Page 3 *ITEM 5 – SILBURY HILL: OPTIONS FOR STABILISATION (EHAC 2005/4)

5.1 The Committee was asked to consider three options for the future of Silbury Hill following the collapse of the vertical shaft: 1) do nothing; 2) fill the voids; 3) support the voids.

5.2 It was confirmed that the first option did not include any backfilling at the top of the mound as extra weight on the polystyrene blocks would have a detrimentaleffect further down the shaft. Filling the tunnels would cover the earlier Merewether tunnel as best as possible although it was unclear how much of this had been backfilled already.

5.3 Health and Safety checks would need to be in place particularly in the case of unauthorised public access.

5.4 It was agreed that work already carried out had advanced our understanding of Silbury Hill. There were complex issues and a legacy from the Atkinsonexcavations. The meeting at Devizes on 28 September 2004 had been useful in demonstrating the extensive research EH had undertaken and that EH had taken on board a wide spectrum of opinion.

5.5 The Committee agreed that the paper presented was helpful in enabling them to reach a decision and that EH had taken time to consider the various options. Option 1 presented an unknown risk to the monument; option 2 had problems with further intervention and concerns about the additional boreholes; and option 3 posed minimum intervention in terms of furtherdamage to the monument. The variation to option 3 which proposed keeping the tunnel accessible for visitor access was a concern because of the frequentair exchange. There was, however, a need for monitoring the stability of the repairs over a timescale yet to be defined.

5.6 The key was in not moving too quickly but in taking time to consider the best option. There was no planning application that necessitated the need for speed, but there was a lot of local interest and frequent radio coverage.

5.7 It was acknowledged that there were still a lot of unknown factors and risks in further intervention. For option 3 to be recommended, there needed to be a demonstrable link to management-critical research objectives. The cost of the project would be in the region of £600k plus on-going maintenance costs. It was suggested that a combination of all these factors meant clarifying the uncertainties before doing anything further.

5.8 Costs would be spread over a number of years, as it would be impossible to find the money in the short term. A five year plan could be considered and it was possible that other organisations (or companies) might share the costs ifthey wished to join in and be seen to be helping with conserving the environment.

5.9 The area is also a Site of Specific Scientific Interest (SSSI) and, as such, consent will be needed from English Nature for potentially damaging operations. None of the options, however, sounded intrinsically damaging to the chalk grassland interests of the SSSI. EH should maintain close contact with the Conservation Officer at English Nature, Patrick Cashman.

5.10 The comment was made that there was not enough engineering advice in the report and that it was necessary to establish how, for example, the timber and iron from previous interventions would perform under further construction work. A paper on the engineering side of the project would be welcomed.

5.11 <b>The Committee agreed that: i) Option 3 was preferable, more engineering advice was needed andthere was no need for haste; ii) the site should be open for monitoring purposes and in situ preservation would be one of the key strategies in future. </b>