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Rig spotters' delight

Foundation work for a four-level basement at a new Marks & Spencer's in central London showcases an impressive array of sophisticated piling techniques. David Hayward reports.

Lunchtime regulars at the Swan Tavern in the City of London might claim to know a fair bit about piling. The pub's upstairs lounge window overhangs one of the city's most congested construction sites, currently home to four of foundation contractor Bachy Soletanche's largest rigs.

Locals will have witnessed large diameter augering being used to break up underground obstructions, construction of a secant piled perimeter retaining wall, plus sinking of king posts, plunge columns and vast under-reamed piles.

Rig spotters might marvel at the hardware: a Soilmec R622, a BG30, an HT55 mounted on a C60 Casagrande crane base and a KCA100 rig on a NCK Ajax crane. The keen observer might question why casings vanish downward through pre-cut slots and holes in a ground level concrete slab. However even an informed foundation engineer might find it difficult to explain fully the view to fellow drinkers.

This is because the impressive array of piling operations is being carried out through an existing 7m deep, two-level basement. And the 450mm thick basement slabs cannot be removed until a new 221m long secant piled perimeter retaining wall is in place, just 100mm inside the existing wall.

'The slabs act as horizontal props to an original concrete retaining wall around the sides of the old basements,' explains Bachy Soletanche contracts engineer Ian White. 'To demolish the slabs before completing our secant piled wall and a new concrete retaining wall above would risk settlement in adjacent roads and office blocks.'

The compact, 3,000m2 corner site in Fenchurch Street is destined to be home to one of Marks & Spencer's largest new department stores. Main contractor HBG will provide its client with an eight-storey building above ground and four levels below. Project consultant is Ove Arup & Partners.

The old basement slabs are heavily propped and headroom is low. There was no alternative but to cut 1.5m wide access slots through both slabs, directly above the new secant piled retaining wall. Another 30 or more square holes were needed within the new basement area to allow load supporting plunge columns to be formed.

All slab cutting, plus the positioning of temporary steel plates over the top, was planned to be completed by HBG before Bachy arrived on site to start its £2M contract in March this year.

'But the extent of the work proved greater than expected and initially we were given only partial access', White explains. 'We had hoped to have the first three rigs fully operational within a week but, in reality, it took longer.'

Bachy Soletanche's first task was to remove numerous concrete obstructions below basement level, bequeathed by the builders of the Barclays Bank that previously occupied the site. Concrete bases, a 5m thick tower crane foundation and even a concrete filled pedestrian underpass were all broken up by large diameter augering.

This done, work could start on the secant piled wall, consisting of 304 piles up to 31m long. The secant piling will form the lower part of the new retaining wall with a new insitu concrete retaining wall up to 1m thick built over the top.

Typically every third pile has a steel king post cast into it, protruding 7m above the pile top into the basement area. Short flat-jack props, placed horizontally in the narrow gap between king post and existing wall, will aid the latter's support during demolition of the old slabs.

Some half dozen old bank piles were in the way of the underground secant wall. Instead of trying to remove these Bachy Soletanche has incorporated them into the new wall, by pressure injecting grout to ensure a watertight seal between old and new piling.

Equally complex are the 33 load bearing plunge columns scattered around the site and being threaded down through both basements. Their purpose is to provide a steel column frame around which construction of the department store can be started simultaneously both upwards and downwards from ground level.

Each column, up to 23m tall, is positioned using a guide frame and embedded in a 1200mm diameter pile, bored a maximum 16m deep. Steel columns extend upward through both basements to rise above ground level.

To complete the piling jigsaw, 11 under-reamed piles are needed to later carry additional above ground steel columns. These 900mm diameter bored piles contain an under-ream up to 3m wide, the condition of which is determined remotely using Bachy's prototype penetrometer system.

This underground congestion of piles is matched on the surface where, in the early weeks of the contract, there was little space to store anything. A 'holding bay' was set up in a haulier's yard behind King's Cross Station on the other side of the City and the daily arrival of a lorry laden with rebar and pile casings became the norm.

At £400 a round trip through congested streets, the economic timing of such journeys was important and White soon developed a 'just in time' arrival system despite the vagaries of London's traffic.

More stressful now for the site team is the contractor's efforts to make up for lost time. Daytime noise constraints remain in place but, following discussions with environmental health officers, Bachy is allowed an extra hour's piling work early morning and a similar extension until 7pm.

The evening ruling was imposed originally for the benefit of the adjacent Swan Tavern, but it appears site noise does little to distract the regulars from the serious work of enjoying a pint.

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