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Notes from the underground

The glory on a world class prestigious project like the British Museum Great Court is generally taken by those responsible for what appears above ground. But underneath and literally underpinning the complete structure is highly complex and intricate geotechnical engineering no less elegant in its concept and execution.

Existing foundations under the reading room consisted of a weak, brittle mass concrete ring beam with a compressive strength of only 8N/mm 2. Aside from strengthening the foundations to cope with additional load from the new roof, structures and services, construction of a new basement adjacent to the reading room posed immense challenges.

Ground conditions consisted of around 5m of Taplow Gravel on London Clay found about 4m below footing level. The new basement was to go down to clay. Assessing the soil parameters to accurately model ground movements by determining ground stiffness was essential.

The choice of construction method was also highly dependent on the soil parameters. But, regardless of the method chosen, space constraints would require machinery to be dismantled and reassembled. Contiguous piling was ruled out because a continuous flight auger rig would have been too big, while an alternative tripod rig would be too slow and limit piling to no closer than 1m from the reading room, reducing the basement area.

Minipiling with ground anchors was also examined but would be expensive and slow. 'With this, we were afraid of punching shear. If one of the cast iron columns failed because of this, the whole building would fall down, ' says Buro Happold project manager Steve Brown.

Jet grouting was chosen in the end, with ground anchors to restrain the ground from movement. Ground engineering specialist Keller was awarded the £800,000 contract, beginning work in August 1998. But detailed design then led to a decision to replace grouted dowels as anchors within the London Clay, with a grout jetted block to limit movement during construction.

The jet grouted underpin was to be constructed in four rows of jet grout columns positioned at 1,000mm centres, with one row to project into the clay. Complex monitoring equipment was installed to measure movement, with simple green/amber/red-for-danger readouts available instantaneously on site, and in Buro Happold and Keller's offices.

Total ground movement was in the end 5mm vertically and 9mm laterally, comparing well with predictions.

A clear instantaneous monitoring system enabled control of the construction throughout, preventing excessive movement with potentially disastrous consequences. Brown says success also depended on cooperation between Mace, Buro Happold and Keller. Only an imaginative approach and collaboration with Keller's experts made it possible. DF

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