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Geotech special: Cannon Street cunning

Innovative foundation designs are offering substantial time and cost savings during construction of a central London office development. David Hayward reports.

At first sight, the several rigs currently sinking over 200 minipiles deep in the bowels of London’s Cannon Street Station would appear to be forming conventional foundations for the new eight storey office and retail complex, now in the early stages of construction at this busy train interchange.

But a second glance at the unusual rectangular block layout of the pile groups would trigger questions from inquisitive geotechnical engineers. And the answers would doubtless release at least a muffled “wow”.

Two innovative foundation concepts

World-leading designs and cutting-edge technology head up the impressive claims from specialist subcontractor Expanded Piling and the consultant team in their description of two separate and innovative foundation concepts − rectangular “solid earth” caissons and settlement reducing piles which enable the reuse of existing foundations.

The £360M futuristic-styled Cannon Place offices, being built by main contractor Laing O’Rourke for the UK arm of American property developer Hines, will generate massive 1,600t loadings which transfer down columns, through the station’s platforms to its undercroft.

To support around a third of the columns, the geotechnical team has created a novel, costsaving and relatively small alternative foundation to the original solution, which involved hand digging huge concrete caissons.

Groups of 300mm diameter minipiles have been arranged into average 3.5m long rectangular-shaped boxes each enclosing virgin London clay. This competent ground is not excavated but left undisturbed and then contained by a 1.5m thick concrete pile cap covering the entire 2.8m wide foundation box.

Up to 24, close-centred minipiles in each group work in conjunction with the block of clay they enclose to create a “solid” caisson capable of carrying the full 1,600t loading on each column.

Victorian origins

The need for such innovative design thinking has its origins in Victorian London. Cannon Street Station was built in 1868 with a classic curved iron roof over an open, multi-arch brick viaduct, carrying rail lines from a river bridge across the nearby Thames into the heart of the train terminus.

This 140-year old viaduct remains in use today and runs above much of Expanded Piling’s enclosed worksite deep beneath the station.

“This must be one of the most congested and restricted piling sites that we have yet faced.”

Matt Smith, Expanded Piling

Offered just 2.6m headroom, the piling team must work around − but not interfere with − existing concrete columns and ground beams squeezed either side of the viaduct’s brick foundations.

These old supports are the remnants of a recently demolished 1960s office block which had occupied roughly the same site that Cannon Place will above the station roof.

As a further complication, the priceless ancient walls and floor slabs of a Roman governor’s palace lie buried beneath existing foundations. This scheduled ancient monument is guarded by an onsite team of five archaeologists and its presence means there are strict controls on the positioning of new foundations.

“This must be one of the most congested and restricted piling sites, above and below ground, that we have yet faced,” says Expanded Piling construction manager Matt Smith. “To have formed hand-dug caissons − the original foundation proposal − would have proved very difficult if not totally impractical.”

Protecting Roman palace walls

With Laing O’Rourke as main contractor, its in-house geotechnical specialist, Expanded Piling, was brought in early during the design stage three years ago. The design team of Laing O’Rourke consultant Foggo Associates and geotechnical advisor GCG, soon rejected the hand-dug caisson option on space and health and safety grounds.

Instead it developed the minipile technology advanced by Expanded Piling’s own consultant Byland Engineering. These smaller, rectangular shaped unexcavated caissons considerably reduce the impact on the various Roman palace walls.

Each caisson’s 35m deep minipiles are sunk through a preassembled timber frame containing steel guide tubes.

“The piles work as a group and shear loading on them is gradually transferred to the surrounding ground.”

Tony Taylor, Foggo

This ensures already demanding one in 200 verticality tolerances are achieved and often bettered, offering maximum design efficiency and added protection against damaging the important archaeology.

With piles positioned at 500mm centres, some 60% of the caisson perimeter remains undisturbed clay. The foundation resists loading largely through the shear resistance of its perimeter.

This is improved where the clay is undisturbed, creating stronger, smaller foundations compared to conventional concrete-filled caissons.

“The piles work as a group and shear loading on them is gradually transferred to the surrounding ground,” explains Foggo project designer Tony Taylor.

“We believe it is the first time that piles and plugs of ground have acted together to form this caisson-style foundation.”

Quicker, easier and safer

Laing O’Rourke project director Andrew Veness agrees that the new-style foundations are substantially quicker, easier and safer to construct compared to hand-dug caissons. And it is claimed that the space, time and money saved through such innovative foundations have made a significant contribution to the project’s overall viability.

It is providing a solution to site constraints that could otherwise have rendered the development unbuildable.

“We believe it is the first time that piles and plugs of ground have acted together.”

Tony Taylor, Foggo

Eleven of these caissons will support 30% of the new office complex with parts of the rest carried on equally innovative foundations based on the reuse of existing, now redundant, 50 year old underream piling.

These 2.3m diameter underream piles, with vast 6m square bases, had supported Cannon Place’s predecessor; the recently demolished 15-storey 1960s office block. These unused bases still monopolise the ground beneath Expanded Piling’s crowded basement work site and reuse of 20 of them as foundations for Cannon Place has led to the project’s second geotechnical innovation.

Although new overall loadings on these underreams will be similar, the design team was concerned that predicted settlement characteristics of some of them would be different. Its solution was to design 36m long settlement reducing piles sunk in between the underreams to carry any major load differential from the new building. This would ensure that additional movement of the reused underreams would be minor and within allowed tolerances (see box).

Innovation in reused foundations

Around half the load from Cannon Place will be supported on existing, but redundant, large underream piles.

Miners hand excavated the 6m square bases, working at the bottom of a 2.1m diameter shaft.

These underreams were installed 50 years ago for the recently demolished 1960s office block occupying roughly the same roof area as the new building.

Reuse of these foundations was conditional on the design team ensuring that the currently unloaded underreams would neither heave nor settle unacceptably when reloaded.

Guaranteeing that they will carry roughly the same loading has led to the installation of rarely used settlement piles.

Ten of these 750mm diameter piles are being sunk alongside existing underreams and linked to them by a concrete ground beam.

Desk analysis of real loadings from the 1960s structure, and those expected from Cannon Place, highlights a roughly 300t loading increase on several foundations.

The 36m-long settlement piles are designed to carry this increased weight before loadings transfer across to each underream.

The piles have no factor of safety and are designed to limit their capacity in geotechnical terms to about 300t each.

Readers' comments (1)

  • more details of this impressive building can be found on http://www.canonplacelondon.com

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