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The rolling stones

Foundations Avebury stone circle

Authenticating a 4,500 year old standing stone is no easy task. Ruby Khan reports from Avebury, Wiltshire, where engineers are stabilising a monolith which has lost its foothold.

Something strange was afoot at the Avebury stone circles in Wiltshire, UK, in April.

Druids appeared in white robes, drums were ceremonially beaten and a group of archaeologists with spoons appeared. Then one of the standing stones began to move.

But this was not the work of some ethereal wonder. It was movement caused by carefully jacking a 50t standing stone back to its original vertical position.

The events were part of a ú50,000 restoration project for the 4,500 year old stone circles, funded by Britain's National Trust.

In 1999 the Trust approached consulting engineer Mann Williams to assess whether the leaning stone was in danger of toppling. Four years of consultation with archaeologists and druids followed before the method statement for the stabilisation of the stone was finally approved.

Avebury's stone circles cover over 11ha, an area 14 times the size of its better-known cousin, Stonehenge.

The site comprises an outer stone circle and a pair of inner circles to the north and south.

The two standing stones in the centre of the north circle are all that remain of the Cove, believed to have been a focal point for ancient rituals.

The Cove's two sarsen standing stones are thought to have been dragged to the site between 3000BC and 2200BC by people using leather straps to haul them over logs used as rollers.

The smaller of the two stones, affectionately named 'Little Nell' by the engineers, was found to be leaning 15infinity when compared to 19th century photographs which showed it lining up vertically with an adjacent cottage wall.

It was unclear from these photographs whether the larger of the two stones, known as 'Big Bertha', had moved, so it had to be treated with similar caution.

Mann Williams decided that sonic logging tests had to be carried out to see how deep the stones were embedded below ground. This would enable the consultant to assess the extent of the problem.

Results revealed that Little Nell was founded 1m underground. The need for stabilisation work became clear when the centre of gravity was calculated to be outside the stone's base.

Further geophysical investigation suggested that Big Bertha had a shallow, tapered base. At best it was embedded 1m in the ground; at worst, less than 300mm. There was a risk that Bertha was in imminent danger of toppling over, so fencing was erected immediately to protect the public.

Since the site is of prime archaeological interest, Mann had to devise a stabilising scheme which had almost zero impact on the stone and surrounding ground.

A thrust block could have been cast into the ground to provide the reaction for jacking but archeological considerations precluded this.

The solution was to construct a scaffold platform perpendicular to both stones with sufficient kentledge to press the scaffold legs into the ground.

Anchoring the legs behind ridges in the undulating ground provided further resistance.

Contractor Ellis erected two support frames of RMD props off the platform and around the stone, with jacks resting on rubber pads against the leaning side of the stones. This prevented the jack heads damaging the rock and protected ancient lichen growing on it.

A prop was also installed in front of the stones to prevent them falling forward. The prop was connected to the front jack using a stiff tension ring which could be mobilised to transfer the out of balance load back to the main support frame.

'We decided to use screw jacks instead of hydraulic ones because we couldn't risk hydraulic oil spilling onto the stones. We also tried to use traditional methods of engineering where possible, ' said Mann Williams project engineer John Mann.

The frames had pinned connections at strategic locations to allow them to deform as jacking took place and the stone rotated into the vertical position.

Since the underlying ground was clayey chalk, Mann decided to use hydraulic lime concrete, poured into a trench at the base of the stones to secure them after the repositioning operation.

The mix design ensured the concrete had a similar strength to the ground, so that any natural creep could be accommodated.

The choice of concrete also meant that if archaeologists wanted to re-excavate the ground, the material could be chipped away easily without damaging the stones.

This solution won favour among the local druid population and National Trust as it used calcium based materials, which were natural to the site.

Excavation at the foot of each stone began the week before Easter weekend, once the supports had been put in place.

Enter a team of six archaeologists with spoons carefully digging a 1m deep trench against the leaning side of Little Nell.

With the support frame in place contractors were also able to excavate at the foot of Big Bertha. This revealed that the stone was much more stable than first feared, embedded over 2.5m in the ground. With no danger of it falling over, it was enough to backfill the trench with lime concrete. 'We will continue to monitor it for any signs of movement over the next five years, ' Mann says.

Jacking of Little Nell was completed in a few days and the trench in front of it was filled with lime concrete. Lime grout was used to fill the gap left on the opposite side of the stone after it had rotated. When the concrete had gained enough strength the props could be removed.

As the base of the stone was revealed, so too was the mystery behind the stone's movement.

It emerged that the footings of an adjacent cottage had broken into and undermined the chalk sockets which cradled the standing stone, making it unstable.

Mann believes that when the National Trust demolished the cottages in the 1950s, the chalk socket was damaged again, increasing the stone's instability.

Contractors removed the props last month, once the consultant was satisfied the stabilisation work had succeeded.

Now druids and engineers alike can touch the stones again.

INFOPLUS www. nceplus/magazine

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