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Thanks a million

MORE THAN £1M was saved on a London project after a finite element analysis by consultant Card Geotechnics (CG).

The firm is providing geotechnical advice to structural engineer Walsh Associates over the design for a new development at Tally Ho Corner in Finchley, north London.

It suggested a load-bearing raft rather than the piled foundation originally proposed and used numerical modelling to prove this was a technically viable alternative.

The mixed development, which has two basement levels, includes a theatre complex, shopping and leisure facilities, a bus station and a 14-storey residential tower. The developer is Chiltern Investment Properties and the main contractor is Gleeson.

CG's remit includes foundation design, groundwater control and construction of a contiguous pile wall around the site footprint.

Boreholes showed the site to be typically underlain by 10m of glacial till with many silty partings with elevated pore pressures. This was underlain by 2m of glacial sand and gravel. London Clay was encountered at about 12m. The bottom of the proposed basement lay near the base of the glacial material at a depth of 8m.

CG suggested a raft because under the proposed building loading, the bearing capacity of the glacial till and London Clay on which the raft would be founded was not critical, but settlement and differential movements of the substructures were.

To assess the likely settlement of the raft both during and after construction, CG did a series of finite element analyses of cross-sections through the basement structure using Sage-Crisp software.

This allowed time-dependent interaction of the soil and structure to be modelled under the complex loading patterns.

To model a series of possible construction sequences and to obtain an indication of the relative deflections of the raft, time analyses were done on two cross sections and one long section through the site.

Two-dimensional results of the analyses were assessed in combination with more traditional design tools and extensive in-house experience.

This provided an indication of the displacements of the raft and perimeter retaining walls in three dimensions.

The results were also used to assess the likely impact of the construction works on a deep sewer which runs under part of the site at a depth of 37m, to satisfy its owner Thames Water.

Walsh Associates worked closely with CG in refining the design and construction sequence to restrict the differential and total settlement of the raft and to minimise the impact on the sewer.

Output from the finite element analysis was used to help in the structural design of the raft itself and additional geotechnical analysis provided the best design for the contiguous piled retaining wall.

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