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Every thing, London

Development of new goods handling facilities for world famous store Harrods involved some extremely complex geotechnical engineering. Max Soudain reports.

A baby elephant bought for Ronald Reagan by the son of King Zog, the last king of Albania, an alligator bought for Noel Coward, aeroplanes, yachts made to order and houses built - at one time it was claimed Harrods could get anything for its customers.

While customers and their requests may now be a little more conventional, the store is still renowned for stocking a huge variety of weird and wonderful products.

Surprisingly, all deliveries to the massive London department store, however exotic, have to come in via a single loading bay at the back of the building. Only one lorry can unload at a time which makes things very congested, explains Harrods director of technical services Paul Crofton.

A pressing need for new goods handling facilities were not the only problem - some of Harrods' backroom operations also had to move out of two nearby buildings that were being redeveloped.

The solution was found opposite the rear of the store on Basil Street, on the former site of Kensington Crown Court. This building was demolished, retaining the original facade, and replaced with a new structure with a sevenstorey basement. New facilities will include nine loading bays, car parking, warehousing, kitchens, offices and apartments.

The lowest basement level is linked to the store by a tunnel beneath Basil Street.

Merchandise will be distributed to the shopflorr via a new lift shaft in a lightwell inside Harrods.

The £72M development is being carried out by Kvaerner's construction group, with Kvaerner Cementation Foundations and Kvaerner Cementation tackling the underground work, Kvaerner Trollope & Colls acting as main contractor and former sister firm Cleveland Bridge handling structural steelwork. WSP Consulting Engineers is responsible for the design of the new building on the Crown Court site.

Geotechnical work (excluding the tunnel) is valued at £7. 5M and obviously is a major undertaking in this prestigious area ofLondon, crammed with exclusive retail and residential property. 'The key issue is ground movement, ' confirms KCF area manager Martin Kenwright, Kvaerner installed more than 2,500 instruments linked to its Surpoint+ system (Ground Engineering, April) to monitor ground movement. The electrolevels, extensometers, inclinometers,3D retro targets, piezometers and telltales produce 35,000 data records every week, although Kenwright says some redundancy was built into the system to check results.

The seven-level basement of the new building was built by top-down methods. After underpinning the embassy building on one corner of the site and installing temporary support for houses backing on to it, KCF installed a 242m long diaphragm wall around the footprint of the building. The 60, 800mm thick, 28m deep panels are between 2. 8m and 6. 7m long - 'they were generally limited to 4m long, ' says Kenwright - and were formed under bentonite through the underlying Thames Gravels and into London Clay beneath.

He adds that the operation was very complicated, as there was no repetition ofpanel design, with a varying layout of slabs, ramps and sumps.

To allow top-down construction, 39 bored plunge column-bearing piles were installed.

These are between 1,200mm and 1,800mm in diameter and up to 45m deep. Steel I-beam plunge columns up to 29m long were then placed in the wet concrete of the piles at 23m below ground level. KCF used its patented Cemloc system to position the 800mm by 300mm steel columns to very tight tolerances, with accuracy in plan at ground level of+/10mm and at cut-off level +/- 20mm. Vertical tolerance was set at +/- 10mm. 'Brain surgery, ' says Kenwright. Once in place, the bores were backfilled with foam concrete and the temporary casing pulled.

Tension piles were then installed to reduce uplift and prevent heave of the base slab and 36, 1,050mm to 1,500mm diameter bored piles were put down to 40m, cut off typically at 15m below ground level.

The ground level slab was cast and a miniexcavator used to dig down to the base slab level,23m below, with floor slabs generally cast every 3m. The base slab is 900mm thick.

Construction of the 23m deep shaft in Lightwell X5, for excavating the tunnel under Basil Street, was extremely complicated.

Kenwright explains that work had to be carried out without affecting the retail operations, so noise and vibrations were strictly controlled. During opening hours, materials, plant and equipment had to enter and leave the working area via a small entrance alongside the loading bay at the back of the store, but only after 3pm.

Kenwright says the main technical challenge was to minimise settlement during construction of the shaft and the tunnel. The Harrods building is a cast iron structure and its fascia is very sensitive to movements, so before shaft excavation could begin, underpinning was carried out. The working area was only 15m by 4m with a headroom of 2. 5m. Kvaerner used two electric rigs, which had to be winched down a ramp to the piling area. Nineteen settlement reducing and 21 temporary support minipiles were installed to support three columns and to reduce settlement on the wall next to the shaft. These are up to 24m long, 285mm diameter in the gravel and 235mm diameter through the London Clay.

Support for the three new lift shafts in the lightwell is provided by 24, 285mm and 235mm diameter minipiles, up to 40m long and sleeved over the top 18m to avoid heave, passing within 1. 6m of the tunnel.

Once the minipiles were in place, the 14. 5m by 4. 5m watertight cofferdam for the shaft could be built. This comprises a hard/ soft secant wall, made up of 110 male and 110 female raking minipiles (again 285mm and 235mm diameter). The male piles are 30m long and the female piles 8m long. Kenwright says these were installed with 'exceptionally' high tolerances and were surveyed at three levels (at 7m, at the tunnel level and at the base of the piles) during installation to ensure they did not interfere with the tunnel construction and top-down shaft building.

Compensation grouting to reduce settlement during tunnel building was carried out from a temporary platform within the completed shaft. Kenwright explains the original plan was to excavate halfway down the shaft and grout but instead the shaft was completed to allow tunnelling to start two to three weeks ahead of programme. Some 51 tubes a manchette up to 47m long were installed to inject 104m 3of PFA based grout and 25m 3of bentonite/cement grout into the London Clay.

Empirical assessment of settlement was carried out by the Geotechnical Consulting Group using building damage prediction charts developed by Boscardin and Cording.

Analysis produced lower and upper limit boundaries for settlement prediction, with maximum predicted settlement between 30mm and 35mm. Actual settlement was somewhere in between, says Kenwright.

The 70m long tunnel was excavated by hand in the London Clay, using pilot tunnels.

The main tunnel is 5m diameter, lined with precast concrete segments, and the holding tunnel beneath Harrods is 7m diameter, lined with spheroidal graphite iron segments.

Major underground work is now finished and the shell and core of the new building will be finished at the end of the year. By the end of 2001, Harrods will have its new handling facilities, allowing baby elephants and alligators to make their way safely into the store, two by two ifnecessary.

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