Silbury Hill; English Heritage Conservation Project, Investigation & Temporary Stabilisation

This section describes the initial phase of the English Heritage Silbury Hill Conservation Project. This phase encompasses the temporary stabilisation work and the investigations undertaken to provide information which would allow decisions to be taken on the long-term conservation of Silbury. The material set out below and reproduced in the attached pdf file is taken from McAvoy, F 2005 'Silbury Hill, Wiltshire, An Assessment of the Conservation Risks and possible Responses arising from Antiquarian and Archaeological Investigations Deep into the Hill'.

The English Heritage Silbury Hill Conservation Project was initiated to address issues resulting from the opening of a hole on the summit of Silbury in May 2000 (Figs 1, 2).


Figure 1: overview of the opened up shaft on the top of the Hill (photo English Heritage)


 Figure 2: detail of the opened up shaft (photo: English Heritage)


The hole was created by infill falling into an open part of a vertical shaft that had been originally dug in 1776 (Figs 3, 4). This is the first known investigation deep into the centre of the Hill, and other significant investigations, this time involving horizontal tunnels at the base of the Hill occurred in 1849 (the Merewether tunnel) and in 1968-69 (the Atkinson tunnel).



 Figure 3: plan of Silbury Hill showing investigations and borehole locations



Figure 4:  schematic part-section of Silbury showing investigations and voids


The hole that opened on the summit was approximately 13m deep and 2.25m wide and the immediate on-site response was to construct a protective scaffold and steel capping (Fig 5) and then to carry out a remote inspection using a suspended video camera (Fig 6).


Figure 5: the protective cover over the shaft (photo: F McAvoy & English Heritage)


Figure 6: the open shaft (photo: F McAvoy & English Heritage)


A desk-based assessment of the potential for archaeological recording and investigation was prepared (McAvoy 2000a) and a programme of investigation initiated to determine the cause of the collapse of the infill and how, and what, remedial work could or should be carried out. This programme involved studies of documentary sources, cartography and aerial photography.


English Heritage staff from its Regional, Archaeological Projects, and Conservation Engineering teams joined a team from G Daws Associates (specialists in rock mechanics and confined space working) and made a physical inspection within the open shaft in August 2000. An outline dimensional drawn record was made, and further photographic images were obtained.


The inspection showed that there was a large cavity leading off to the south-west from near to the base of the open part of the shaft (Figs 4, 7).


Figure 7: the ‘cavity’ on the south-west side of the shaft (photo: F McAvoy & English Heritage)


This undercutting cavity presented very real challenges for any stabilisation and remedial work (McAvoy 2000b) and whilst these were being considered there was a major collapse during December 2000.


This collapse occurred in two stages. First the undercut mound deposits fell into the cavity beneath. This created a steep-sided ovoid hole to the surface of the Hill, leaving three of the shaft sides more or less intact at its northern end (Fig 8).


 Figure 8: the collapsed ‘cavity’ and shaft

The sides of this hole then collapsed (apart the northern side) to leave a crater on the summit that was 3.8m deep and 7.2m by 5.5m wide (Fig 9).


Figure 9: the crater left after the second stage collapse (photo: English Heritage)

The nature of the investigation and stabilisation programme was reconsidered. Direct access into the crater was not possible on Health and Safety grounds but Archaeological Projects made a remote record of the shaft location and of the deposits exposed in the sides of the crater (Fig 10). A watching brief was also carried out during the erection of a new fence around the crater.


Figure 10: the crater (looking north, photo: F McAvoy & English Heritage)

English Heritage Archaeological Science staff carried out geophysical survey on the summit using earth resistance, ground penetrating radar in February 2001 (Linford and Martin 2001) and electrical imaging on the summit and on parts of the adjacent slopes in June 2001 (Linford 2001).


In between these two surveys Archaeological Projects carried out some small-scale excavation adjacent to the crater (Fig 11).


Figure 11: excavation on the summit (photo: F McAvoy & English Heritage)


The surface of the Hill was mapped by English Heritage Archaeological Investigation staff as part of their analytical earthwork survey of Silbury and its immediate landscape (Field 2002). This work brought together a deal of research on the history of Silbury and its archaeological significance.


For comparative purposes vertical photography of the mound taken in 1968 was re-processed by the English Heritage Metric Survey team.


Archaeological Projects staff located and recorded the position of the entrance to the 1968 tunnel.


It was felt that a seismic survey was the only practical technique that offered the potential to provide information on the internal condition of the Hill and a brief was prepared (English Heritage 2001b) for external contractors to tender against. In advance of the survey, which would take some time to initiate and deliver results, measures were taken to stabilise the crater and prevent the erosion of its sides. Cementation Skanska, who were also commissioned to carry out the seismic survey, undertook this work.


The sides of the crater were lined with a mesh anchored to the surface of the Hill, and the crater was filled with large polystyrene blocks, above a geo-membrane (Fig 12) and below a capping layer of chalk (Fig 13). This provided a lightweight, robust and tamper-proof support.


 Figure 12: stabilising the crater (1), geo-membrane and polystyrene blocks.


Figure 13: stabilising the crater (2), final chalk infill.

Physical access to the deposits in the sides of the crater became possible during this process and these were recorded by Archaeological Projects.


After stabilisation, preparation for the seismic survey commenced in August 2001. Skanska’s method for executing the survey was to drill four boreholes from the plateau at the top of the Hill with material in the holes recovered as intact cores (Fig 14).



















Figure 14: left, drilling on the summit by GB Geotechnics; right, extracted lengths of core (photo: F McAvoy & English Heritage)


Cross-borehole data was then collected utilising an airgun source and in-hole hydrophones.  In-borehole to surface source data was collected utilising an airgun source and a network of geophones on the surface of the Hill.


One of the boreholes (no.4) encountered a void where it intercepted a western lateral tunnel dug in 1968 (Fig 15) and was replaced by borehole no.5. The void was inspected using a down-borehole CCTV camera and the images obtained were recorded onto videotape.



Figure 15: steel arches from the 1968-69 tunnelling revealed in borehole no. 4 (photo: English Heritage)


The initial seismic survey results were reported in November 2001 (Kirkbride 2001) and independently assessed (Worthington 2002a). Further seismic survey of a geophysical anomaly on the north side of the Hill took place in February 2002 (Kirkbride 2002a) and the cross-borehole seismic survey was repeated in April 2002. The final report of the seismic investigations was prepared in August 2002 (Kirkbride 2002b) and independently assessed (Worthington 2002b).


The geophysical anomaly on the north side of the Hill was examined through four cores (Fig nos.8-11) taken by Archaeological Science staff (Fig 16) in August 2002 (McAvoy 2002).


Figure 16: hand-coring on the north side of the Hill (photo: F McAvoy & English Heritage)

A brief was prepared for further geotechnical investigation (English Heritage 2002) and Cementation Skanska drilled two boreholes on the summit in March 2003. Borehole no.7 was drilled through the centreline of the 1776 shaft with an adjacent borehole (no.6) to provide comparative information on the composition of the mound and to test a geophysical anomaly. Borehole no.7 encountered a void in the 1968 tunnel at the base of the shaft that was inspected using a down-borehole CCTV camera with the images obtained recorded onto videotape (Fig 17).

Figure 17: steel arches from the 1968-69 tunnelling revealed in borehole no. 7 (photo: English Heritage)

The results of this geotechnical investigation were reported upon in April 2003 (Kirkbride 2003) and independently assessed (Chandler 2003).


The cores from borehole nos.1-7 have been recorded by, and are housed with Archaeological Projects (Fig 18).

Figure 18: recording the core from borehole no. 8 (photo: English Heritage)

One core, no.5, has been fully processed to recover environmental evidence and an assessment of the results is in preparation. Another core, no. 6, has been partially examined to further the study of the burial environment within the Hill.


This study was reported upon in July 2004 in a paper that also considered biological preservation issues in relation to options for remedial works (Canti et al 2004).


The entrance to the 1968 tunnel was partially re-exposed in November 2004. An inspection of the backfilling showed that this had been competently carried out for at least four metres inwards from the entrance.




Canti, M G, Campbell, G, Robinson, D, and Robinson, M 2004 Site Formation, Preservation and Remedial Measures at Silbury Hill, CfA Rep 61/2004


Chandler, R C 2003 Report on Geotechnical Aspects of the Stabilisation of Silbury Hill, Project Archive


English Heritage, 2001b Silbury Hill: Brief for Geophysical Investigation, Project Archive


English Heritage, 2002 Silbury Hill: Brief for the Commission of Further Geotechnical Investigations to the Shaft and Geophysical Anomaly A2, Project Archive


Field, D, 2002 The investigation and analytical survey of Silbury Hill, Arch Investigation Rep Ser, AI/22/2002, English Heritage


Kirkbride, M A 2001 Silbury Hill, Wiltshire, Report of Tomographic Survey November 2001, Rep no C100975/CR1, Cementation Skanska


Kirkbride, M A 2002a Silbury Hill, Wiltshire: Report of Surface Tomographic Survey, April 2002, Rep no C100975/CR2, Cementation Skanska


Kirkbride, M A 2002b Silbury Hill, Wiltshire: Final Report of the Geophysical Investigation, August 2002, Rep no C100975/CR4, Cementation Skanska


Kirkbride, M A 2003 Geotechnical Investigation of Silbury Hill, Avebury, Wiltshire, Rep no C2002/CR3, Cementation Skanska


Linford, N and Martin, L 2001 Silbury Hill, Wiltshire: Report on Geophysical Survey, February 2001, CfA Report 21/2001


Linford, P 2001 Silbury Hill, Wiltshire: Report on Geophysical Survey, June 2001, CfA Rep 65/2001


McAvoy, F 2000a Silbury Hill Shaft, A Desk-based Assessment of the Potential for Archaeological Recording and Investigation, CfA Archive


McAvoy, F 2000b Silbury Hill: Evaluation Report and Options for Further Work, CfA Archive

McAvoy, F, 2002 Silbury Hill, Wiltshire, Report on the evaluation through coring of the physical and geophysical anomalies on the northern aspect of the mound, CfA Archive


Worthington, M H 2002a Assessment of the Results of a Seismic Tomographic Survey at Silbury Hill, Wiltshire, Project Archive


Worthington, M H 2002b Assessment of the Results within Cementation Skanska Report no. C100975/CR4: Silbury Hill, Wiltshire, Final Report of Geophysical Investigation, August 2002, Project Archive



30 Nov 2010, 05:44