The London Underground metro network’s history started with the world’s first underground metropolitan railway (hence ‘metro’) in 1863, many of the tunnels of which are still being used. It follows that there has to be a heavy programme of maintenance and development, including protection against water.

“Waterproofing technology is currently evolving,” says Dr Keith Bowers, “and is driven mainly by new build, with the situation on maintenance requirements fairly steady.”

Some of the refurbishment work on London Underground has been associated with links to new works for Crossrail, a programme to help cope with an urgent need for increased capacity.

Last year was the busiest ever on London Underground with 1.1bn journeys, a 10 per cent increase on 2009.

The need to increase capacity is likely to continue into the foreseeable future, as extra capacity seems to be absorbed quickly. A major aspect of the programme to increase capacity is to create a more efficient flow of people to and from the trains, including the new Northern Ticket Hall at Victoria; currently the busiest station.

London Underground has its own standards covering the level of water control required. These require waterproofing to ensure the absence or removal of any water drips above passenger access ways in existing tunnels and over track and electrical equipment. Lower down in the running tunnels the requirements are more tolerant and permit the diversion or ‘tanking’ of water so that it does not cause a nuisance and can be drained away. In new works the aspiration is to create an ‘entirely dry’ tunnel, says Bowers.

“LU is currently updating its tunneling and waterproofing standards and plans to issue revised versions of all of them by the end of 2011. The latest London Underground technical specification [see Tech spec, right] is T0006 covering materials and workmanship in deep tube tunnels. London Underground standards focus on ‘performance requirements’. In other words, will the system do what we want of it? Nonetheless, the standards, specification and guidance notes all provide information on specific approaches that have been found to work well in the railway environment.”

Membrane pros and cons
In comparing sheet and sprayed or ‘liquid-applied’ membranes for refurbishment Bowers says, “Both systems have pros and cons. Sheets are quite efficient large, simple shapes such as highway tunnel linings. They only tend to leak if abused, by a puncture for example.” Sheets can be awkward to install where tunnels are geometrically complicated such as in station passageways.

Also with sheet membranes, voids on both sides of the membrane mean that the one behind can fill with water. If the water leaks out somewhere, the ingress point may be nowhere near the external source of the water, so it can be difficult to rectify. Sprayed membranes can bond to the linings without leaving hidden water paths thus making repairs easier.”

“On the other hand,’ he continues, “sprayed systems are not much use when applied to running water. In such cases the water has to be diverted first using sheets or stainless-steel channels.”

He adds, “A further complication is that currently available sprayed products can have very different characteristics. There are possible solvent problems, for instance, meaning some may pose health risks if used underground in confined spaces or near connections to areas of public access. Membrane performance in case of fire must also be considered.”

Other approaches
“Another way to improve water-tightness,” says Bowers, “is to use high quality concrete in linings, with appropriate additives in the mix as necessary. However, the main problems with water paths are usually at the lining joints, so relying on the concrete alone is not acceptable.”

Bowers says that, whatever the waterproofing system, London Underground favours ‘reinjectable’ grout pipes at high risk areas such as tunnel junctions so that any leaks can be rectified more easily later in the structure’s life.

Green Park
One example of an ongoing refurbishment on London Underground is the step-free access project at Green Park station, where the contractors and consultants Joseph Gallagher, TubeLines and Capita Symonds used both sheet and sprayed-on membranes. BASF’s Masterseal sprayed on waterproofing membrane was used at the top of the new passenger access shaft, and sheet membranes in lower sections.

The project was the subject of a BTS meeting in March and will be reported in T&TI in the near future.

Membrane application
Isolated compartments behind the sheet membrane have to be created by welding water bars to it with the full encirclement of tunnels and shafts. The compartment must be no longer than 10m.

There is a special requirement for the interface between superficial geological deposits and the London Clay where the barrier must consist of at least three separate water bars.

Before applying sprayed waterproofing membrane, the surface (assuming sprayed concrete) has to be cleaned of surface contamination such as dust oil and loose particles. The surface has to be smooth enough to permit an even, continuous spray of the membrane.

If the surface is too rough for full or economical coverage, a regulating layer of sprayed concrete has to be used with aggregate not greater than 4mm. If an application of sprayed membrane is defective the section has to be resprayed with a minimum overlap of 200mm over the boundaries of the defect.

The sprayed membrane has to bond well with the primary lining to prevent movement of groundwater behind it. This bond must be better than 0.5MPa strength, tested to the relevant British Standard.

Also water movement between the membrane and the secondary lining has to be restricted so that any necessary leak sealing treatment can be effective. In a similar way to sheet membranes, compartmentalisation of lining may be used to prevent water migration longitudinally.