<< Feature Articles >> Silbury Hill : Time for Care in the Community
Submitted by Heritage Action on Monday, 07 March 2005 Page Views: 1225
Site Watch
As may now be widely known, English Heritage revealed sad details about Silbury Hill in British Archaeology magazine a few days before Christmas. Major losses to the archaeology are now inevitable, whatever repair strategy is adopted. We feel that the timing and means by which the announcement was made was disappointing and we hope that full details will now be put on the English Heritage website, as previously indicated.
Repair options....
• Repair option 1: Filling the tunnels and voids with chalk slurry (grouting them).
• Repair option 2: Re-boring the Atkinson tunnel and then filling it and the voids with compacted chalk.
• Repair option 3: Re-boring the Atkinson tunnel, re-supporting it and leaving it open for study.
In each case, the central shaft would be re-filled only when the base had been fully stabilised.
Issues arising....
It is made clear that each solution will cause further damage, to a degree that may not be fully quantifiable. It is suggested however that the options that involve re-boring the tunnels may be more damaging than grouting them and “may trigger further collapse“.
Certain additional pros and cons are cited. Grouting might not reach all the voids and might damage the anaerobic conditions and biological deposits at the centre of the hill. By contrast, tunnelling would protect these deposits and provide research opportunities.
It is therefore certain that the decision is not simply an engineering issue and will demand hard choices about archaeological priorities. Tunnelling may offer research opportunities relating to the biological deposits within the small inner mound, 'Silbury 1', at the cost of extra disruption to the larger outer phases, 'Silbury 2 and 3'. Conversely, grouting may offer greater protection for Silbury 2 and 3 at the expense of extra damage to Silbury 1.
In essence therefore, this may involve a simple but fundamental decision upon the way in which this generation wishes to bequeath Silbury to the future. As such, we feel it is a matter that should properly involve a fully informed public debate and the sooner all the available details are published by English Heritage, the better.
We cannot sensibly hold a view at present – until the scale of the respective risks is laid out to the full extent that is possible, who could? However, as a starting point for debate we offer our initial instincts about the issues:
First, we suspect we are not alone in being initially horrified by the thought of boring what would be a new and larger tunnel into the side of the hill since it is an echo of the regrettable errors of the past.
Second, we felt from the start that the grouting repair option should at least have been the lead area for investigation and that remains our view. In our opinion, its complete rejection at an early stage was a mistake, and cost much time.
Third, as a matter of principle, we feel the most disruptive options should be regarded as last resorts, particularly if the scale of likely damage is open-ended. They should be seen as acceptable only if viable alternatives cannot be found.
Fourth, while the preservation of the biological deposits is an important consideration to be weighed in the final decision, this generation’s ambition to study them should not cloud the decision. If preserved, and if required, that could equally well wait for a century or two.
Fifth, there is a statutory duty to preserve and a statutory duty to learn, and common sense dictates that the former must be regarded as imperative and the latter dependent on the first. The pursuit of both simultaneously is not necessarily a sensible interpretation of duty.
How the decision is made....
The wider 'informed public' must be enabled to properly weigh the issues. We have concerns that this position has not yet been reached.
In particular, much is being made of the fact that the inner mound 'Silbury 1' must be protected from damage because of its biological deposits, and this is as it should be.
However, the downside – the fact that this may necessitate accepting losses elsewhere – has merely been acknowledged. It has not been quantified, and indeed may not be quantifiable at present. A balanced debate requires strong and informed advocacy on
behalf of the vastly larger overlying building phases 'Silbury 2 and 3', to balance the enthusiasm being shown for a research agenda for Silbury 1. Every call for preservation and study of the latter might be a call for extra damage to the former. It must be made crystal clear what is the nature and scale of any proposed trade-off.
This potential for imbalance is made worse, in our view, by the fact that English Heritage called for submissions about the repair options from the September peer group attendees together with suggested avenues for research. To us, this creates a clear danger that there will be an inherent majority against grouting and in favour of tunnelling in the total submissions received. Particularly since wider submissions were not invited.
Already, eloquent voices are being raised in support of the research agenda and 'Silbury 1', and not only within English Heritage. Professor Richard Bradley sees Atkinson’s “negligence” in failing to leave adequate records from his tunnelling as amounting to unfinished business which English Heritage has an ethical duty to complete. Elsewhere, Mike Pitts has commended tunnelling as the “most exciting” of the repair options. In each case, in our opinion the 'preservation priority' should also have been given equal prominence.
How the issues are presented
We feel that some parts of the details so far offered to the public have inadvertently produced an unbalanced picture.
A potential future loss of 13% of the 'Silbury 1' primary (inner) deposit is suggested, and this is indeed a shocking prospect. Converseley, the quoting of a potential future loss of 0.44% of 'Silbury 2 and 3' self-evidently seems far less concerning.
Yet is it? Should these figures be presented as percentages, when there are vast differences in the sizes of the respective areas of the hill? Had the potential losses been presented in simple volume terms, the reader might have gained a quite different perspective. Based on these percentages, the potential loss to 'Silbury 1' is 38 cubic metres. Whereas to 'Silbury 2 and 3' it is 1715 cubic metres! Needless to say, the latter may involve surface disfigurement.
English Heritage’s interpretative comment, that the percentage figures show the impact “will be most significant on the poorly understood inner mound (Silbury 1), where biological preservation is exceptional” is questionable at the least, yet on the percentage basis provided this fact would escape many readers.
We believe that only if every relevant factor is highlighted can the wider community reach a rounded view of the issues. Thus, if we were acting as advocates for Silbury 2 and 3, we would point out that, so far as we know, the importance of the biological deposits in Silbury 1 was not mentioned or included as a factor in the planned programme until September 2003. By then, many interventions into them had been made and there are currently two plastic-lined boreholes into them. So the concerns that grouting may destabilise biological deposits by “introduction of air into otherwise well-sealed anaerobic environments” should be seen against this, not in isolation.
We must stress once again, we aren’t taking sides. Grouting may not be the answer and may involve an unacceptable number of new boreholes – who yet knows? We simply say that everything must be given an equal public airing. The perfect preservation of the whole of the 'inner' biological deposits themselves may or may not be the proper main priority. We would strongly maintain that an ambition to study them certainly isn’t. Only an open discussion of all the issues in a balanced fashion is likely to lead to a decision in which all can have confidence.
Proposal....
We have a proposal which might enable the wider concerned public to take a meaningful role in the decision making and to gain confidence in the final decision. We suggest that a useful first step would be the publication of a purely engineering solution, one that shows the most efficient and least disruptive means to stabilise the hill, and nothing else. Once this has been done, then a useful yardstick will exist, a blue print showing a least damaging solution.
We see such a yardstick as essential to any balanced judgement. If particular priorities or particular research avenues are to be proposed then their true cost in terms of additional cubic metres of loss or increased risk elsewhere in the hill ought to be on public display, so far as is possible. As things stand, the published repair options are in an incomplete format. They offer no means to compare true costs, and merely describe some means to some ends. As such we can see no way in which they can be properly weighed by the public or even by those in charge.
We see this as a once-in-a-generation matter, and a grave decision. As such, it warrants absolute clarity and the widest possible dissemination of all relevant factors.
Note: Further discusssion in the Forum
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