Conserving Silbury Hill

Temporary stabilisation works to the top of the Hill in progress. In June 2000, a large hole suddenly appeared on the top of Silbury Hill, Europe's largest prehistoric artificial mound and a key feature of the Avebury World Heritage Area. It was soon clear what had happened: the head of an old excavation shaft, dug in 1776 to penetrate to the heart of the enigmatic Neolithic monument, had begun to collapse. English Heritage immediately embarked on a programme of research to understand why this had taken place and find out if the situation was likely to get any worse. As soon as we understood the nature of the collapse, we began work to arrest it and return the ancient monument to its former glory.

Archaeological examination of the freshly exposed deposits showed that the top of the shaft had slumped and been filled up again several times before. In other words, the collapse facing us in June 2000 was clearly part of a recurrent problem which might occur again. So we began a multi-disciplinary programme of investigation to determine the reasons for the collapse and to test1 the overall stability of the Hill. The investigation, which included documentary research, detailed surface mapping and analysis, geophysical survey (seismic tomography) and geotechnical investigation (coring), as well as excavation-style recording, has now been completed. The top of the Hill has also been temporarily stabilised by filling the hole with polystyrene blocks, which provide a robust support to the sides but do not increase the pressure on the collapsed material within the shaft.

As a result of our investigations, we are relieved to be able to conclude that Silbury Hill is a very robust structure with no major defects that might threaten its stability, due largely to the way it was originally constructed 4,500 years ago. We have, however, identified two causes for concern. Subsidence within the 1776 shaft occurred because it was originally only partly backfilled, with the lower part being left open. Although material has gradually accumulated in the shaft so that the space is wholly filled today, there was some deterioration of the shaft walls in the past. Secondly, while further collapse will not occur, there could continue to be some minor settlement of the material within the shaft, though nothing like as severe as the collapse that occurred in June 2000.

Our investigation showed that it was not just the central shaft that was suffering. Another tunnel, excavated at the base of the mound in 1849, had also been left open and, contrary to assurances given at the time, a tunnel excavated in 1968 was inadequately filled and consolidated. There have been small-scale collapses of the walls and roofs of these tunnels. This process has already happened with the outer part of the 1849 tunnel, as depressions recorded on the surface of the hill testify. We are confident that these problems do not pose a threat to the overall stability of Silbury. 

With the help of expert external advisers, we have considered a range of options for remedial works to the Hill. We have discussed these options at a seminar involving interested individuals and members of relevant local and national organisations and they have been considered by English Heritage’s   Advisory Committee and Commissioners. In order to preserve the long-term stability of the Hill, whilst minimising further damage to the Hill’s unique archaeology, English Heritage have decided that the best option would be to remove the inadequate backfilling to the tunnels and properly backfill them. 

Following this work, the temporary capping to the top of the shaft would be replaced with chalk, and a programme of monitoring put in place. English Heritage staff  have now drawn up a detailed brief setting out the objectives for the work and the constraints under which the appointed contractor would operate. Tenderers have been  asked to produce a method statement demonstrating how they would meet the brief. Their submissions have now been assessed by the Project Board and a contractor appointed. English Heritage staff will be working with the appointed contractor to develop detailed repair plans. Depending on the outcome of this development work, English Heritage hopes to commission the full repairs to the Hill in spring 2007.

Please note: access to the summit of the mound is not permitted, to prevent the severe erosion and damage to rare plant species that too many visitors will cause.

Our main website for Silbury Hill is at www.english-heritage.org.uk/silburyhill

Find out more about the landscape investigation at Silbury Hill.

Report 61/2004
Site Formation, Preservation and Remedial Measures at Silbury Hill. (PDF 1.2Mb)
MG CANTI, g Campbell, D Robinson and M Robinson
Centre for Archaeology

Silbury Hill, Wiltshire
English Heritage's Investigations into the Collapse at the Top of the Shaft and the Stability of Silbury Hill. (PDF  177Kb)
Rob Harding, English Heritage, Revised February 2005

Silbury Hill Review (PDF 111Kb)
Interpreting the seismic tomography data  By Professor Michael Worthington  And  
Silbury Hill: geotechnical work and investigation of voids  By Professor Richard Chandler

Silbury Hill Assessment  (PDF 5Mb)
An assessment of the conservation risks and possible responses arising from antiquarian and archaeological investigations deep into the Hill
Fachtna McAvoy, Archaeologist, English Heritage Fort Cumberland, Revised May 2005.