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Rewriting history Restoration of the venerable Round Reading Room at the British Museum required extensive underpinning.

MILLENNIUM PROJECTS

Work on the £97M British Museum Great Court project began in March 1998. Its centrepiece, funded with £30M from the Millennium Commission and £15.75M from the Heritage Lottery Fund, is the restoration of the Round Reading Room, the original home of the British Library.

Designed by Norman Foster & Partners, the domed structure will have a new exterior of Portland Stone, a new basement level and the Great Court in which it sits will be topped with a 6000m2 steel and glass roof.

The main thrust of the geotechnical work was the formation of jet grouted columns to underpin sections of the Reading Room and the buildings surrounding the 92m by 73m courtyard. This allowed excavation of the 5m deep basement in the southern half of the area and strengthened existing foundations to take loads from the new roof.

The first stage of the project was to demolish the redundant book stacks that obscured the Reading Room for much of its working life and almost filled the courtyard.

While this was going on, geotechnical contractor Keller Ground Engineering began to move on site. All the plant, equipment and materials had to be dismantled and craned in over the front of the museum. Keller used two jet grouting rigs and one mini piling rig for the contract and set up a batching plant outside the front of the building.

Site investigation had to be carried out before demolition with access restricted to just four boreholes, one in each corner of the courtyard. But consultant Buro Happold's geotechnical group was confident of the geology, mostly fill overlying Thames Gravels and London Clay.

Before work could start, detailed finite element analyses were carried out by Keller and Buro Happold - the latter was also structural engineer for the project - to finalise jet grouted column design. Originally, ground anchors were going to be used to provide additional support for the jet grouted block but modelling showed there was no need to tie back. Analyses also reduced the number of jet grout columns.

The Reading Room's copper dome is supported by a skeleton of 20 cast iron ribs, with an interior papier-mache lining. The ribs sit on stone and brick foundations, in turn supported by a concrete slab between 1.8m and 3m thick.

More than 200, 1.2m diameter columns were installed under the southern edge of the foundations at 1m centres to form a large block to prevent settlement and lateral movement of the reading room.

Grouting was carried out at foundation level, with columns keying into the underside of the foundation slab and into the London Clay about 5.5m below. The front row, which acted as a cut-off for the basement excavation, was vertical with columns behind angled under the foundations.

Keller used a triple fluid jet grouting system, with a high pressure air shrouded water jet to cut the column and grout injected through a separate nozzle below.

A double pass was used to deal with the stiff London Clay and to ensure column integrity was maintained across the boundary between the gravel and the clay. As the tool was withdrawn, jetting was carried out, but without grout injection. Just above the boundary of the clay and the gravel, the jets were switched off and the tool returned to the base of the hole. All three jets were then switched back on and jet grouting carried out.

One of the main problems was preventing movement during grouting. The first few columns produced small amounts of heave in the Reading Room, but pressures were adjusted to prevent this. However, heave was well within acceptable tolerances - 10mm vertical and 7mm lateral - and showed good contact and bond of the columns with the underside of the foundation slab. Spoil generated from jet grouting was reused to form a cut-off wall around the rest of the excavation perimeter, minimising waste and allowing dry basement excavation.

Extensive instrumentation linked to data loggers was used for real time monitoring of ground movements. To monitor settlement, water cells were placed on the stone pads beneath the Reading Room's cast iron columns on the southern side, one was installed in the middle of the building to act as control and one at a remote location in the courtyard.

Tiltmeters were also been placed on the cast iron columns above the main area of jet grouting. Precise levelling and geodetic surveying was carried out on the outside of the building. Inclinometers, piezometers and rod extensometers were installed by Keller for the basement excavation.

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