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Holding back the years

NCE looks at two projects where engineers are striving to stall the effects of old age on vulnerable structures.

Down to a fine art

London's Somerset House pushed Georgian construction technology to its limits. Dave Parker sees how today's structural engineers are creating a modern gallery inside the historic but ageing fabric.

After 200 years as government offices and archives, Somerset House is undergoing a transformation into London's Louvre. Galleries, art collections and restaurants will be housed in what is still a largely unmodified major Grade 1 listed Georgian building.

For the first time since the mid 19th century, the public will be allowed back on to the riverfront roof terrace, and the huge central courtyard, once a Civil Service car park, will stage open air opera and theatrical performances.

Central to the £30M project is the conversion of the building's grand river frontage into a home for the Gilbert Collection. In its prime location between the West End and the South Bank, Somerset House should become a magnet for art lovers across the world - provided major foundation problems can be sorted out.

Structural engineer tackling this demanding job is Oscar Faber. Principal engineer Mike West explains the problem: 'The architect opted to found the main river frontage on top of the massive Tudor river wall which separated the gardens of the original Somerset House from the Thames. Foundations on the landward side are much shallower, so the main building is tilting northward.'

But the real problems lie with the Embankment building below the terrace. Here the foundations were found to be inadequate soon after the building was opened in 1786.

Architect Sir William Chambers provided 'parking space' for the horses and carriages of Somerset House bureaucrats on the ground floor of a barrel vaulted structure 'nearly as long as the Titanic', with a mezzanine floor above. Its outer wall, in those pre-Victoria Embankment days, was founded in the muddy foreshore of the Thames itself.

While this design allowed Chambers to provide a Great Arch in the frontage through which Navy Office barges could pass to tie up at a dock inside, sadly, the technology of the time was hardly up to the task of stabilising this outer wall.

'The upper sediments are very soft,' West explains. 'Below this there is rather better clays and silts, but Chambers couldn't get down that far. All he could do was dig down as far as he could, drive elm piles and lay an elm grillage on top.'

Unbalanced forces meant that serious cracking developed in the brick arches above the stables even before the building was completed. Wrought iron ties were installed, but settlement continued and the ties were constantly breaking and having to be repaired. As the building moved, the lead waterproofing between the arches and the terrace above began to stretch and tear.

Temporary salvation came in 1870, with the completion of the Victoria Embankment along the foreshore. This helped to balance the forces on the outer wall. At the same time much heavier ties were installed across the arches 100mm below their Georgian predecessors and asphalt was poured on top of the lead in another attempt to waterproof the terrace.

Soon after, having lost its access to the river, the Navy Office moved out. For several decades the ground floor of the Embankment building became variously a document archive and a printing works .

'Considering its proximity to the river and lack of a damp-proof course, the building was remarkably dry,' West comments. 'This was largely down to the use of lime mortar, which we have also specified almost exclusively for the refurbishment. We'll be taking other precautions against rising damp, of course.'

But the real environmental problem of converting this space into a modern gallery is the pollution from traffic on the Embankment, says West. And with full air conditioning considered essential, the challenge was to find space for plant rooms, ducts and trunking without having to convert an unacceptable number of the largely original Georgian offices nearby.

Oscar Faber's answer was to insert two very long concrete boxes below the existing ground floor of the Embankment building to act as plant rooms. Air will be taken in from the least polluted area available - the lightwell behind the building's main frontage - and exhausted towards the Embankment. Ducts and trunking have to be squeezed through and around the complex masonry of the Tudor wall and the Georgian structure above.

A new first floor has also been added using steel and timber. Bordered by glass with minimal connections to the main fabric, it will appear to float in mid air.

But its complex design was not the problem. Providing sufficient headroom below the large reinforcing ties added in the 1870s was the biggest test for engineers.

'Victorian clerks had to put up with ducking between the ties,' says West, pointing out that visitors to the gallery cannot be expected to do likewise. 'We had to find a way of strengthening the structure so these ties at least could be taken out.'

Site investigations above the 300mm thick brick arches discovered several layers of failed waterproofing under the terrace. 'Removing all this makes it just possible for us to insert a reinforced concrete 'staple' which takes the load off the arches and reduces side forces by 60% or so,' West explains.

This staple consists of an insitu concrete slab cast on top of very thin Omnia precast bridge deck planks - total thickness is only 400mm - and groups of 25mm thick stainless steel bars inserted down the spandrels of the main arches.

The reduction in loads makes it theoretically possible for all ties to be removed, although at the moment it is hoped the Georgian ties can be retained. And extending the staple over the full width of the terrace also significantly reduces the load on the much smaller arches above the river front loggia.

The outer edges of the new slab also pick up load from the stone balustrading, which needs extensive strengthening to meet modern safety requirements. Stainless steel channels within the slab will connect to stainless steel bars passing through the solid diestones between sections of balustrade. All will be joined by hidden stainless steel rods and yokes.

Above the slab the architect has just 350mm below the terrace surface to install a complex stainless steel drainage system. A sheet waterproof membrane bonded to the top surface of the staple acts as the final defence against water penetration.

Main contractor John Doyle began work on site early last year. Excavations were preceded by an intensive archaeological investigation - although little was found, says West. 'There are references in literature to the King's Barge being kept inside the Great Arch, so we expected to find some sort of dock below the floor. But there was almost no trace of anything except a mud bottom.'

Once completed, visitors to the Gilbert Gallery will enter from the south via the newly exposed Great Arch. West explains: 'Most of the arch has been hidden for years by the Embankment. The plan is to excavate the Victorian fill beneath the arch down to the original foreshore level then construct a glass bridge from the pavement into the ground floor of the gallery, which will allow visitors to get a proper view.'

A row of 600mm diameter continuous flight auger piles had to be placed each side of the arch before excavation could begin. Currently, with the ground floor removed and the walls cross-braced, John Doyle is well advanced with the excavation for the plant rooms. At terrace level, beneath elaborate weather protection, concreting of the staple is under way. Progress is good, says West, despite the inevitable constraints of working within a listed building.

He adds: 'This project will set the standards for all future work at Somerset House. We still have to finalise our plans for solving the differential settlement of the main river frontage - but it will have to be done as discreetly as possible.'

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