Piling for a massive new housing development in south London involves seeing what the UK’s largest CFA rig can do
Amid the forest of piling rigs and cranes that has sprung up on the south side of the river Thames in London’s Nine Elms, one machine stands out: a Casagrande 850 continuous flight auger (CFA) rig, complete with 34m mast. Miller Piling bought the rig earlier this year, and it is already proving its worth on the firm’s £3M contract to install the foundations for a showpiece housing development.
Embassy Gardens is developer Ballymore’s massive housing and business scheme on land adjacent to the site of the new United States Embassy, currently under construction on a former industrial site behind the New Covent Garden fruit and vegetable market near Vauxhall. The decision by the US government to relocate its embassy from central London has prompted a massive regeneration programme in this unprepossessing area of south London, which includes – at long last – the redevelopment of Battersea power station.
Embassy Gardens occupies nine separate plots over an area of 6ha, and will eventually house 1,982 apartments, as well as business space, shops, cafes, bars, restaurants, a hotel, a health centre, a public park and sports pitches.
Construction of the first stage of the development, known as the Capital Building, began last year. The piling was split into two sections: Miller initially installed 534 piles in the east half of the plot earlier this year, before moving out to enable concrete contractor PC Harrington to make a start on the buildings’ cores, then returned in mid-August to complete the piling on the west side.
“We knew the risks and there were no surprises. It means we didn’t have to reinvent the wheel”
Colin Murphy, Miller Piling
Miller Piling operations director Colin Murphy admits that the first phase was “a steep learning curve”, thanks to a combination of difficult and varying ground conditions, working at a low level – inside a 4m deep basement excavation – and a high water table. But having overcome these obstacles, Miller was well placed to tender for the second phase of piling – something Ballymore recognised when it reappointed the firm.
“We knew the risks and there were no surprises,” says Murphy. “It means we didn’t have to reinvent the wheel.”
Risk and reward
Miller’s reappointment on phase two was on the basis of sharing risk and reward, which Murphy says is “definitely to the benefit of the job”.
The piling at Embassy Gardens involves installing a total of 999 bearing piles that transfer load into the ground through skin friction. They are all either 600mm or 750mm in diameter and between 25m and 35m in depth, depending on the depth of the underlying clay and the loads being imposed by the building above. “Our design team works out the diameter and depth depending on the loadings,” explains Murphy. “The top 7m or 8m is obsolete in design terms.”
This obsolescence is due to the fact that over most of the site the top 7m or 8m consists of a 3m layer of made ground that Murphy describes as “really soft and mushy”, then gravel and silty clay. It is only when the piles get into the London clay below this layer that the skin friction develops.
Miller had initially hoped to use CFA piling for phase one, which was carried out inside the basement excavation 4m below datum. However, the high water table meant the ground was permanently saturated, and the CFA piles were being undermined, so Miller had to rethink its methodology: the piling mat was reconstructed at a higher level, and the firm switched from CFA to rotary drilling with cased piles, using three piling rigs, three mobile cranes and two vibrators.
The entire Nine Elms area has a history of industrial use, and the Embassy Gardens site is no different. At one time it housed a gasworks, and when Ballymore took over, the massive concrete base of an old gasometer was still sitting in the ground. This base measures 30m in diameter and 2.5m in thickness, and sits at a depth of around 9m below ground level, so digging it out before piling started would have been a major task – as would backfilling the space it would leave behind. As the base is unreinforced, Miller instead decided to leave it in place, and pile through it.
During phase one Miller used a rotary rig with a rock auger to drill through the concrete and install cased piles. “It was slow, but it worked,” recalls Murphy. But, by the time it came to phase two, the company had bought the latest addition to its fleet, the Casagrande 850 – the largest CFA rig in the UK – and Murphy was keen to use it.
“We said we’d try it because the machine has a good pull down force,” he explains.
This turned out to be a smart decision. The machine has proved capable of drilling right through the concrete in less than 20 minutes, and can install between eight and 10 uncased piles a day to a depth of 34m. This compares with just four piles a day using three rotary rigs on phase one. “Because we’re not using a crane and vibrator, and there is no need for casings and rock augers, it is much quicker,” says Murphy. “It has saved a lot of time and effort.”
As a result of its success on the Embassy Gardens site, Miller has already had enquiries from the developers of two neighbouring plots for the CFA rig to be used on their developments.
Although the Casagrande 850 is ideal for the piles that have to be installed through the concrete slab, Miller is still using a rotary rig for the piles on the remainder of the site. “CFA is efficient in the obstruction areas, but because of the overbreak it wouldn’t be efficient for other areas,” explains Murphy. “There are a few other obstructions that we can get through with the rotary head.”
During both phases of the piling contract Miller has had to contend with an interesting geological feature – a v-shaped dip in the level of the clay that runs right across the site. Here the upper level of the clay plunges from around 8m below ground to more than 20m down. The feature is thought to be glacial, although it could be artificially created, as this section of the Thames riverbank has been home to many small harbours over the centuries.
Miller knew the approximate location of the feature, then probed down using long casings to determine the exact clay profile.
Two other features that have to be considered during the piling works are definitely manmade: an existing Thames Water sewer, which crosses the north west corner of the site, and the line of London Underground’s Northern Line extension, yet to be built, which will run beneath the south west corner of the new development.
In both of these locations, any piles that are within the zone of influence of the tunnels have to be installed inside sleeves to ensure that no load is transferred through the ground into the tunnel walls.
Miller is using 6mm to 8mm thick steel sleeves lined with bitumen to make sure there is no contact between the pile and the ground – effectively creating a gap in the friction action.
Miller is now reaching the end of its three month contract to install the foundation piles for the second half of the site, and is being swiftly followed by concrete contractor PC Harrington, which is building the frame and cores for the new buildings. And even though there are still months of construction to go before any of the new apartments are ready, the flats that have been released have all been sold, confirming the developer’s confidence that Nine Elms is, its marketing material says “London’s most anticipated riverfront development”.