Waterproofing cross passages

Interfaces in construction projects often introduce risk. And cross passages are, by definition, interfaces between main tunnels.

We asked some industry specialists for their views on waterproofing cross passages: what has changed, what goes wrong and where should more attention be given. The consensus was that, while waterproofing  cross passages in traditionally constructed tunnels remain relatively straight forward, there are some new issues to think about for those that connect TBM tunnels in softer ground.

One of the 38 cross passages that link HS2’s Chiltern Tunnels.

“Challenges have increased where you have soft ground and the groundwater table is above the tunnel roof and hence above the cross passage roof,” says Kurt Zeidler, director at Gall Zeidler. “You need to avoid groundwater inflow into the tunnel for durability and operational reasons and to avoid lowering the groundwater table. Hence you need to make them more-or-less watertight.”

For cross passages between TBM tunnels, there is also the interface between two different lining types to contend with – often a weak spot, according to our experts. And there could be contractual interfaces too since different subcontractors could be responsible for building the main tunnels and the cross passages.

Higher durability

Asset owners are demanding longer and longer design lives for their tunnels: 120 years, and even more than that, are not uncommon. That means that the waterproofing strategy, installation and materials must be robust enough to enable those long lives.

“The most influencing aspect in tunnel structural durability is water leakage,” says Zeidler. “Waterproofing is one of the most important elements in underground construction to make sure structures are safe and durable.”

Yves Boissonnas, global key project manager at Sika, says that clients’ aspirations for tunnel lives of 100, 120 or even 150 years are not always sufficiently translated through into their specifications.

“The client should clearly state to which standards he wants the life expectancy to be proved,” says Boissonnas. “If he only asks for 100+ year durability without defining the tests, then he might get a membrane that will not disintegrate within this lifespan – as was asked – but which will very probably not stay flexible enough to guarantee the watertightness expected and hence the durability required.”

Boissonnas adds that manufacturers are using tests and standards designed to prove 100-year lifetimes and need to extrapolate their results to state that their product will last for the required 120 or 150 years. “We are already getting some subjective interpolation,” he says.

Norbu Liebetrau, head of sales at Rascor, also highlights problems with testing requirements, albeit from a different angle. He observes that increasing durability demands have led to the need for more rigorous testing, but he warns that some of the required tests are unnecessary, because they have been set by specifiers who don’t properly understand waterproofing technologies.

“These excessive requirements can inflate costs and delay projects, highlighting a gap between theoretical planning and practical execution,” says Liebetrau.

Ground freezing on London’s Silvertown tunnel for cross passages beneath the River Thames.

He calls for more consultation between those planning projects, manufacturers and waterproofing experts. “This approach not only streamlines the construction process but also improves the overall quality and sustainability of tunnel projects, bridging the knowledge gap between planners and practical construction demands,” he says.

Liebetrau highlights the design for the 38 cross passages for the Chiltern Tunnels in the UK part of the HS2 project, where Align JV is the main contractor. This is a good example of where durability requirements and waterproofing have been extensively considered and stipulated, he says.

“In planning the Chiltern Tunnel’s cross passages beneath the groundwater level, planners incorporated the highest expected groundwater levels over the tunnel’s 120-year lifespan, including the impacts of climate change,” says Liebetrau. “HS2 has defined specific requirements for materials and durability, including criteria for water tightness and concrete crack widths, to ensure a construction lifespan of 120 years.”

In a paper for HS2’s learning legacy site, engineers David Morgan, Yuan Le and Thomas Schultheis of Gall Zeidler explain that the secondary lining for the Chiltern Tunnel cross passages has been designed as a second line of defense to the groundwater should the sheet membrane behind it fail: “The secondary lining is considered a second water barrier with the aim of inhibiting groundwater ingress in the event of breaches in the waterproofing system (primary measure).”

The paper goes on to say that the secondary lining has been designed with a maximum crack width of 0.2mm for the external face, aiming to reduce the rate of groundwater penetration in the event of a breach in the waterproofing system. It also details the compartmentalisation of sheet waterproofing and re- injectable grout tubes – which are covered later in this article.

Enrico Pavese, projects coordinator at Renesco, reports that materials specifications sometimes don’t make sense. He notes that he has seen a wide range of materials, systems and termination details specified, especially in countries where there are no specific guidelines or involvement of specialist contractors in the design.

“Sometimes this leads to technical solutions and the adoption of products which do not fit the purpose, for example the use of HDPE for cross passage waterproofing,” says Pavese.
“In North America is still quite common seeing plate termination specified in the contract drawings when tape termination is normally a more efficient solution less subjected to defects.”

Slowing the flow

For cross passages through soft ground below the water table, the first step is to find a way to excavate safely. The flow of ground water has to be reduced and the ground may need stabilising, depending on the soil type. Pre-grouting, dewatering, ground freezing or some combination of these methods may be required.

More challenging ground requires a more sophisticated approach to ground water control, says Andreu Ferrero, technical services manager for De Neef, part of GCP Applied Technologies. “Traditional methods are not necessarily going to be sufficient,” he says. “You have to apply more modern technology and be more inclusive of failures as a possibility in your design response.”

In selecting the right grouting materials, it may be necessary to consider long-term durability too, says Liebetrau. “When using injection methods with materials such as polyurethane, urea silicates, or acrylates, several factors must be considered,”
says Liebetrau. “These include the environmental compatibility of the materials, injection under the hydrostatic pressure of groundwater, exposure to varying temperature levels, and other relevant environmental conditions.”

Pre-grouting is being deployed on the Chiltern Tunnels by specialist KVJV – Keller and VSI International – to close fissures in the clay before the cross passages are excavated. Special tunnel lining segments installed at the cross passage locations have been cast with areas clear of reinforcing bars so that KV JV can bore through them before grouting.

Grouting technology and applications have significantly evolved over the last decade. Around two years ago, Rascor refined its approach with the RASCOhybrid HMS317 grouting material, which sees polyurethane, supplied by Rascor, mixed on site with a cement suspension through a bypass system.

“This method ensures that the two components only blend during application, providing a unique solution particularly effective for managing loose materials behind lining segments and addressing substantial water ingress in cross passages,” says Liebtrau. “It significantly minimises washout and also offers flexibility in adjusting the early strengths and viscosity of the grouting material, adapting dynamically to the challenging conditions encountered in cross passages.”

One of the challenges in North America is that engineers are not up to date with current grouting technology, says Ferrero. “There’s a disconnect between general engineering, design, and the availability of modern technology. Designers need to know there are options for these problems we see,” he says. “We spend a lot of time educating people on modern technologies.”

For instance, says Ferrero, some engineers are not aware of acrylate technology which functions similarly to acrylamides, but which are not toxic. “Acrylamides are banned in Europe, but we still use them in North America,” he says. “Acrylates have extremely low viscosity so they can permeate tight soils and you can control the reaction times.”

As a result of this lack of knowledge, De Neef is often asked to trouble shoot – rather than being consulted early in the process to provide solutions that would have avoided the problems occurring.

“We get called in a lot of times when they have already dug a hole and the excavation is failing,” he says. “You can build form work and inject behind it but it’s very difficult and it always costs more than doing a pre injection process.”

He adds: “The point of weakness is often at the interfaces between sections. As you break out the tunnel wall, there can be leaks and soil erosion behind the tunnel segments, voids opening up, and that can be a problem. Pre grouting those areas outside the excavation limits is an important first step.”

Ground freezing

Artificial ground freezing, once considered a last resort solution for treating soft ground, has been deployed more frequently in recent years as more underground structures are constructed in difficult ground.

It is most commonly used for cross passages, according to Dr Christian Gilbert, director of civil works and structures at Systra and a specialist on the subject, who gave a presentation on ground freezing at the British Tunnelling Society meeting in January 2024.

Ground freezing is often used in conjunction with grouting to reduce ground water flow as much as possible before the energy-hungry freezing begins. Additional grouting may be needed during the ground freezing if the water does not behave as expected.

Among the examples of ground freezing for cross passages presented by Gilbert were the Paris Metro Line 14, where the ground was very fine sand with an 80m head of water. Here, the initial grouting regime had not slowed down the flow of water sufficiently, so more grouting was needed once freezing was underway.

Ground freezing was also deployed on the Silvertown Tunnel in London – also the subject of a recent BTS presentation – which runs beneath the River Thames, to limit water inflow for four of the seven cross passages between the main tunnels, executed by specialist contractor Züblin. These four cross passages ran through granular Lambeth Group beds.

For two cross passages at either end of the tunnel that did not sit under the river, and ran through London Clay and Lambeth Group beds, dewatering was deployed. Specialist WJ UK used a combination of surface wells and in-tunnel wellpoints. The seventh cross passage was excavated through ground that had already been treated for the TBM breakthrough.

Termination details

There was agreement from our panel that the area requiring most attention during design and construction is the point where the cross passage and the TBM tunnel meet. Not only must the waterproofing layer for the cross passage – whether sheet or spray-applied – connect to the segmental lining of the main tunnel; all other potential water paths must be shut off too.

The traditional termination detail, where a sheet membrane connects to the segments around the opening, involves bolting a compression gasket onto the segments and clamping the edge of the cross passage membrane to the gasket with a metal plate. The pressure in the gasket must be twice the water pressure to be resisted. The gaskets can be installed on the cut face of the segments or on the back face, which requires more excavation.

A more recent development sees the use of a tape, which is fastened to the segment opening using epoxy resin. The waterproofing membrane is then welded to the tape.

Installers like the tape method as it is faster to install. But some designers are less enthusiastic.

“I am not always convinced,” says Zeidler of the tape system. “I can see that there is a big advantage in the time taken to install it. Instead of several stages of work for the clamp and metal plate, there are only a couple of stages so it’s faster. But it requires utmost attention to the installation including cleanliness, avoidance of ‘wrinkles’, adequate bond between the back of lining and the WP tape, and I would never refrain from using an additional line of defense such as re-injectable grouting tubes.”

“The termination details are all done with tape in the UK,” reports Sarah Langley, head of infrastructure for Sika UK. “With the clamp system, it’s difficult to get a uniform pressure because of the shape of the opening and where you can drill into the segments to get a fixing. Typically, the compression isn’t the same all the way around, so there are points where the water leaks through.”

The mechanical gasket system doesn’t take as much hydrostatic pressure as the epoxy tape either, says Langley: “The bond of the epoxy has been tested to over 12 bar. The limits are the substrate rather than the glue. The concrete would fail before the bond between the tape and concrete would.”

Pavese says that tape terminations have several advantages over mechanical ones and points out that tape systems have been in use since 2014 when the first application was on the Finnetunnel in Germany.

Mechanical terminations require a design calculation involving into account water pressure and the cold flow behaviour of the clamped material, says Pavese, which is not required for tape. “This is not required with the tape termination since the system adhesive plus tape is able to withstand over 10 bar of water pressure,” he adds.

Attention to detail when applying the tapes is vital, says Eneritz Ochoa, UK project manager for Renesco: “The joint sealing – via injection – is crucial to ensure the tape termination watertightness. These works are specific and unique and require an experienced subcontractor,” she warns.

With the right approach and experience, the tape system can be used where ground freezing has been deployed, as Renesco did on the Silvertown tunnel. It can also de used where there is high water pressure – up to 15 bar, says Ochoa.

For either system, preparation of the segment surface is important. For the epoxy tape, Langley has this advice: “The back of the concrete needs to be smooth and dry, surface mat damp. Epoxies are moisture tolerant but cannot be used with running water.

“Typically, you grind it off slightly to remove backfill grout and expose the aggregates to get a bond into the concrete – not just the laitance on the back of the segments or into backfill grout because that would introduce a path for the water.”

On a practical note, Jorge Monserrat, senior tunnelling engineer at Dr Sauer and Partners, raises this point: “Sometimes, there is not enough room to install the termination detail around the TBM opening,” he says. “You need 400 or 500mm from the primary lining to the corner, so you need to over excavate to give the installer more room.”

The engineers should iron out details like this early, before they get to site, advises Langley: “Take into consideration whatever break you need to give the correct widths either side of the opening for detailing,” she says. “It’s not just the joint for membrane to segment, you include a safety factor. And you need space for the injectable hose around it plus a hydrophilic strip to contain the injection if you need to do one. This prevents water infiltrations around the connection through cracks or along the gaskets.”

Boissonnas floats the idea of Sika’s fully bonded membrane for cross passages. Developed decades ago for basements, it has been adapted for tunnels and used for that purpose in South Korea only for around 10 years. This, combined with the taped termination detail, could provide a more reliable system, he says: “With fully bonded membranes,

you don’t have water migration so it’s easier,” he says. “If the membrane is punctured, the concrete lining will provide the watertightness. In case of a crack at the same location, you just repair that.”

One detail which is sometimes overlooked, says Zeidler, is the gap between the termination gasket and the gasket in the segment joint. These gaps can be filled with resin, an additional gasket or a flexible sealant – depending on the water pressure and whether any movement is expected.

“The choice of the material for the gap filling between the gasket and the membrane is crucial for the proper execution of the tape termination detail,” says Pavese, explaining that it must have low viscosity in order to fill all the gaps, no shrinkage once cured and flexibility to accommodate movement between the cross passage and segments.

“Based on our experience, three-component PU resin is the most appropriate choice for this kind of application,” continues Pavese. “However, due to the limited access, limited framework time, and the small amount of resin required for the termination sealing, it is important to evaluate the technology and the equipment required for the application of each specific product which can affect the productivity – and therefore costs.”

Treatment of joints is key, says Langley: “You need to pay attention to the lateral and cross joints,” she advises. “It is not possible to see where the segment joints are, so they need to be raked out, sealed and then injected. Sometimes the segment gasket is quite a long way in, or it can even be chewed up. There should not be anything coming through laterally if the pre injection has worked.”

Vanessa Dos Santos, tunnelling and mining manager at Protan, agrees that systems using tapes are faster than the mechanical ones but warns that the quality of the tapes used is not always rigorously controlled: “The issues with tape are movement and the quality of the tape. If there is any structural movement, that connection can be a bit risky and tricky,” she says.

“Every manufacturer has the membrane and the mortar tested but we find that with the tapes they have not been tested and don’t have certification. I’ve seen tapes that have no certification being used on major projects here in the UK. My advice would be to make sure that the tapes have been tested by an independent laboratory.”

Protan has developed a new type of tape, launched last year, which uses the same quality materials as the waterproofing membrane, with holes in it so that the epoxy mortar can pass through them. “This means that the tape is effectively sandwiched between two layers of epoxy. It becomes more of a mechanical fix, being more robust overall,” says Dos Santos.

Spray-applied membranes

Spray-applied membranes are less often used for cross passages in soft, wet ground, for the simple reason that they cannot be applied to a wet substrate.

“Even if the surface looks dry, as soon as you spray on the membrane and close off the air, then you have water behind it before the membrane bonds so you get blisters and curing problems,” says Zeidler. “That happens with all the spray on membranes that are available. They are very sensitive to temperature changes, humidity and substrate moisture.”

Spray-applied membranes come into their own where the geometry of a cross passage is more complex, meaning that more cuts and hand welds would be needed should a sheet membrane be used, says Hannah Cross, UK major projects manager at Normet. “The other benefit of a spray applied membrane is if there’s a leak, it can be easier to visually identify, and you can inject locally.”

Where spray-applied membranes are used, the problem area is the same as for sheet waterproofing: the connection to the TBM tunnel segments. “There is not enough care taken with the connection detail between the running tunnels and the cross passages which is why generally you get a failing at that joint,” says Ritchard Hood, director sprayed concrete process at Normet.

Hood talks us through this detail for a tunnel constructed in Hong Kong: “After the primary lining was installed, they made a 50mm-deep cut in the extrados of the segment. A geotextile drainage fleece was then inserted into the saw cut and bonded into place and then bent and fixed onto the cross passage. They then added a secondary barrier of hydrophilic joint sealer into the saw cut.”

Water seepage can be dealt with, says Hood, as it was in Hong Kong. “We used a geotextile drainage fleece to channel the water into the invert and had a temporary sump underneath the walkway concrete for the cross passage out into the main running tunnel,” he explains. “Once the 28-day strength had built up for the secondary lining, we put a packer into the drainage hole and filled it with cement grout.”

A new development from Normet could lessen problems due to moisture. Normet has developed a new version of its spray-applied membrane which cures far more quickly than its existing one.
This new product takes around 30 minutes to cure where its predecessor Tamseal 800 would take around three hours.

“That means that your resistance to water migration is much better,” says Hood. “We are just about to launch it and have had a lot of interest.”

“Faster curing time will give the customer a faster cycle time, with the second membrane layer, if even required, and the secondary concrete layer installed sooner. And that potentially means reduced costs too,” says Cross.

Belt and braces

As a result of both the demand for longer durability of tunnels, and the difficulties of always creating a watertight joint between cross passage and running tunnel, many of our panel argue that back-up measures are needed in case of a failure in the waterproofing system.

“You always have to think whether something could go wrong during construction and you need to be able to repair the system. That should be an integral part of the design,” says Zeidler. “I have seen tunnels in other countries where they have used membranes but have not allowed for repair. Once the lining is in place and there’s a leakage, you don’t know where the breach is. With a sheet membrane, if you don’t have compartments, water can travel for hundreds of metres.”

Compartmentalisation sees sheet waterproofing divided up into sections, separated by water bars which are welded to the membrane. The position of the bars will usually coincide with the positions of the construction joints in the secondary concrete lining, with a hydrophilic strip cast into those joints.

Creating compartments means that any leak will be contained in one part of the tunnel. Injectable hoses connected into each compartment allow grout to be pumped into any area where there is a leak.

Contractors don’t always get this right, says Langley. “People don’t always co-ordinate that correctly,” she says. “We’ve had to install additional water bars at times because the positions haven’t been lined up.”

Monserrat highlights this too: “It’s important how you manage those construction joints where the water bars will be and the positions of the compartments.”

An alternative to compartmentalisation, which is sometimes used in other parts of Europe, says Dos Santos, is using a vacuum system. This has two layers rather than one, welded together to create compartments. Hoses into each compartment are then attached to a vacuum pump. If any water is found, the same hoses can be used to inject resin.

Another important area where back-up measures are recommended is that tricky interface area around the termination detail. “There should be an injectable hose around it plus a hydrophilic strip to contain the injection if you need to do one,” says Langley.

Boissonnas suggests that it may be the concrete rather than the termination detail that causes leaks. “As everyone knows, concrete is a cracked material,” he says. “Small cracks at the segment surface cannot be avoided and these cracks offer a waterway around the tape fixation. Therefore, backup measures with re-injectable hoses must be installed to allow the repair of any leakages.”

One trend Dos Santos has observed in Europe is a move to re-injectable hoses, rather than single-use ones. “Now in the UK and a lot of other countries people ask for re injectable hoses because they can inject more than once. There is a difference in the quality of the hoses depending on whether they are single or re-injectable.”

Change in mindset

The combination of more challenging ground conditions and design lives means that there should be a more rigorous approach to the design and installation of waterproofing, especially at tricky areas such as cross passages between TBM tunnels.

“Cross passages are one of the most challenging areas of tunnel waterproofing, they are very specific and unique,” says Pavese. “Therefore, specifications should properly address who is entitled to perform such kind of works, providing clear criteria on how to select the proper waterproofing sub-contractor.” Without this approach, a company without the necessary competence could be employed on price, leading to expensive and complex remedial works.

There is often a defeatist attitude towards waterproofing, according to Zeidler “I keep hearing these throwaway comments such as ‘a tunnel is never dry’, which is not very clever,” he says. “If you start at that point, you will never achieve a dry tunnel.”

Ferrero has observed a similar attitude: “The thought process is ‘get it done and when it leaks, we will come back and fix it’,” he says. “It’s a financially driven mindset, maybe because projects are awarded to the lowest bidder and the contractor will try to squeeze every last dollar out of it. But it’s a false economy.

“When you create something that does not have issues, you do not have to come back and pay to put things right. And the owner does not have to deal with ongoing maintenance and repairs.”