Ground engineering work for a new housing development in Beckton has benefited from early contractor involvement and previous client experience.
Work underway in east London is set to transform a former industrial area into a vibrant new community with apartments, shops and offices plus paths through to Beckton District Park. However, poor ground conditions mean that significant foundation work is needed before the new homeowners can take up residence at the Bellway Homes development in 2020.
Keller’s early involvement in the contract with consulting engineer RLT, along with the ground engineering contractor’s past work with Bellway, have benefited the foundation solutions for scheme. The combination of early involvement and past experience enabled pile testing torefine the design and the techniques tominimise costs and reduce the risk of future settlement. The site was previously occupied by East Ham Industrial Estate, a 1960s development with mostly automotive businesses, so neighbouring householders must be looking forward to the change as much as new residents.
According to architect Stockwell, the development was planned to create 390 homes, plus shops and office space split over five blocks ranging in height from four to 12 storeys.
“There are two parts to the project for the ground engineering work –there’s CFA piling for the apartment blocks and vibro concrete columns (VCCs) to support the roads and parking areas,” says Keller business development director Derek Taylor.
Work on the VCCs started in mid-August and finished in early October, while the main piling work started in early September and is on schedule for completion in early November.
The piling completion date could be pushed back or a second phase of piling added when design of the remaining two blocks is finalised.
“Ground conditions are typical for the area with a thin veneer of made ground over alluvium, River Terrace Gravels and London Clay,” explains Taylor.
There are considerable peat layers within the alluvium which led to the use of VCCs for the roads and parking. Other developments nearby have piled foundations but the pavements, roads and parking areas all show signs of differential settlement as a result of the soft ground.
Nonetheless, the use of VCCs meant that some additional testing was needed as the local gravel was thin. As a result Keller undertook cone penetration tests to verify the presence of sufficient thickness of gravels to support the expected 275kN loads.
“The soft ground means that we have to be careful how quickly we undertake adjacent piles”
“VCCs are an enlarged base pile which we previously used for another Bellway development in Waltham Abbey,” adds Taylor.
The 600mm diameter enlarged base carries all the load in end bearing and requires a minimum of 3m to 4m thickness of gravels. At the top, the VCCs are supporting a slab or road using a 900mm diameter float-finished head to a precise tolerance to allow the slab to be placed with reduced span.
In total, 1,400 VCCs have been constructed at the site. The weak ground also presented challenges for the pile design, but the preliminary pile testing helped to alleviate concerns and optimise the design.
The weak alluvial soils and gravels are underlain by London Clay that is also relatively weak, which meant that 600mm diameter CFA pile working loads were less than can normally be achieved in this geological sequence in east London.
“RLT director Neil Turnbull was designing for high pile loads but using the conventional factor of safety of 2.5 the loads would require 750mm diameter piles,” says Taylor.
Turnbull suggested early appointment of a piling contractor to involve them in advance testing. Using information from Keller’s pile testing programme, Turnbull was happy to approve a safety factor of two, which meant the diameters could be restricted to 600mm. Taylor describes this as a win-win for all parties as smaller diameter meant lower costs and less muck away in this busy part of east London .
The test piling was undertaken in late August and involved three piles –two 28.5m deep and one to 20m – that were tested to 150% capacity. “The ground where we undertook the test piling was worse than the ground investigation suggested,” says Keller piling supervisor Aaron Middleton.
beckton piling 1
“As a result, the deeper pile did not perform as expected, with 30mm of settlement. The shorter pile performed better with just 0.6mm of movement. The third pile at the same length as the first was undertaken to prove the design and only settled 17mm. All the piles were in specification at working load though.”
Fortunately, the ground conditions found over the rest of the site have been better than at the test pile location. Each of the piles goes to a different depth depending on the expected load with the deepest going to 31.3m, but some are just 17m deep.
In total, Keller is installing 633 piles for the three apartment blocks as well as 16 anchor piles for the tower cranes. Keller currently has two rigs onsite – a Soilmec SF65 and a Soilmec SR95 – but Middleton says that the scheme may switch to using just one asphasing the work is complicated.
“The soft ground means that we have to be careful how quickly we undertake adjacent piles,” he says. “At the moment both rigs are completing 16 piles a day but I think we can increase that to up to 20 piles a day with just one rig, which is a more cost effective approach and easier for sequencing.
”Having two rigs might only save a week on the overall timescale.”
The peat layer is up to 4m thick so there is a high risk of overbreak and Middleton says that 25% overbreak has been standard on this site. The cages being used are 7.5m to 8m long and are being fabricated on site.
“Each cage is specific to each pile, so fabricating them on site makes it easier for sequencing as every pile is bespoke.”
While there was at least another month of work to complete when GE visited the site, Middleton seemed confident that the early challenges on the scheme were behind him and he was not anticipating further issues.