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Clean cut construction

Huge underreamed piles have provided load capacity for an office complex being built over a live London Underground railway.

With piles next to a live Underground railway line in central London, and strict restrictions on working hours, noise and vibration, a minimum risk solution was needed for the Cardinal Place office development for client Land Securities. The District Line bisects the site in boxed tunnel 3m above the basement piling platform.

Winning the work after a detailed tender process, Expanded Piling has just completed 155, 1,500mm and 1,800mm piles for principal contractor Sir Robert McAlpine. Many are constructed with up to 6.3m diameter underreamed bases, which Expanded believes are among the largest ever formed. The bases give a safe working capacity of over 2,100t.

Piles were designed by consultant Arup. Work began in May this year following successful preliminary load tests on straightshafted piles, with deflections achieved under static load testing to 1,400t well within specified limits. Ground conditions were typical of London with fill and Terrace Gravels overlying London Clay.

As the previous building with its own substantial foundations spanning the Underground tunnel had only been partially taken down, the tight piling programme followed demolition closely.

The rigs occasionally had to core through reinforced concrete obstructions, where deep excavations immediately next to the tunnel could have jeopardised its stability.

Expanded used a casing-led method when drilling immediately next to the Underground railway to avoid ground disturbance that could undermine the tunnel.

It chose two of its most heavyduty hydraulic Casagrande rigs to install the temporary casings, up to 11m long, in one length.

Avoiding numerous lifting operations speeded up the process that would have been needed if segmental casing had been used.

The District Line effectively isolated one portion of the site, with no direct access. This meant the 130t piling rig and its attendant crane had to be lifted in from an adjacent street using a large mobile crane. A special access platform over the tunnel, designed and built by Expanded, provided a safe route for the removal of excavated materials and support for a pumped concrete pipeline to form the piles.

Working next to the tunnel, both pile installation and the movement of equipment over it were the subject of further method statements, which followed stringent standards set and approved by Metronet SSL Rail - formerly London Underground.

At tender stage Expanded devised a way to reduce construction times for the 6.3m diameter underreams with a new underreaming tool, weighing about 8t.

Its design took advantage of the available torque and lifting capacity of the company's latest hydraulic rigs and achieved cutting times 'not previously thought possible'.

A key aspect in speeding up the process was avoiding the need for manned descent to clean and inspect the pile bases. Sir Robert McAlpine, Arup and Expanded worked together to refine the methods to prove the pile bases were sufficiently clean to provide reliable pile behaviour under service loads.

Avoidance of remoulded material relies on experienced personnel and on monitoring the position of the underreaming tool so that it returns to exactly the same depth after each cutting/emptying cycle of the tool. Monitoring of when the cutting arms of the tool are retracted is also fundamental to cutting accuracy.

To help achieve this, an innovative device was fitted to the underreaming tool, which transmits a VHF radio signal back to the piling rig when the cutting arms have just closed.

Next, high quality remote CCTV recordings were made over the base of all underreamed piles to provide initial confirmation that the cleaning process had been successful.

A final check was provided by taking standard U100mm diameter soil samples from the shelf area of the underream. These were returned to the surface and extruded for visual examination for the presence of remould - the deposition and smearing of clay cuttings on to the surface of the base that would adversely affect the settlement characteristics of the pile. The equipment met Arup's stringent specification for ensuring base cleanliness.

Without the need for manned descents, underreamed piles have been shown to be more cost effective, particularly on sites where single piles per column are chosen to speed up the programme.

Communication on site was also improved by using an extranet website which allowed all project documents to be shared.

Expanded completed its multimillion pound contract last month - ahead of its 24 week programme.

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