(Sorry, I know I said that I wouldn't post on HH again, but feel this is rather important. Nigelswift, Goffik & Moth know I have been in correspondence with English Heritage about Silbury after driving past one day and seeing the fence down. I don't know if anyone has had this statement from Dr Kevin Brown of EH, but it arrived in my in-box today. I didn't know where to post it so have posted it here, if it is irrelevant or you want to post it on the HA site then please do. Thank you. Nat)
POSITION STATEMENT
In June 2000 English Heritage began work to arrest the collapse of the head of a central shaft, originally dug in 1776, that created a large hole on the top of the Silbury Hill. The top of the Hill has been stabilised by filling the hole with polystyrene blocks, providing a robust support to the sides whilst not increasing the weight on the collapsed shaft fill.
Evidence from sides of the collapse showed that there was a history of the top of the shaft slumping and being refilled. As there was clearly a recurrent problem with the fill of the shaft, a programme of investigation was begun to determine the reason for this and to establish the overall stability of the Hill.
The investigation, which included documentary examination, archaeological recording, detailed mapping, geophysical survey (seismic tomography) and geotechnical investigation (coring), has now been completed.
We believe that Silbury is, due to the nature of its original construction, a very robust structure with no major defects which threaten its stability. We have however identified two current areas of concern.
Subsidence within the 1776 shaft occurred because it was originally only partly infilled, with the lower part being left open. Although the shaft is now wholly filled there has been some past failure of shaft walls and, whilst further collapse will not occur, there could be minor settlement of the shaft fill and displaced mound material.
An earlier tunnel dug at the base of the mound in 1849 had been left open and, contrary to assurances given at the time, a tunnel dug in 1968 was inadequately filled and consolidated. There has been localised collapse of the walls and roofs of these tunnels. This process has already happened with the outer part of the 1849 tunnel, as evidenced by depressions on the surface of the hill.
We are confident however that these problems do not pose a threat to the overall stability of Silbury and are now considering what remedial work needs to be carried out in the medium to long-term to address these concerns. We have commissioned a risk assessment to consider all the findings from the various investigations, identify the options for work to the Hill, evaluate the risks associated with each option and provide a method for deciding between them. This work is allied to a research strategy which aims to increase our understanding of the environmental conditions within the Hill.
