Column plunging accuracy at Tottenham Court Road Station suggests that clients could demand more on future schemes
In an age where every square millimetre has the potential to earn money it is essential to maximise the floor space in today’s property developments – and that extends to the record-breaking basements too. While this may seem like another difficult demand for those involved in such projects, Bauer believes the bar can be raised.
“Even the worst column achieved 1:1,000 verticality, while the best was measured at 1:13,000.”
Alex Cartwright, international operations director, Bauer
Three years on from installation the plunge columns formed at Tottenham Court Road by Bauer and Keller for the London Underground upgrade are still the largest in Europe – and also possibly the most vertical too. The contract called for the columns to be constructed to a verticality of 1:400, compared to the industry standard at the time of 1:200, but recent results have shown that the average tolerances were well inside the limits.
When the £1bn upgrade of Tottenham Court Road Underground station is complete in 2016, passengers will benefit from better access and greater capacity to meet the demands of Crossrail services from 2018. Development of the area above the station is planned and the plunge columns play a key role in this.
“The excavation for the station has now revealed the true accuracy we achieved in 2011,” says Bauer international operations director Alex Cartwright.
“The average verticality of the plunge columns has been measured as 1:3,000. Even the worst column achieved 1:1,000 verticality, while the best was measured at 1:13,000.”
“The basement at Tottenham Court Road is very deep and is being built from the top down. The site is not only very small but is also surrounded by sensitive buildings – some on shallow foundations – and the speed of construction was also a concern, which is why the plunge column option was selected.”
The columns are 32m in length with 10m embedded in the top of the piles and an exposed length of 22m. Each column weighed in at 50t.
No specific loads were known for the over site development at the time of construction and the columns were designed to carry “anything”, according to Cartwright.
Cartwright says that using a bespoke hydraulically controlled plunge frame fitted with four lasers to cross check the verticality during the installation helped achieve the high levels of verticality.
Cartwright believes that such accuracies are vital to future basement projects as it will allow developers to maximise the space available within their buildings for rental. “Tolerances demanded on column plunging schemes are getting tighter but I believe that the work at Tottenham Court Road shows that developers could now be more demanding,” he says.
“The accuracy at Tottenham Court Road also means that designers can select more slender columns to further increase internal basement space as the high degree of verticality means that there is a lower risk of buckling,” adds Cartwright.
“There are no second chances when you are installing plunge columns but this project demonstrates what is possible and I’d urge the industry to start to routinely offer higher tolerances. When I started plunging columns, 1:200 was the best capability and, in 2008, we offered 1:400. Now I’d suggest that 1:600 should be the minimum.”