Dropped and propped: piling on Ludgate Hill
Piling at Ludgate Hill has provided tourists taking open top London bus tours this summer with a new attraction, but the team on site have been firmly focused on fast tracking the work.
With the Old Bailey on one side and Ludgate Hill on the other, you would expect foundations work for a new mixed use development to experience challenges when it comes to logistics of getting materials onto site.
But for development of 1 and 2 New Ludgate, delays to the start of work have meant that the site team have the whole site to play with and the real challenge has been constructing the new foundations between existing underground structures.
Land Securities’ new £260M development will link two distinct buildings together with a new public square with 3,000m2 of restaurant and retail space at ground and basement level and 35,000m2 office space above.
Construction of the £1.35M foundations work has recently been completed by Keltbray, clearing the way for main contractor Skanska to start work on the above ground work with the aim of finishing the scheme by April 2015.
The new development will be eight to 10 storeys high and will replace three 1960s buildings that varied in height from six to 12 storeys above ground with a two storey basement car park.
Demolition of these was undertaken by Keltbray in late 2010 and piling work was due to follow on in early January 2011 while the demolition progressed, but Land Securities decided to halt the development after demolition.
It was partly the phased demolition and piling work that enabled Keltbray to secure the foundations element of the scheme rather than it going to Skanska’s own Cementation Skanska business.
“The demolition took the site back to basement level but left the retaining walls, which were propped at 4m to 5m centres using vertical soldiers to existing basement level and raking props secured either to existing pile caps or new thrust blocks,” says Keltbray project engineer David Kwong.
“The remaining pile caps and slabs were exposed and broken out to 3m below ground level.”
The propping design was undertaken by Keltbray’s design business Wentworth House, which also undertook the pile design elements of the scheme.
The propping positions were planned in conjunction with Keltbray to ensure that the piling rigs could operate in the space left between them.
Although the existing buildings have been demolished, a new substation – Seacole House – has been built on part of the site and 1 and 2 New Ludgate will hug it on three sides.
“The geology at the site comprises made ground over water-bearing River Terrace Gravels and London Clay,” says Keltbray piling operations director Lee Cain.
“The thickness of each and the depth to the London Clay varies across the site with the depth being up to 7m at the northern end but only 2m to 3m at the southern end.”
All the pile designs were carried out to Eurocode 7 (EC7). Keltbray managing director, piling, Stuart Norman, admits this may not sound unusual now, but the design work was undertaken in 2010 when Eurocodes were only just being adopted and even then only on major projects.
“There are some peculiarities to the design as a result of our early adoption of EC7”
Stuart Norman, Keltbray Piling
“There are some peculiarities to the design as a result of our early adoption of EC7,” says Norman. “For example, the alpha value we adopted was 0.6 rather than the standard value of 0.5. Under EC7 this wouldn’t normally be allowed without a test pile to prove the assumption, but with the delays to the schedule we were able to install a preliminary test pile that allowed us to derive an actual alpha value of 0.57.”
In total, Keltbray has installed 780 piles for the new building, including 64 to support the four tower cranes that Skanska will use during the construction phase.
All the piles are 450mm in diameter but have been constructed using a mix of CFA and rotary techniques.
“We used rotary rigs near to the props as the size of the rig allows these to be installed without problems but we are using CFA in the open areas for efficiency,” says Kwong.
The piles are cased through the gravels to avoid issues with the groundwater.
Despite the previous buildings being supported by piled foundations, none of them is being reused, due to Land Securities’ concerns regarding the warranty of the existing piles and issues related to their capacity.
The piles presented Keltbray with a challenge when it came to setting up and meeting the 3D spacing plan while threading the new piles in between the existing ones.
The piles vary in length from 20m to up to 29m at the northern end of the site where the depth to the top of clay is greater.
Reinforcement cages using 16mm bar are being fabricated on site to reinforce the piles to 9m depth.
Only the 64 that will support the cranes have the addition of a full length Dywidag bar through the centre.
Concrete for the piles is a C32/40 DC2 mix that is being supplied to the site by Hanson Euromix from its Silvertown plant.
Keltbray moved onto site to start the piling work in mid-July and was scheduled to finish as this issue of GE went to press, based on the original programme.
But because the piling has not had to fit around the demolition work, the company has managed to get ahead of its plan and Kwong’s team completed the work several weeks ahead of schedule.
The fast tracking of the scheme is also partly thanks to Keltbray being able to operate a large piling rig fleet on the open site.
At the peak of work the company had three CFA rigs – two Soilmec CM50s and a CM70, one Soilmec R312 rotary rig and two mini piling rigs – a Klemm 708 and Hutte 205 – to undertake the work in the areas that were tightly confined by the propping.
Piling has gone very smoothly and the only nerve-wracking issue for the Keltbray team was the delays in pile testing until 70% of the piling work was completed.
Nonetheless, they need not have worried as under the reaction frame testing undertaken by PMC, the piles only settled by 4mm at the 1500kN working load and 7mm at 1.5 times working load.
Parts of the site have now been handed over to Skanska’s ground works subcontractor Mitchellson which will undertake construction of the pile caps for the concrete framed structure that is planned for 1 New Ludgate and a raft for the steel framed structure of 2 New Ludgate.