Waterproofing a large underground structure is demanding at the best of times, but when that structure is going be built under an airport taxiway and must have a service life of 120 years – then your waterproofing solution had better be good.
That was the challenge facing contractor Ferrovial Construction when embarking on the Kilo Apron Development (KAD) substructure at Heathrow Airport.
The project, completed this summer, involved the construction of a massive 100m x 190m x 15m subterranean facility, located airside, between Heathrow Terminals 2A and 2B.
Designed for a 120-year service life, the project creates the infrastructure for the future Terminal 2 baggage handling system and early bag store as well as the tracked transit system that will connect passengers from main processing in Terminal 2A into Terminal 2B and, in future, Terminal 2C.
The internal depth of excavation was close to 18m, most of it within London clay. The perimeter retaining diaphragm walls were 24m deep.
Protecting the subterranean structure against water ingress throughout the 120-year design life of the facility was a priority.
Ferrovial worked closely with the project’s structural engineers and potential suppliers to specify, design and develop a waterproofing system that would achieve watertightness class I in accordance with EC 1992-3 (meaning that any leakage would be limited to a small amount with some surface staining or damp patches deemed acceptable).
The original design consisted of a secant piled wall with an internal reinforced concrete wall lining. This would have entailed construction of an in-situ reinforced concrete wall up to 12m high fixed into the secant piles around the whole perimeter of the basement – more than 500 linear metres in total.
The alternative solution, to adopt a diaphragm wall construction, avoided the need to install formwork and overcame the need to drill into the secant piled wall.
The system chosen for the waterproofing comprised a combination of several products supplied by the UK division of French manufacturer Soprema.
Although its products have been available in the UK since the mid-1990s, Soprema is a relative newcomer to these shores, having only established a UK presence in 2012.
The diaphragm wall was cast in 1.2m x 5.5m x 24m deep sections. At the joint between the wall and the base slab, a three-layer protection system was designed consisting of a double set of Soprema’s hydrophilic synthetic Sopraswell NG and Sopraswell ITS injection tube system.
By swelling on contact with water to fill any gaps, Sopraswell NG ensures effective protection against water ingress, regardless of any structural movement or heave. Sopraswell B, a single-component hydraulic caulk, was also used to seal pipe penetration, poured wall joints, cold joints, sheet pile joints and pre-cast segments.
It took around five months to cast the 500 linear metres of diaphragm wall, with work sequenced to enable rapid progress. The subterranean accommodation was then further excavated and ground anchors installed to a depth of 40m to counteract the effects of heave.
Protecting the slab from hydrostatic pressure was crucial, explains Ferrovial’s senior design manager Victor Guasch Vela: “Once the excavation reached the deepest point, a no-fines concrete layer was cast as a first protection and drainage layer under the slab with a network of perforated HDPE pipes to channel any water ingress through the project’s internal drainage system.”
This provides first protection against water ingress and releases any hydrostatic pressure that could build up over the life of the structure. Internal gullies were also installed to account for any washing regimes, prior to casting of the 1.5m-thick reinforced concrete basement slab.
The waterproofing system for the top deck had to provide a robust solution that enabled backfilling and had the resilience to withstand heavy aircraft traffic. Soprema’s Antirock Bridge membrane was chosen for its mechanical resistance, bonding capabilities, and speed of application – the latter thanks to the Mini Mammouth machine, an automatic applicator developed by Soprema itself.
The Mini Mammouth is a self-propelled machine with a metre-wide gas-fired heating head which simultaneously peels, welds and presses Soprema membranes in rolls of up to 10m.
“Antirock B, being a single layer application installed with the Mini Mammouth, saves a tremendous amount of time on application,” explains Soprema UK’s technical manager Mike Vaczi. The machine automates an otherwise labour-intensive task, and can install up to
1,000m2 of membrane in a single day.
Antirock B is also very tough, which enables other trades to carry on while the installation is in operation, says Vaczi: “When the Antirock B is in place and fully bonded it is not easily damaged by normal foot traffic.
“On top of this Antirock B is 4mm of flexible SBS bitumen, so not a thin layer of waterproofing by any means,” he adds. “Once it is fully bonded and sealed at the overlaps and then covered, there is literally no chance of water penetration.”
The design and application of the Antirock B membrane is key to delivering the 120-year lifespan required by the client, says Vaczi: “It is a different situation to a basement in the sense the Antirock B waterproofing has been fully installed onto the concrete base.
“Antirock is very durable and impact resistant and because it is covered with further backfill after installation it is not subject to UV degradation and so is expected to last for the full term of the building design life.”
Soprema’s Geoland HT drainage separation layer was applied to protect the waterproofing membranes from the risk of puncture during backfill and to help horizontal drainage.
Finally, as a substitute for the final concrete layer on the vertical areas, Soprema’s Tiltex 12 was used. Consisting of a concrete-sand mix sandwiched between two layers of geotextile, this provides a thin protective layer for the waterproofing layers and, after wetting, cures to concrete with a compressive strength of 40Mpa.
“This was an extremely important component of the specification,” says Vaczi. “It helped to accelerate project delivery by avoiding the use of on-site cast concrete, and it ensured a reliable installation during the winter construction programme.”
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