CIVIL engineers have returned to Silbury Hill to begin further exploration of the prehistoric mound. It's almost three years since an ancient shaft at the core of the hill re-opened leaving a gaping hole.

English Heritage was forced to act after the old shaft began to collapse leaving a massive crater in the centre of the 4,800-year-old monument.

Now international civil engineering consultants Cementation Skanska have been asked by English Heritage to carry out further tests on the hill.

In 2001 the company used pioneering new methods to create a three dimensional picture of the inside of the hill.

It bored four holes from the top of the hill to its base to allow sensitive recording equipment to be lowered inside the mound to provide a 3D image of the hill.

The material removed during the drilling also allowed archaeologists to see how the hill had originally been constructed between 2800 and 200BC. As a temporary repair measure English Heritage had the crater in the centre of the hill packed out with polystyrene capped with a layer of chalk. But local archaeologists have called for permanent repairs to be carried out to the hill.

Plant and mobile offices from Cementation Skanska arrived at the foot of the hill this week.

The engineers prepared a mesh surface on the southeast slope to enable equipment to be winched to the top of the hill.

English Heritage explained that further tests were needed at the centre of the hill to ensure that the old shaft was adequately filled in.

In a statement English heritage said: In 2001 we carried out works to arrest a collapse to the head of a shaft which in 1776 had been sunk by the Duke of Northumberland to the centre of the hill.

"Following this work a geophysical survey of the whole hill was carried out on our behalf by Cementation Skanska.

"Reassuringly they reported that the hill was a robust structure which was basically stable but identified certain areas which required further investigations."

The work that started this week is on the 18th century shaft.

"As part of this assessment we intend to test the consistency of the backfilling in the lower part of the shaft by drilling a borehole through it from the top.

"Another borehole will be drilled nearby as a control. The information gathered will help us design the long-term remedial work."

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