effect a central collapse of a saturated 125 cubic metres has had upon it.
Absolutely, that is why it is being asked where current thinking places the point(s) of entry of water into the structure and what measures are being taken to counter the problem? Answers please English Heritage.
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ocifant
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Re: Silbury's structural integrity
Aug 09, 2007, 13:23
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I don't think comparing the size of the voids with the size of the hill as a means to illustrate how small they are is valid.
Quite, this is something of a red herring. Even minor subsidence in a structure can result in major damage. A few cracks in the foundations of Durham Cathedral would be seen as minuscule compared to the total mass of the building - left unchecked they could well be enough to bring down half the structure.
Ok, some rule of thumb maths here.
Let's say a standard 4x4 car (I drive an X-Trail for instance) is 1.8 metres high. It's approx 4 metres long, by 3 wide. I make that a volume of some 21 and a bit CuMtrs.
Multiply by 6 to get a rough figure somewhere near 125 CuMtrs.
That's a lot of space! Hardly a small area...
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Littlestone
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Re: Silbury's structural integrity
Aug 09, 2007, 17:31
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Multiply by 6 to get a rough figure somewhere near 125 CuMtrs.
In other words, the central void is about the size of five single garages or one four-bedroom detached house? Looked at in those terms that really is very frightening.
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jimit
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Re: Silbury's structural integrity
Aug 09, 2007, 17:53
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"Ok, some rule of thumb maths here.
Let's say a standard 4x4 car (I drive an X-Trail for instance) is 1.8 metres high. It's approx 4 metres long, by 3 wide. I make that a volume of some 21 and a bit CuMtrs. "
3M wide, 9ft 10 ins in old money!!! Are you sure?
Jim.
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ocifant
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Re: Silbury's structural integrity
Aug 09, 2007, 18:11
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I did say rule of thumb! :-)
Ok, take 2M x 4M x 1.8, give s afigure of 14.4CuM.
That makes approx 8.5 carloads of space! Even worse. Almost half as many cars again!
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Robert Carr
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Re: Silbury's structural integrity
Aug 09, 2007, 18:45
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ocifant wrote:
Ok, take 2M x 4M x 1.8, give s afigure of 14.4CuM.
That makes approx 8.5 carloads of space! Even worse. Almost half as many cars again!
shhhh... they might turn it into a multi-story carpark :-)
Hey peace Ocifant. I like maths too.
http://www.hg420.com/gallery/showphoto.php?photo=93542
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juamei
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Re: Silbury's structural integrity
Aug 09, 2007, 22:35
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Don't understand...
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