Major piling jobs are often associated with installing hundreds of piles but one complex project in the City of London called for just 15 and took eight months to deliver.
Installing 15 piles for a new development might sound like a simple, straightforward job, however, it was anything but simple for Cementation on the 21 Moorfields development in London. The piles had to be installed from a steel grillage above Moorgate tube station and slot accurately into envelopes of space within the station, while not disrupting passengers.
The piles will eventually support 17 storey building – known as the Glacier – which was designed by Wilkinson Eyre for client Landsec. The building has already been let to Deutsche Bank despite piling work only just being complete.
Installation of the piles started at Easter 2018 but enabling works contractor Mace has been involved with the project for years and discussions with Cementation and designer Robert Bird Group over the foundation solution also started some time ago.
The result of all this planning means that passengers have probably not been aware of what has been happening above their heads.
Source: Wilkinson Eyre
“Passengers have not really been aware of the work as the trains are usually noisier than the piling operation,” says Mace senior construction manager Mark Jackson.
Cementation Skanska pre-construction director Julian Mansfield adds: “Considerable time was spent on the planning of this project and as a result very little has been changed since moving onto site. We were first involved in the discussions for the project in 2014.”
The site was previously occupied by three “finger” blocks of eight story office buildings from the 1960s, which were demolished by Keltbray between February 2016 and March 2017. The offices were built on a deck over the platforms for the Hammersmith and City, Circle and Metropolitan lines at Moorgate Station.
The recently completed piles will support the new building in a similar manner.
“Landsec bought the oversite space and also bought 20 pockets of land within the station below from London Underground to create space for the new foundations,” says Jackson.
One pocket is within the Crossrail site and the supporting column – known as the mega column transfers its load on to three piles joined by a pile cap – for the new development was designed by Mott MacDonald and built by Bauer during the Crossrail work in 2013.”
Robert Bird Group director of geotechnical engineering Tim Hartlib: “The scale of the interface with London Underground meant that we needed to have a protocol in place for the piling work. We are also just 3.5m from the Crossrail exclusion zone so a lot of finite element analysis was undertaken to prove that were was no impact on either station from the work.
“The deck that we’re working from is supported by the foundations of the previous building.
“We initially looked at piling from the retained deck slab but the weight limit that created meant it wasn’t possible.”
The result is that a steel grillage has been built for the piling platform which is supported by the former building’s foundations.
Hartlib adds: “The tower crane is supported on the existing piles and some of them will also be used for the new development. There will be more load going through them in the temporary case than in the permanent case. The loads acting on the existing piles will always be limited to 80% of the historic dead load though.
“The new piles are located on either side of the building and there will be a structural span of 60m.
“We looked at placing a median support to reduce the span using support within the platform but the area was not wide enough and the station had to remain operational throughout.”
The new building will be supported on a total of 15 new piles installed under the current programme together with a mega column that is founded on the adjacent Crossrail box. This column is supported by three 2.4m diameter piles that were constructed as part of the Crossrail contract.
“There has been lots of dialogue with the structural engineering team over the differential settlement,” says Hartlib. “We have looked at the envelope of worst case settlements to ensure that all the steel, the trusses and façade work within the tolerance.
“The tolerance at the top of the pile is 40mm, whereas on a standard pile the tolerance would be nearer 5mm to 10mm. These piles are more heavily loaded so there is more of an envelope for the tolerance.”
The 15 piles are rotary bored 2.4m or 1.8m diameters and all extend to 58m to 61m from grillage level to give 2m to 4m penetration into the Thanet Sands.
The combined base and shaft capacity is 55MN, which Hartlib believes makes them the most heavily loaded piles in London and, possibly, in the UK. Ascertaining the ultimate base capacity was key and this has been adopted as 20MPa.
All the piles are base grouted with eight circuits on the 2.4m piles and four on the 1.8m diameter piles.
Cementation Skanska senior project manager Deon Louw says: “The top 5m acts as a column through the underground station and this section has been lined so all the load is placed below the station level.”
The piles are being installed using a temporary casing to a depth of 2m into the London Clay to ensure there are no obstructions and then the permanent liner is installed in two sections with the annulus filled with grout for support. The permanent liner has a 2.5m diameter which is a tight fit in places.
There are four cage sections per pile, each measuring 15m in length. Concrete pours take up to 17 hours with base grouting is carried out after the pile is completed.
“A number of other solutions were looked at but they were all variations on a theme and the design was limited by the number of piles we could get in and where,” says Louw.
“On the south side of the site, the piles are going into an old platform area that was used for Thameslink, while the pile locations on the northern side where the result of Landsec’s negotiation with London Underground to make use of back of house and mess areas.”
Other options were considered. “A minipile cofferdam solution had potential here but it just could not offer the loads needed,” says Mansfield.
Hartlib adds: “It was easier for logistics to develop one solution for all the pile locations.”
Jackson explains: “We had to create a number of holes and pockets through the existing steelwork and precast concrete slab of the retained deck, which is also the roof over Moorgate station.”
This included bringing in Martello Piling to construct two minipiles to assist with a section of existing steelwork and slab that was cantilevered to open up the position for pile 10. These minipiles had 540mm diameters and extended to 28m.
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The grillage that creates the piling platform was a bespoke design that sites on the previous pile positions and columns of the station. It is formed from 1,500t of sacrificial steel.
“Despite using the grillage, there were still limitations on the equipment that could be used for the new piles and the logistics,” explains Jackson. “For example, you could not put the crawler crane and the piling rig next to each other so planning has been essential.”
Harlib adds: “We were fortunate that there were good pile records from the previous development.”
Investigations were undertaken to check that the records matched the as built foundations.
Jackson adds: “Part of the challenge was just working out how to bring the piling rig and crawler crane onto site.”
The result was use of a remote control self-propelled modular transporter (SPMT) to bring the rigs in via Fore Street Avenue and onto a temporary steel bridge structure to span over existing basements and onto the grillage. The same approach has recently been used to take the equipment off site again.
“The limited space and access challenge means that we used just one piling rig on the project – a brand new Liebherr LB36,” says Louw.
Even the removal of spoil has called for innovation with development of a special skip for the piling rig to spin off into that can then be moved to the eastern end of the site for wagon loading.
Another innovation developed for the scheme is the steel plates that lock around the liner at grillage level to prevent anything falling onto the station below.
Positioning of the grouting pipes also called for further innovation to prevent damage during construction. The tubes a manchetter have been positioned on the inside of the cage with a cone inside to guide the tremie.
“We developed a special cage lifting frame for the end of the cage to avoid the tubes a manchette from being damaged during the lifting,” says Louw.
Monitoring has been key during the construction and will continue during the next phase of work. “We monitored all the piles using our Cem Optics fibre optic system to monitor strain and integrity,” says Louw.
Hartlibs adds: “It is unusual to have all of the piles instrumented but we will use the system to monitor the performance of the piles as the building goes up.”
Piling was completed in early December and final stage of the grouting work has now been finished and once the building is complete, it will be Deutsche Bank’s office for at least the next 25 years.
“A number of different options were considered for the test pile including Osterberg Cells, a bespoke method to load a single pile and a normal test pile with a reaction pile,” explains Hartlib.
“We had some concerns about the results of using Osterberg Cells in Thanet Sands and there was limited space to use conventional methods, so we developed a bespoke system that used a two storey high steel test truss that reacted against four permanent piles to place a 50MN load onto a 1.2m diameter pile that was bored to 60m.
ny 21 m pile test truss 148
“The approach meant that we have to install five piles before we could carry out any testing but we planned the test so it used the more lightly loaded piles for reaction so there was an amount of built in redundancy in the pile design for those locations.”
The testing was carried out between 10 and 12 September 2018.
The load test structure weighed 200t and featured four pairs of 800t jacks and nine load cells to transfer the load to the test pile. Additional kentledge was used in addition.
“We had to cast 16 57mm diameter bars with 3m to 5m embedment into the top of the four reaction piles to allow the test frame to be attached,” says Louw. “These were positioned using a frame and a cold joint at 3.5m below the top of the pile with the remaining concrete poured once the bars were in position. Couplers were positioned at the final concrete level to allow the bars for the testing to be easily removed later.”
According to Mansfield, the original plan was to not carry out a test. “The reliance on the Thanet Sands to carry the load meant that it was necessary,” he says.
According to Harlib, the loads placed on the pile crept up during the design process which added to the need to carry out a pile test.
There was also discussion around how to undertake the test too. “We had concerns over Osterberg Cell results in Thanet Sands and consulted with others who had undertaken such test in the Thanet Sands to fully understand the issues,” says Mansfield.
Hartlib says: “The testing was a nerve wracking stage and I was here until 3am during the test.
“We looked at the risk if the test did not go according to plan but even with the more heavily loaded piles, the soil parameters we were using for design were well within the limites used for design elsewhere in London.
“The prediction for the results at the maximum test load was 150mm to 300mm of movement but the reality was 75mm settlement.
“This gave the green light for the rest of the piling to get underway the following day but we are working on full interpretation of the results to ensure we can learn from the process.
“The benefit of the better than expected result is that structural engineers are using the information to reduce truss sizes and the full results will be essential for this process.”
According to Hartlib, if there had been a problem on a pile then there is no option here to move the location and try again. Jackson says that the piles have often been referred to as “one hit wonders” as a result.
“With this in mind, we sat down at the start to consider every scenario and the what ifs of all situations, including a terrorist attack on the station itself,” says Hartlib.
“The concern over delays led to use of new equipment and new Kelly bars and we developed a strategy to manage the pile construction by extending by 2m if there was a delay in concreting.”
Louw explains the solution: “The cage design was modified to allow it to be easily extended by 2m if this as necessary.
“The piles at two locations have been extended by 2m but not due to delays, it was because of variance in the Thanet Sand level at those locations.”
Hartlib adds: “We have worked with GCG on the pile design and they carried out an impact assessment on the tunnels to double check that there was not an issue. We have also worked with Landsec’s consultant Byrne Looby to review the designs. The project has been massively collaborative.”
The collaboration resulted in a strict approach to the planning of the piling. “The piling was carried out on a four day cycle to deliver one pile a week,” says Louw.
“On Mondays we bore through the London Clay and into the Lambeth Group under dry conditions, which is about 46m below ground, before flooding the bore with bentonite at the end of the day. On the Tuesday we dig another 10 to the top of the Thanet Sands. Teams from Robert Bird Group, GCG and Byrne Looby visit site on a Wednesday morning to confirm that the Thanet Sands have been reached and sign off on the final depth of the pile. Once that is confirmed, we change the bentonite before digging to depth, undertaking a base hardness test and placing the cage on the Thursday. Placing the cage can take up to six hours and concreting usually starts by 7pm and can take 12 hours to complete.”
Verticality was key for the piles and the team used a Jean Lutz system that is more commonly used on diaphragm wall construction to check the verticality of the piles during construction.
“One did deviate from vertical but we were able to give Robert Bird the full details as piling progress so they could calculate bending moments that prove the deviation was not a problem to the capacity at that location,” says Louw.
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Getting set to pile
Mace undertook the enabling works at the site, which included managing asbestos risks and relocating station facilities to allow the pile positions to be used.
“We first moved onto site in June 2015,” says Jackson.
Each of the pile locations was hand dug to 2.5m to ensure there were no obstructions, particularly from utilities. Diversion of utilities at the station took a considerable amount of time at the start of the enabling work. Landsec placed a services contract direct with London Underground to relocate the services that obstructed the pile positions but all these needed to be mapped before the diversions could be planned.