The project aims to protect one of the most important archaeological finds in London this century while making it possible to build 1,600 square metres of office space on prime development land.
In a complex operation, engineers will build a platform over the ruins to take the weight of the new building and, later, scoop out a storage space for archives underneath, effectively leaving the amphitheatre suspended in mid-air.
The space containing the amphitheatre will become a museum featuring pottery,
animal bones and pipes recovered from excavations before building work began. Visitors will be able to look down on the remains of Roman London from elevated
David Alsop, director of Oscar Faber, consultant engineers for the pounds 38m Guildhall Yard East project, said the whole venture has been exciting: 'It's a major challenge. We have brought together ideas and techniques that have never been done before on the one site.
The City of London Corporation decided to build on the east side of the square outside the Guildhall in 1986 to replace a gallery destroyed during the Second World War.
About 3,500 paintings owned by the corporation have been in storage ever since. The new complex, which will also include offices, will open
However, it emerged that the place where the corporation intended to build was the site of an unrecorded Roman amphitheatre, buried some 19ft down.
Permission to proceed with the project was given only after a public inquiry during which the corporation gave an
undertaking that it would construct the new building in a way
which would not disturb the remains. It also promised not to start work until archaeologists had conducted a thorough excavation of the site and removed artefacts.
Now, after six years of delay, the work has got underway. The central engineering conumdrum was devising a method of sinking the 41- metre (130ft) concrete support piles for the building into the ground without damaging the amphitheatre.
Placing a 100-tonne piling rig directly on the site to drive the supports into the ground would have shattered the remains.
The solution has been to build a platform or transfer deck on top of temporary piles which were dug with much lighter machinery and which did not disturb the remains.
These smaller supports are designed to carry the combined weight of the 750-tonne platform and the main piling rig during the second phase of the operation - sinking the principal piles for the new building.
After the weight of the platform has been shifted from the temporary piles to the main supports, it will be strong enough to carry the weight of the new gallery.
Enormous customised steel girders are being brought into London from a steel manufacturer in Bolton to build the platform. Each piece, the largest 12-metres (40ft) long and weighing 22 tonnes, is swung into place by a large hydraulic crane and then bolted on to the existing framework.
Building over the south-west corner of the site has just begun and builders are gradually edging across the remains to the north side, bordering on
Once the platform is in place, the piling rig will be brought on to it.
It will sink concrete piles 41 metres (130 ft) into the ground to support the final building. Then the whole platform will be bolted on to the permanent piles and the temporary piles removed.
Normally the temporary piles would be of identical size and placed at regular intervals, but this is impossible on the Guildhall site.
Instead, engineers have varied their size and tucked them in between ruins of the former entrance to the amphitheatre. Contractors will build a supporting platform under the remains and hollow out a further two floors. These will be used to house the boilers and other machinery needed for the building.
On top, they will build the spilt level art gallery on the first two floors and, above that, four floors of offices which will be used by the corporation.
In the footsteps of Roman giants
Six years ago, when a Victorian building adjacent to the Guildhall was demolished, the archaeologists from the Museum of London moved in. They had an inkling that they might find something important.
They uncovered the east gate of what was once a magnificent Roman amphitheatre - evidence that the Guildhall site has been an important focus for settlements on the north bank of the Thames for nearly 2,000 years.
Nick Bateman, site supervisor for the museum's archaeological service, said: 'It is undoubtedly a hugely significant discovery, not just for London, but for the whole country. The dig began in 1987. 'We quickly realised the remains were part of an amphitheatre, said Mr Bateman.
English Heritage classified it as a scheduled site, which meant it could not be disturbed. A public inquiry followed and work halted before excavation resumed in 1992.
Mr Bateman said: 'What we are looking at is development from the Roman period right through to the present-day Guildhall. It is the whole story that is told by this historic site that is important.
'For instance, how does a very important civic centre like Guildhall relate to an equally important building that is 20 feet below it?
The Saxons thought that the Roman remains were the work of giants and possibly used them as foundations for buildings during the middle ages.
The remains are a great example of the Romans' brilliance as civil engineers. They invented hydraulic concrete, a water and cement mix which enabled them to build the huge arches and viaducts seen on the Continent. After the decline of Rome, the technique was lost to the West until the mid-19th century.