As tunnelling technology has developed over the years, so the methods of waterproofing tunnels, in more difficult environments, had to advance with it.
Today, there is a multitude of proprietary materials, systems and products available to the tunnelling engineer – so how do you choose which is the best?
Malcolm Lenaghan, sales and business development manager for central and eastern Europe at Grace Construction Chemicals, says that the choice of waterproofing solution depends entirely on the application. "You have to look at the tunnelling method, the prevailing ground conditions, the level of the water table, the design of the tunnel linings – all these things and many others will influence the choice of waterproofing system," he says.
Grace is one of a number of large construction chemicals firms which manufactures and distributes a range of waterproofing systems, from concrete additives, through flexible membranes to waterstops and water-bars.
Tunnel waterproofing technology can be divided into three categories, says Lenaghan: high quality waterproof concrete, requiring additional waterstops or sealants between segment joints; liquid membranes which are poured, painted, sprayed or injected to seal tunnel linings; and flexible sheet membranes which are fixed mechanically to the tunnel lining to prevent water ingress.
Grace is perhaps best known for two of its sheet membrane products. Bituthene is a self adhesive, cold-applied rubber bitumen membrane which has been used world-wide to waterproof tunnel ceilings for many years. Pre-Prufe is a high density polyethylene sheet product which forms a mechanical bond with the concrete lining for waterproofing the walls and floors of tunnels and, according to Lenaghan, "turns Bituthene technology upside down".
Grace has supplied the waterproofing systems for several high-profile cut-and-cover tunnels in recent years, among them the Hatfield Tunnel on the A1 north of London and the Seoul Metro in South Korea. "We wrapped the whole thing up with a million square metres of Bituthene," recalls Lenaghan.
Choice of membrane
The choice of membrane is of course influenced by tunnel characteristics. And, according to Lenaghan, the barrier systems like Bituthene and Pre-Prufe have reached saturation point in the UK and is used extensively in most former British colonies. "But mainland European countries have their own systems and have always resisted products like Bituthene. In Germany and Austria they use high quality concrete and build in injection hoses so that resin can be injected in the appropriate places to cure any leaks."
Henry Fenner, waterproofing specialist with Swiss chemicals giant Sika, is not convinced this is the case. He says that, in a driven tunnel through ground with high water pressure, the two most appropriate waterproofing solutions will be fair concrete with a waterproofing additive, or a sheet membrane.
Fenner divides tunnel waterproofing systems into two types: the ‘umbrella’ and the ‘submarine’. The typical cut-and-cover tunnel with negligible groundwater pressure needs only an ‘umbrella’ waterproofing system to prevent water seeping in from above or through the walls. A driven tunnel through ground with a high water pressure requires the ‘submarine’ treatment.
Sheet membranes are certainly used extensively throughout mainland Europe. Sika’s own Sikaplan PVC membrane was developed in the mid-1960s and Fenner claims that upwards of 3Mm² are used every year.
"With an open cut tunnel in ground with water pressure, you can use all types of system, including bituminous felt. But you can’t use this material in a driven tunnel. However, in a driven tunnel with no water pressure, you can use certain sheet membranes, like Sikaplan," says Fenner. The typical solution in a driven alpine tunnel these days is to first shotcrete the rock, then install the inner concrete ring followed by a geotextile drainage membrane and finally the 2mm or 3mm thick waterproof membrane.
Different factors
Although Fenner believes there are certain generalities which can be stated with confidence, he agrees with Grace’s Lenaghan that the precise choice of waterproofing system is ultimately dependent on the particularities of the tunnel in question – and there are many variable factors to influence the decision.
"You must not forget that not all tunnels actually need waterproofing – even if they are in ground with high water pressure. Irrigation tunnels don’t need waterproofing; nor do most sewers – it’s really only transport tunnels which do. Worldwide, this is only about 5% to 8% of the total," says Fenner.
Another natural phenomenon to take into account is climate, particularly temperature fluctuations. Freezing is the biggest enemy, as the damage caused by frost cycles can be extensive. With frost likely to penetrate as much as 400m from a tunnel entrance, waterproofing is likely to be a significant consideration in cold climates. The most apparent damage is spalling and cracking of concrete tunnel liners which not only permits water to enter the tunnel but can also initiate reinforcement corrosion and weaken the tunnel structure.
Even where the integrity of the tunnel itself is not affected by freeze-thaw cycles, the risk of large icicles forming below the tunnel ceiling creates a danger for the trains or vehicles which use it. Icicles can also cause short-circuiting of overhead power cables.
In many developing countries, funding is a major determinant. If labour is cheap, then maintenance and remedial work might be preferable to an expensive waterproofing solution that minimises maintenance requirements. Consequently, many tunnels are built without any waterproofing at all, with drainage used instead to carry away water from inside and reduce water pressure on the outside.
Similarly, in a metro system, for example, a certain amount of moisture ingress might be acceptable in the running tunnels but not in the station platforms. In this instance, minimal waterproofing is required for the majority of the tunnel work.
Of increasing influence in waterproofing specification is design life. The Channel Tunnel linking England and France was one of the first tunnelling projects to be built for a 120 year lifespan; but this has created a precedent that has seen many clients specify 100-plus years. Clearly this demands very high standards of construction, including waterproofing.
The growth of build, own, operate, transfer (BOOT) construction procurement methods has encouraged this trend. Where the construction consortium also expects to have to maintain the tunnel in later years, the incentive to spend more on initial construction is high.
"The Italian private highways were among the first to use full membrane waterproofing in their alpine tunnels," observes Fenner. "That’s because the owners pay for both the construction and the maintenance. Only about 4% of raw construction cost is waterproofing, well spent if you can save a lot of money later on".
While long service life is increasingly being specified, some people question the value of this. John Hester, director of tunnelling with UK contractor Balfour Beatty, asks: "Who would want a 120-year tunnel life? Developments in transport systems around the world are changing so fast that nobody can say what transport infrastructure we will require in 120 years’ time. Just look at the developments during the 20th century. Nevertheless, it is a requirement these days, and we have to satisfy it."
Balfour Beatty has been involved in some of the most prominent tunnelling projects of the past decade, notably the Channel Tunnel. More recent projects have included the Heathrow Express project and the Jubilee Line extension, both crossing beneath the river Thames, but using minimal waterproofing. "Both were concrete lined with the segments bolted and gasketed. They are through London Clay which is impervious to water," says Hester.
Huge project
By way of contrast, Balfour Beatty is currently working on a huge project to pipe water to the city of Los Angeles in California, and where there is plenty of waterproofing work.
"We’re doing a tremendous amount of pre-grouting: drilling ahead of the tunnelling machines and pumping grout into the rock ahead to seal the ground," says Hester. This tunnel passes through a massive seismic area, crossing the San Bernadino fault, and the works have to be able to cope with massive ground movements and are sealed to prevent water loss. The tunnel itself is a 4.5m diameter welded steel pipe, but this in turn runs through a waterproof concrete annulus.
As Lenaghan of Grace says, waterproofing is a matter of "horses for courses". Sika’s Fenner is even more cryptic: "What does the customer actually need? It’s often a question of degree: how dry is ‘dry’?"
