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Diaphragm walls: Saving space

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Work is underway on what is probably London’s largest basement but, while the space may be completed next year, it is likely to be a number of years before it is pressed into service. GE visited Heathrow to find out more.

Managing break out of 50,000m3 of concrete, construction of 502 linear metres of diaphragm wall and 350,000m3 of earthworks for a new 200m long, 100m wide and 16.5m deep basement is always going to be a challenge. But place that structure airside in the middle of a busy airport taxiway, close to operational buildings and the challenge is more than just technical – there are logistical and security issues that need to be carefully managed too.

These are exactly the problems that Heathrow Airport’s Kilo project is currently contending with in a bid to future proof baggage and passenger transport facilities at Terminal 2, as well as maximise aircraft stand space. Ferrovial has been working on the scheme for two years with an estimated £6M invested in enabling works so far but the ground engineering work on site by Cementation Skanska only got underway in May this year.

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The site is close to operational aircraft stands, terminal buildings and baggage haul routes

The need for this scale of work may not be immediately obvious to the casual observer but the £300M Kilo Apron and Substructure projects brings together two business cases that combine a number of objectives.

“The project will create the through Kilo taxi lane and new pier stands through demolition of the Eurobuildings that linked Terminal 1 with Terminal 2 but blocked the taxiway and added to airfield logistics,” says Ferrovial project lead Antonio Izcara.

“The Eurobuildings were 260m long and were and operational pier built in 1996. They the first link between Terminal 1 and Terminal 2.

“The project will also add extra stand capacity with seven new stands capable of servicing the largest aircraft, including the A380.”

The other driver for the Kilo project is to safeguard space below ground for baggage facilities for Terminal 2, which currently using Terminal 1. The new baggage hall below the Kilo taxiway will create storage for 10,000 bags under the early baggage storage plan.

“Terminal 2 currently handles 20,000 bags a day,” says Heathrow Airport senior project manager Simon Newland. “The project is part of the airport’s masterplan for 2035 and combines with other safeguarded space under Terminal 2B.”

Around a third of the safeguarded space is for a track transit system to take passengers from Terminal 2 to Terminal 2B. In the future is could also connect with Terminal 2C, if and when that is built.

“Work on the Kilo project started in August 2016 and the first of the new stands were handed over in December last year with the next three due for completion later this autumn and the whole above ground element operational by 2023,” says Newland.

“The main risk is the deep excavation for the £190M substructure as the work closely interfaces with the airport operations with the site close to baggage handling routes and aircraft movements. The silos for the bentonite are just 50m from the airfield and the airfield boundary is only 10m away on the south side of the construction site.”

The basement structure measures 200m by 100m and will be excavated to a depth of 16.5m, although the diaphragm wall will extend to 24.5m below ground. The diaphragm wall will be tied back with a single row of anchors with an internal beam fixed on the anchors to limit wall movement.

“The geotechnical work on site is being undertaken by Cementation Skanska but the detailed design has been undertaken by Ferrovial with specialist input from GCG,” explains Izcara.

The original plan had been for 840 linear metres of diaphragm wall with the structures for the baggage area and transit system separated but the move to a single excavation removed the need for propping.

The depth of the diaphragm wall has also been optimised from original plan for it to extend to 34m depth to the current plan of 24.5m

Bearing piles to tie down the structure have also been removed from the design and the structure will now use self-weight to overcome hydrostatic uplift pressures.

Ground water management was also reviewed and the planned sheet pile wall cut off around the site has been replaced with overlapped waterproofing within the structure and drainage around the outside.

“There were concerns over vibration on operational parts of the terminal buildings resulting from sheet pile installation,” says Newland.

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Ferrovial worked with Heathrow Airport to reduce the diaphragm piled wall from 840 linear metres to 502.

The floor slab and roof slab have also undergone a value engineering exercise. The original plan was to have a 1.5m thick cast insitu concrete floor slab with 200 11.8m high columns with a 1.45m thick concrete cast insitu roof slab. The base slab remains the same but the columns are now to be formed from steel sections and the roof slab is a composite design that is just 0.45m thick.

“The redesign, which embraces off site manufacturing, reduced man hours on the project by 100,000 and means we need 20,000m3 less concrete,” adds Newland.

The current ground engineering phase of works is being overseen by Ferrovial project lead for diaphragm walls and excavation Ross Brophy.

Brophy describes the ground conditions as good. “They comprise of 5m to 7m of gravels over London Clay with ground water about 4m below ground level,” he says.

Work on site is progressing well with the diaphragm wall construction scheduled to be completed by mid-September.

There are 75 panels planned and, when GE visited site in August 46 had been completed. The panels are excavated using grabs and 6.75m to 4.3m wide, depending on the location, and 1.2m thick.

The diaphragm wall will tie in with existing below ground structures from Terminal 2B.

“The team is on track to complete seven panels this week using the two rigs on site but we have achieved eight panels in some weeks,” says Brophy. “The sequence between adjacent panel is between one and half and two days.

“There are only 11 bentonite silos on site, which in theory, could allow up to four panels to be opened at one time with up to 190m3 excavated per panel.

“We are using stop ends in 15m and 8m sections with a built in water bar to provide the connection between panels.

“The wall has a verticality tolerance of one in 200 and in-cab controls on the cranes are being used to control and monitor the work.”

The 3m wide, 1100mm deep cages come in two sections – one 18m long and the other 7m – which are spliced on site using a band system for both speed and operative safety. The cage also features casing for the ground anchor installation and polystyrene spacers where the base slab will be connected to reduce the amount of breaking out that will be required.

“We have an airside concrete batching plant which is operated by Tarmac, which does help with the logistics of delivering concrete to site but we are still competing with demand from other apron work at Heathrow,” says Newland.

The diaphragm wall is being formed using C40 concrete, which was taking up to six hours to pour but has been reduced to four hours as the work has progressed.

“We have been operating a back shift to prepare the panels and allow a double pour to be undertaken on some shifts,” adds Brophy. “The de-sanding takes around 2.5 hours.

“Cementation is using a bespoke platform for the work with a deck for safety and special holes for the tremie pipe.”

Cementation will return to the Kilo site in 2019 to install the 330 ground anchors.

Testing using 50m anchors with a minipiled thrust block is currently underway to refine the design but Brophy expects the anchors to be up to 40m long – although this may be reduced to 30m – at 1.75m centres.

According to Brophy, the start date and time it will take to install the anchors is more about the pace of excavation work.

The anchors will be located 3.5m below the roof slab, but that is up to 6m below the current ground level and up to 13m of excavation will be needed to enable the ground anchor installation.

“We also have to factor in space and access for destressing of the anchors at the end of the project,” explains Izcara.

Ground movement is a major concern, according to Newland. “The site is very sensitive to ground movement,” he explains. “Just 20mm of differential settlement could close a taxiway.”

As well as the ground anchor trials, Ferrovial is carrying out tests on a number of different systems for breaking out the top 3m to 4m of the diaphragm wall. Systems currently being tried include use of Brokk demolition machines and chemical bursting systems.

Now the diaphragm wall work is nearing completing, Ferrovial is already looking to the next stage and has just announced that it will undertake the bulk excavation work itself in order to reduce the interfaces with subcontractors to keep the work on schedule.

 

Safe and sustainable

Heathrow Airport has a strong focus on both safety and sustainability throughout its operations and its construction work is no different.

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The diaphragm wall grabs unload directly into the dump trucks to keep the site clean

The Kilo team is proud of its approach to safety and has no Riddor incidents with over 2M hours worked on the scheme and just one LTI.

The drive to improve safety has also led the team to look carefully at construction operations and the equipment being used on site. While the use of Caterpillar 730 ejected articulated dump trucks for handling the arisings from the diaphragm wall work may be efficient, it is also safer.

“Avoiding tipping operations means that there is less risk,” says Newland. “Managing the logistics of the muck away has been challenging. The site is an island within the operational airport and the last time there were earthworks on this scale were for Terminal 2B five years ago but that was a main construction site and not isolated in the way the Kilo site is.”

The trucks scrape the arisings into the “swimming pool” area that separates gravel from the London Clay and allows each to be disposed off site with minimal mess. Cementation is loading the trucks directly from the grab to keep the construction site as clean as possible.

Of the excavated earthworks, around 50,000m3 of material will be reused at Heathrow and the remainder will be used for remediation purposes elsewhere.

In line with Heathrow Airport’s overall policies on sustainability, the project has also kept a keen eye on its credentials and says the carbon footprint of the work over the last two years amounts to 35,000t of CO2.

Fuel efficiency of machines used on site is also a focus and several hybrid excavators have been put to work and their efficiency benchmarked against conventional machines also working on the site. Where possible, the site is using electric vehicles and solar powered tower lights.

All the concrete being broken out for the project will be reused within the site and the piling mat will also be recycled once the diaphragm wall work is completed.

 

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