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Somerset House rises again

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This summer, a magnificent new museum opens in Somerset House. We examine the hidden work that made it possible

Somerset House, completed in 1809, was built on the site of a sixteenth- century palace which had fallen into disrepair.

Designed by Sir William Chambers, architect to George III, it was one of the first purposebuilt government office buildings in Europe. Grouped around a very large central courtyard, it originally contained 50,000m 2of office space.

After years of settlement - some of which originate from the time of construction - remedial works have been carried out to safeguard the structure. The Thames-side South and Embankment Buildings are now dramatically restored - converted into spacious galleries to house the Gilbert Collection of decorative arts, together with a grand public entrance from the Embankment through the newly excavated Great Arch.

The problem of settlement was compounded because Chambers had decided to found the riverside wall of the South Building on an existing Tudor wall that originally separated the gardens of the old palace from the Thames. As foundations on the courtyard side of the Tudor wall were set at a higher level, the building began to tilt soon after it was built.

In contrast, the Embankment Building was founded on elm piles capped with an oak grillage. Although by Georgian standards these foundations were very deep, they were bedded on fluvial silt, causing the building to tilt towards the river, away from the South Building. Architects of later major buildings lining the river bank learned from Chambers' problems and provided substantial concrete raft foundations.

Following three collapses of the brick vaulting during construction of the Embankment Building, Chambers introduced small-section wrought iron bars to tie the brick vaulting together below the springing lines. These snapped over the next 80 years (demonstrated by a range of repairs) and the building continued to move.

The movement was largely halted in the late-1870s when the Victoria Embankment was built hard up against the original river frontage of Somerset House. While this supported the outer wall of the Embankment Building, the height needed to gain freeboard over the highest tides in the Thames required the new road level to be set so high that it covered a third of the height of the Georgian facade.

The new road blocked solidly the river entrance to Somerset House, through the Great Arch.

At the same time, the use of the Embankment Building changed from Government accommodation to the storage of national records and, intermittently, the provision of government printing facilities.

Structure strengthened The new building work includes a mezzanine floor to display the Gilbert Collection in the Embankment building. This presented problems because the old tie bars restricted the ceiling height.

The wrought iron bars below the vaulting were replaced with a reinforced concrete 'staple' across the top of the brick vaulting to tie the top of the building together. This was formed by a thin concrete slab spanning the brick vaulting below and pinned to the tops of the supporting walls with groups of 25mm stainless-steel bars deeply penetrating the brickwork spandrels below.

By removing several layers of failed waterproofing, a thick layer of cracked puddled clay and make-up fill, there was just enough room to form the free-spanning concrete slab in the space between the top of the brick vaulting and the historic terrace surface levels.

In the South Building, historic cracks in the cross walls have been lime base grouted under slight pressure.

To strengthen the weakest part of the double-framed Georgian timber flooring, steel T-sections have been inserted into prepared vertical slots in the centre of each bressummer. Along the face of these bressummers the connections to the incoming timber secondary beams have been individually set and then through-bolted, with the hole then sealed with resin to reduce vibrations.

The Great Arch After the Victoria Embankment was built, only the top of the Great Arch remained visible above pavement level. The Victorian fill within the Arch has been excavated down to the original foreshore levels, again revealing the full height of the arch. To enable this to be done, a retaining wall of bored piles was inserted deep into the river deposits along the edge of the pavement so that the deep excavation could proceed safely.

Within the revealed arch, a new glass floor has been installed on a light stainless steel frame over a carefully detailed plated sub frame.This has provided a level pedestrian access across to the Gilbert Collection from the Embankment pavement. Looking down through the glass, one can now see one of the original commissioners' barges used by the Navy Office, on loan from the National Maritime Museum.

The roof terrace The provision of the concrete staple and the removal of the thick layers of old surfacing on the 230m-long roof terrace enabled a new surface to be formed with a period rolled gravel finish, flanked with York stone margins. Beneath this surfacing, a close gril - lage of drains has been laid over an impermeable high-performance waterproof membrane to protect the vaulting below.

The stone balustrading around the Embankment terrace roof was found to be insufficiently robust to take the loadings required for licensing the terrace gatherings. There is the capacity to cater for some 2000 people. A method of strengthening the balustrading invisibly using stainless steel bars was devised by the engineers whereby the bars were threaded through the diestones, firmly hooked into the edges of the concrete staple below and equally firmly bolted down at the top to capture the stone diestones.

Choice of materials One of the reasons that Somerset House has successfully accommodated the imbalance in its foundations is that lime mortars for bedding and render were used almost exclusively. These have allowed the building to flex over time and have also provided vapour permeability throughout the foundation masonry to allow a balance to be established in moisture movement through the fabric.

Consequently, lime has been used throughout the renovations for the brickwork mortar and for the applied renderings. The use of rigid, cementbased mortars and renderings has been restricted to areas where the new works were required to move as engineered shapes within the flexible Georgian fabric. No differential movement is intended to take place in these areas.

As it was inappropriate to insert an impermeable damp-proof course into the thick basement walling, the historic moisture migration route - from the base of the walling set well below the Thames-side water table up to the ventilated surfaces of the walling at basement flooring level - has been maintained.

Where new flooring surfaces have been introduced, the architects have provided a deep layer of no-fines fill under the new ground slabs.

This ensures that any capillary ground water that does reach the underside of the new floor layering will be dispersed and then vented through discrete openings around the floor edge.

As part of Architecture Week, Helen Caroline Jones will be presenting a lecture on the history of Somerset House followed by a tour on Saturday 10 June, 11:00. Contact Somerset House Trust on tel 020 7836 8686

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