Plunge column piles for the Tottenham Court Road Tube station upgrade are among the largest in Europe, and must be installed to extremely tight tolerances.
Adrian Greeman reports.
Every Tuesday and Friday afternoon engineers from foundations contractor Bauer and its joint venture partner Keller take anxious glances at their watches and the busy traffic at the end of London’s Oxford Street at its junction with Tottenham Court Road.
They are looking for concrete trucks in the afternoon rush hour build up.
The ready-mixed deliveries, from a depot near King’s Cross, are for 11 huge 2.4m diameter piles that are part of the massive upgrade works for the Tottenham Court Road Tube station.
These are no ordinary piles, but plunge columns for a big top-down excavation that makes part of the new scheme’s extensive underground hallways and passages.
“Getting the concrete on time is important because once you start a plunge pile there is a sequence to go through that cannot stop,” says Bauer/Keller project manager Alex Cartwright.
There is only limited time for concreting the lower half of the bores, before the mix becomes too hard for the steel column sections to be inserted.
The columns are some of the largest and most accurate to be made in the UK and possibly Europe too.
Verticality tolerences are 1 in 400, against the usual maximum requirement of 1:200.
As a result, they are commensurately more difficult and time-consuming to insert.
“Getting the concrete on time is important because once you start a plunge pile there is a sequence to go through that cannot stop”
Hefty steel sections up to 33m long and capable of supporting future loadings must be positioned and fixed in the pile concrete.
Two sizes are being used, one with a 700mm flange and web length and one with a 600mm flange and web length.
Bored holes for the piles and embedded columns above are 52m deep, the lower 27m forming the pile proper, with 5m of the steel embedded in the top.
“A 2m thick slab base will go over where the piles end so there will be a 26m length of column above,” says Cartwright.
The five-storey deep space beneath another large slab at ground level will accommodate lifts and escalators which will link vast new London Underground substreet ticket halls with platforms for the new Crossrail tunnels which pass at this point.
Tottenham Court Road is one of the interchange points between Crossrail and the Underground here for the Northern and Central Lines.
“We have long been planning a major upgrade here because of the congestion,” says London Underground (LUL) stakeholder manager Ben Coultate.
The old station hall underneath the road junction was sometimes jammed solid with commuters and shoppers, so the space is being increased by a factor of five, with new ground level entrances.
When Crossrail was given the go ahead it made sense to combine this £500M LUL project with the similarly sized Crossrail station, including the big interchange hall which Bauer has been working on.
The escalator and lifts space is referred to as the Goslett Yard Box, a smaller section of a larger scale subsurface development to the south of the Tottenham Court Road/Oxford Street junction.
Diaphragm walls up to 48m deep along an irregular trapezoidal perimeter have already been built under a separate foundations contractor.
“But this is prime space in the centre of London and it is inconceivable you would not have developments above the completed station,” says Cartwright.
And indeed, new property schemes will be completed here by about 2016.
This explains the need for the very large piles and, to allow less disruptive top down construction, the plunge columns.
“The verticality requirement is imposed to allow fewer columns by increasing their load capacity,” says Cartwright.
The key problem is the danger of buckling, he says, which decreases significantly with increased verticality.
“Other design factors affect that, but you can get much more from a section size this way,” he adds.
The accuracy of the column positioning also means that the final base position of the column can be within a tighter tolerance envelope so that smaller adjustments will be needed for later fitting out with equipment and escalators.
“Arguably the necessary footprint for the excavation is reduced.”
But 1:400 is extremely accurate, just some 2.5mm for every metre length of column.
It has been done before, notably on the Shard at London Bridge, but for smaller depths.
Tolerances are so tight that a first requirement was to improve the straightness of the steel columns themselves.
Existing manufacturing tolerances could have eaten up all the allowance for positioning, says Cartwright, and discussions were held with the supply chain to improve that.
Supplier William Hare and its fabrication subcontractor Cleveland Bridge have complied, supplying two section columns that are joined on site with a bolted connection plate.
The next issue is positioning the column itself.
Past operations have used a positioning frame inserted into the borehole to guide the steel as it is lowered into the filled pile.
A plate at the top and the bottom with a guide hole constrains its position.
“To adjust the end there are rollers running against the inside of the hole casing on extendable arms,” says Cartwright.
Using hydraulic jacks these can fine tune the exact end position. At the top, adjustable clamps on the casing align the steel exactly to the grid.
But the piles at Tottenham Court Road station are too large to be cased all the way down.
“At 2.4m diameter it would be difficult and expensive to go down to the base slab position,” explains Cartwright.
Instead, a 10m deep casing is feasible, and this is enough to get through the mixed fill and flood plain alluvial material near the surface to the London Clay series below. “And then we have come up with a way to use the relatively firm clay wall to support the frame.”
Instead of having rollers at the base of the frame, the casing contains four “gripper pads” like smaller versions of those on a rock boring tunnel machine.
The pads XXX on the clay wall of the bore allowing the frame to be adjusted with articulating half scissor arms.
“The excavation can be dry in the top clays but in the lower Lambeth layers, the ground becomes unstable quite quickly and so we do the last 30m or so under bentonite”
These move outwards when the end is pushed with a jack, gripping the walls of the bore.
A plate in the end of the frame with a hole then guides the steel section.
The bespoke frame itself has four hollow vertical tubes at the corners, each with a laser positioned at the bottom and a target at the top.
“It is then a question of using the jacks to position the laser dot, just like Bernie the Bolt, though don’t quote me on that or people will know how old I am!” says Cartwright.
The piling sequence is essentially a two-shift operation.
The first stage is to excavate the top hole section and install the casing, and to be ready with the steel column itself.
The column components are bolted together and stored in a “rat-hole”, a spare borehole on the site.
As well as the two main sections, a shorter reusable follower section is bolted on the top - it has two wing pieces, which can be positioned at any point on the follow-on via a series of closely spaced bolt holes.
These wings are later used for vertical adjustment.
The major work begins in the morning with excavation of the full borehole using a big BG40 rig, which aside from the special purpose BG50 is the largest and most powerful Bauer produces.
“The excavation can be dry in the top clays but in the lower Lambeth layers, the ground becomes unstable quite quickly and so we do the last 30m or so under bentonite,” says Cartwright.
The reinforcement cage, made up from preassembled sections, is positioned and then, in the afternoon, the concreting to fill the lower 26m or so can begin, with a small addition to allow for bentonite contamination.
“This needs to be fairly quick, though modern mix designs with retarders to give more time than before, perhaps up to 12 hours,” says Cartwright.
The bentonite, displaced up the bore nearly to the surface is pumped out and the frame can be lifted in and adjusted to compensate for any off centre position of the casing.
Then the frame has to be adjusted by an operator at the top, working the bottom jacks with a control panel to hit the centre of the laser targets.
“That is usually me,” admits Cartwright.
A big Liebherr 1280 crane lifts the steel, around 65t of it with the follow-on, and it is slid through the top guide to the bottom one and into the concrete.
Progress is monitored with the load indicator on the crane and by video cameras positioned inside the frame.
Fine adjustments to around 50mm vertical are made with jacks under the two “wings”.
“Potentially this could require working into the night, if there are difficulties, which is normally not allowed in this mid-West End location. We had licences for the whole night, one for each of the piles,” says Cartwright. “But we have managed to do everything by 11pm.”
“All told that means we can do two of these a week”
The columns need 24 hours for the slow-curing concrete to reach sufficient strength to withdraw the frame and dismantle the follow-on, along with other clean up operations.
“All told that means we can do two of these a week,” says Cartwright.
Most were done by the time GE went to press and Bauer was preparing to remove its equipment from site, its highly visible yellow tanks and rigs heading for other sites in London.