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Room service

With space at a premium above and below ground, designing foundations for a new London hotel called for some careful planning. GE reports.

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London mayor Boris Johnson’s aspiration to have 40,000 new hotel rooms in the capital by 2026 will soon be 275 rooms closer following successful piling work at one site in the City of London.

Nonetheless, delivering the foundations for the new 23-storey, two basement level, five-star luxury hotel were far from straightforward with the Northern Line directly below and limited space on the site itself.

If the site constraints weren’t enough to contend with, the proximity of Moorfields Eye Hospital added to the need to minimise vibration during construction so that sensitive laser surgery could continue uninterrupted.

Montcalm Hotel Group’s main contractor, SGP Contracts, asked Keltbray Piling to develop a foundation solution to meet the tight constraints at the 151 City Road site.

“One challenge that had to be addressed was location of the Northern line Tube underneath the City Road elevation of the site, 6m behind the wall and with the tunnel crown 3m below formation level,” says Keltbray Piling managing director Stuart Norman.

“The other issue was that City Road acts as the borough boundary between the London boroughs of Hackney and Islington. This meant we were constrained by access and traffic. Space on site was also confined, making moving materials and plant to and from the site a challenge.”

These site limitations, together with the stringent construction tolerance (1 in 200 verticality), meant that a traditional rotary bored system was adopted for the construction of the secant wall using segmental casings, adds Norman.

After detailed review of the ground investigation information, Keltbray engaged its own in-house design consultancy, Wentworth House Partnership, to develop the
geotechnical design.

The proposed design solution was to use combined skin friction and end bearing piles founded in the laminated beds of the Lambeth Group and secant piled walls to deliver retaining structures for the basement levels.

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To justify the piles’ ultimate capacity derived at tender stage, Keltbray installed a non-working test pile, complete with vibrating wire strain gauges as the secant
piled wall was being constructed.

Pile testing on site proved the design but, owing to the requirement to found the bearing piles within the laminated beds, no saving in pile length could be achieved.

However, after a review of the test pile data, a detailed pile group analysis was completed to define an optimum pile layout under the large core within the centre of the site.

Through the use of the “nonlinear” function within the PIGLET pile group analysis software program, the total number of piles under the core was reduced by 10%.

The pile layout was further optimised through analysis of the remaining asymmetric pile caps around the basement perimeter. The perimeter secant wall was also designed to accommodate column loads using the Hovs et al method.

The wall comprised 92 male and female piles with the male piles extending to 22m. The wall is primarily designed with a single top prop at the capping beam level in the temporary condition with double level props along the sensitive City Road boundary alongside the Northern Line.

The bearing piles were installed using temporary casing to support the bore through the backfilled and superficial deposits. The 900mm diameter bearing piles were up to 30m long and it was a requirement of the design to install the concrete within two hours of completing the pile excavation.

A laser survey of the exposed wall has recently been completed with the ability to back-calculate the individual pile verticality achieved. This provided valuable as-built information and showed that a verticality of 1 in 250 was achieved.

Keltbray says the project was completed on time, to specification and with cost savings delivered through design improvements and successful execution of the design requirements on site.

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