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Silbury Hill, Wiltshire

As part of the framework commission awarded by English Heritage south-west region, Health and Safety Associates first became involved with the Neolithic earth mound, known as Silbury Hill in 2000, following a collapse of the 1776 shaft and formation of a large crater in the plateau at the summit of the hill, together with evidence of migration of its grassed surfaces in various locations.

The initial works undertaken in 2001, comprised geophysical investigation operations involving the deployment and insertion of 5 No. boreholes through the depth of the hill, sampling and subsequent seismic survey to investigate the cause. Temporary remedial works were simultaneously undertaken by installing safety netting over the area of the shaft and subsequent infilling of the crater with polystyrene blocks, tied together using steel dowels, and laid upon a geotextile membrane, and making up to original levels with chalky subsoil.

For the following four years considerable research and planning was undertaken by English Heritage and their consultants, in order that long-term remediation measures could be effectively undertaken for the long-term preservation of the hill. English Heritage concluded that the Atkinson tunnel should be re-opened, to permit all the historic interventions within the hill together with all associated and adjacent identifiable voids to be backfilled with chalk, followed by the removal of the temporary infilling of the crater that had been undertaken in 2001 in order to suitably stabilise the hill.

The 2007 project, undertaken between April 2007 and May 2008, comprised the opening up of the existing deep tunnels routed to the centre of the hill below the 1776 shaft, installation of temporary supports/linings and removal/disposal of existing debris, all in collaboration with English Heritage specialist personnel undertaking archaeological investigatory and analytical works of existing groundworks and historic tunnel linings. Importing chalk from a quarry at Westbury, Wilts and its subsequent crushing and both filling into sandbags that were placed both by hand and machine and mechanically compacted within the tunnels and voids commensurate with mixing with lime to form a pumpable chalk paste mixture that could be simultaneously inserted into all voids. Where possible, all historic tunnel steel supports and linings, together with the contractors temporary supports were removed as the filling of the tunnels and voids proceeded.

The turf and chalk infill at the summit of the hill was removed together with geostyrene blocks, geotextile membrane etc.  (previously installed as a temporary measure), and subsequently in filled up to original levels with the pumpable chalk slurry mix topped with chalky subsoil and seeded.

The infilling of depressions on the slopes of the hill, was undertaken by carefully removing the existing grass turves, infilling depressions with the imported chalk and the reinstatement of the grass turves and seeding.    

Photos ©English Heritage