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Made to measure

One of the most complex building projects in Europe is taking shape at Harrods in London. Richard Bennett went shopping for the story.

It is hard to think of a connection between Harrods department store and the construction industry. But some civil engineers are getting used to the rarified retail environment on Harrods new £45M development - even though they may not be able to afford to shop there.

Knightsbridge's famous 'corner shop', which opened in 1849, is being extended to provide 26,000m 3of additional parking, office, apartment and loading space in a new building, on the site of a former crown court just behind the store. Built by the Cementation and Construction divisions of Kvaerner, the building will be connected to the basement of the existing Brompton Road store by a goods delivery tunnel under Basil Street.

The new building has 13 storeys, seven of which have been formed underground with 25m of top down construction.

Above ground the structure will retain the Knightsbridge Crown Court facade, along with new cladding to match the surrounding Edwardian streetscape.

Design of the complex concrete framed structure presented a number of challenges for consultant WSP. A steel frame was rejected due to the complexity of the design, which includes large spans, access ramps and a 'Z' shaped above ground structure. The mixed use requirement - ranging from loading and parking areas to offices and apartments - necessitated three different column grids.

Parts of the first floor had to be suspended to avoid columns in the ground floor loading bay.

'Juggling the load transfer structures to get the load paths right has been one of the most significant aspects of the job, ' says Alan Darling, senior project engineer with WSP.

Top down construction was selected as the only viable method of constructing the seven storey basement. After demolishing the crown court and underpinning the surrounding buildings, an 800mm thick 29m deep diaphragm wall was installed around the site perimeter.

This was followed by piling for the installation of the 39 prefabricated steel plunge columns.

The ground floor slab was cast, and then excavation began below it, with spoil removed through a 'moling hole' cast into the slab. This process was repeated seven times and has now reached the bottom, where mini tension piles are being used to control heave of the floor.

Another technically difficult aspect of the job has been the tunnel and access shaft, which has been sunk below the existing 'lightwell' in the store.

In this environment settlement needed to be avoided at all costs, and noise and vibration minimised. An array of 600 sensors were installed in the ground and surrounding buildings, based on techniques used in the Jubilee Line Extension. These were used in conjunction with computer controlled compensation grouting around the line of the tunnel, using 51 tube-amanchette grout injectors.

Inclinometers, electrolevels, extensometers, piezometers, crack measurements and precise levelling generated 35,000 readings a week. Faced with so much data, Kvaerner Cementation Foundations opted to develop its own software to filter and interpret the information.

Predefined trigger levels were set for each reading, with special emphasis placed on differential settlement.

Prediction of ground movement around the basement excavation was undertaken in conjunction with the Geotechnical Consulting Group, using building damage prediction charts developed by Boscardin and Cording. Analysis of the structures showed that gradients of settlement around the tunnel and shaft needed to be controlled to less than 1:750.

Gradients produced were generally well below this - in the region of 1:1200 - although in some cases the basement excavations caused cracks to open up in the masonry of the adjacent buildings. However, these closed as excavation progressed and settlement troughs widened.

Excavation of the 75m long 5m diameter tunnel was conducted by hand to minimise ground movements, and required constant communication between the tunnellers and grouters. A 2.74m diameter pilot tunnel was driven first and lined by hand with concrete segments.

This drive was then carefully widened out to 5m. Some 49,000m 3of London Clay was excavated by 12 miners using jackhammers. Fears over handarm vibration syndrome meant that low vibration clay spades had to be specially developed and approved by the HSE for use with the jackhammers.

The widened tunnel was lined with 1,400 750kg segments.

These required a bespoke hydraulic erector, built specially by Kvaerner Cementation.

With the shell and core due to be completed by the end of 2000 the job is currently about three quarters of the way through. The major underground construction works are now complete and although there were a few scares, settlement has been within the predicted limits.

Kvaerner and WSP are now turning their attention to finishing the shell of the structure above.

On the spot Name: Alan Darling Age: 52 Qualifications: BSc(Hons) MICE CEng Company: WSP Group Current job: Associate director and project team leader Best thing about the job: The challenge and achievement of solving various design issues on a highly complex building.

Worst thing about the job: Meeting the various demands of your time when time is often not available.

Most useful lesson: The ability to work as part of a team to achieve a successful end result. This is particularly true on this project where the contribution from all consultants, client and contractor have been essential to the project's success.

Advice to young engineers just starting out: Be honest about your capabilities, be keen to take on new challenges and be flexible to gain a wide experience in engineering.

Anything else? I love travelling. Holidays are always a 'voyage of discovery'. I am an enthusiast for exploring old buildings and historical monuments.

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