CIVIL engineers have returned to Silbury Hill this week to begin further exploration of the pre-historic mound.

It's almost three years since the Gazette was the first to reveal that an historic shaft at the core of the hill had partially collapsed leaving a gaping hole in the centre of the man-made monument.

The shaft was sunk in the eighteenth century by miners working for the Duke of Northumberland to try and solve the mystery of why the hill was built 4,800 years ago.

English Heritage has now asked international civil engineering consultants Cementation Skanska to carry out further tests on the shaft.

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 3-D image rather like a surgical CAT scan.

The material removed during the drilling also allowed archaeologists to see how the hill had been originally constructed between 280 and 200-BC.

As a result of the geophysical survey experts came to the conclusion that the massive chalk mound was built in a spiral fashion.

It was conjectured that a spiral walkway would have led around the hill allowing a procession to reach the 120- foot high summit for pre-historic ceremonies.

As a temporary repair measure, English Heritage had the crater in the centre of the hill packed with polystyrene and capped with a layer of chalk.

Local archaeologists have been wondering how and when permanent repairs will be carried out to the hill.

They were taken by surprise to see more plant and mobile offices from Cementation Skanska arriving at the foot of the hill this week. The engineers prepared a mesh surface on the southeast side of the hill to enable equipment to be winched to the top.

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." One investigation to an anomaly revealed by the scans on the northern flank of the hill revealed nothing of significance, said English Heritage.

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," said English Heritage.

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

Silbury historian Brian Edwards, from Devizes, said he hoped the latest investigation would lead to proper repairs being carried out to the mound without any further delay.

Mr Edwards said: "Let's hope the final repairs will be carried out soon.

"The way the hill was left with just a temporary polystyrene filling has caused a lot of concern."

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