Factory-batched Integritank HF ... to avoid variation.
How to achieve dry tunnel vision
Sprayed waterproofing is the most promising methodology for achieving a dry tunnel, says the UK-based Stirling Lloyd, adding that the high-quality waterproofing also ensures reduced cost and faster build speed.
01 October 2010
THE global tunnelling industry has many challenges, both technical and financial, with the latter likely to dominate over the next few years.
While many industries have adopted standard practices in order to overcome specific technical challenges, in tunnelling such a measure is not so easy to follow as every project is distinctive. Therefore, this requires original thinking and an open mind to embrace new techniques and technologies as they emerge.
For instance, sprayed waterproofing is a hot industry topic and a paper recently presented at the North American Tunnelling Conference not only discussed this methodology for achieving a dry tunnel but also how it can reduce build costs and construction time.
Though there are relatively few universal truths in the tunnelling industry, there is one opinion that appears to be shared by most in the sector – all tunnels leak.
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Seamless spray applied waterproofing. |
Mike Harper, development director of Stirling Lloyd, a specialist in the development, manufacture and application of high-performance waterproofing and structural protection membranes and systems, says: “To a specialist waterproofing manufacturer with 25 years of experience in waterproofing all types of civil engineering structure throughout the world, the concept that leaking tunnels are acceptable seems quite odd.”
According to him, tunnel engineers across the world based on their experience have agreed that to a greater or lesser extent that tunnels let water, regardless of what you do.
“Tunnel engineers have, therefore, learned to live with an acceptable level of leakage rather than creating a dry tunnel.
“Over the last eight years we have been asking ourselves whether it is possible to achieve a watertight tunnel and have developed a method for achieving this.”
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Rapid application and cure ... with a sprayed membrane. |
In other industries accepting something that is even nearly watertight is just not acceptable. For example, in the aviation industry having an aircraft that allowed water to penetrate the outer shell would not be tolerated, neither would a submarine that had a few leaks, damp patches or running water. In tunnelling, if water penetrates into any structure it can cause many problems, making it unacceptable for tunnels to let in water. If an aircraft or a submarine can be made watertight so can a tunnel. Whilst it is true that the tunnel environment presents different challenges for waterproofing than some other engineering environments, such as bridge decks or chemical tank linings, if the requirements are clearly understood, effective waterproofing can be achieved.
Waterproofing is an exact science – a structure is either waterproof or it is not. The risks presented by water ingress include short-term maintenance issues which, in the long term, can degrade the fabric of the tunnel itself, shortening the overall life of the asset. Poorly waterproofed tunnels have serious economic and environmental impacts, which is why addressing this issue is so vital.
Waterproofing that works
The decision on how to waterproof a structure is much more important than the lowest initial cost per sq m of material. The crux of the matter is how the waterproofing will perform over the 120-year design life of the tunnel, and what the risks, costs and environmental considerations associated with failure of the waterproofing are.
There are some well established criteria that successful waterproofing systems have to meet in order to effectively waterproof a concrete structure. These have been implemented in the external lining of tunnels, such as cut and cover and immersed tube tunnels, for the last 20 years.
There are five key requirements that can and should be applied to the internal lining of a tunnel of SCL/SEM/NATM (shotcrete lining/sequential excavation method/new Austrian tunnelling method) design to create a dry tunnel.
• Crack bridging capability: This is fundamental to a successful long-term waterproofing membrane when it is intimately and continuously bonded to the concrete substrate. Cracks in new concrete are inevitable at some level, whether from shrinkage during curing or from ground movements. A sprayed membrane needs to be able to bridge cracks that open up or the waterproofing will also crack with the concrete and leaks will appear. Consequently, the product must not only be flexible but also have a very high tensile strength.
• Seamless: Where sheet systems are concerned, the problems of leaking tunnels emanates not from the middle of preformed sheets but from the seams where the sheets have been welded together on site. The more the seams the more likely are the leaks. Complex geometry provides opportunity for even more seams, giving place to more potential leaks. Minimising or preferably eliminating seams is, therefore, the goal. Where sprayed membranes are concerned, seams can still be an issue; the chemistry of the system should be such that a completely seam-free installation is achieved. Although much has been done to minimise the impact of seams, including double seaming, trying to test seams and installing grout pipes to try and stop leaks through seams, these fail to address the root cause, which is the presence of vulnerable seams in the first place.
• Suitability: There are many types of waterproofing membranes in the world, made from a wide variety of base chemistry; some well-known systems such as polyurethane, epoxy and methyl methacrylate (MMA), and less well-known, such as polyureas, rubber emulsions, polysulphides and polymer-modified cements. All have various characteristics that are better suited to some applications rather than others. The confined space environment of a tunnel and the high cost in terms of time of tunnel construction present some challenging requirements for a successful tunnel waterproofing membrane. In addition to it being watertight, the system must have low toxicity and a low explosion risk. The membrane must also be tolerant to moisture, as there is a negative water-pressure environment and some ingress prior to application of the waterproofing is inevitable. The membrane should also cure quickly to ensure there is no costly, unproductive time during the construction process. These requirements have necessitated a specific chemistry design for the product to deal with this particularly challenging environment.
• Control: As with any trade, control of the activity is key. Although some perceive that forming a membrane in-situ is more difficult than forming it in a factory, the material only becomes a waterproof membrane once it is installed and therefore controlling the installation of sheet systems can often be more difficult than a sprayed system. Application of sprayed membrane should be accurate; wet film thickness tests throughout the application will ensure that the membrane is being applied to the correct thickness. The material should be installed by a spray operative as robots will not be able to see if a section has been missed. A system that is applied in two thinner coats is more effective than a single coat membrane; not only is thickness more controllable but the second coat will rectify any potential small defects in the first coat, thus reducing the possibility of any problems. The material should also be simple to use; to avoid on-site variance, the product should be pre-batched and pumped together in fixed ratios. All of this need to be covered by an on-site quality assurance regime and comprehensive training should be given to operatives.
• Proof: A membrane must be 100 per cent effective, whether sheet or sprayed, in order to be watertight and being able to prove this is of utmost importance. Where the ground has been dewatered the effectiveness of the membrane may not be evident until the groundwater table is re-established. Therefore using a quantitative, reliable test method to ensure that the membrane will not leak is essential and should always be undertaken. This cannot be done for sheet membranes; in the factory, it may be possible to test for defects in the preformed sheet itself but they cannot be re-tested for the effects of site damage during installation or by following trades once in-situ. Also, not all seam types can be adequately checked on site, therefore potential for leaks go undetected.
For some sprayed membranes testing is very simple. Spark testing is a non-destructive test method that has been used to great effect in other industries. It tests every inch of the membrane and finds any defects, even one of the size of a pinhole in 100,000 sq m of applied waterproofing, ensuring that the waterproofing membrane is completely continuous. If any defects are identified, these can be rectified prior to the application of the final lining, before it becomes expensive and near impossible to fix. This is the only method of ensuring that the waterproofing integrity has been achieved.
A dry tunnel is possible
A dry tunnel can be achieved through using products that are controllable, suitable, seamless and can be reliably tested. Other areas of the tunnel industry, such as cut-and-cover and immersed tube tunnels, as well as other sectors of the construction industry, have used this ‘best practice’ methodology for many years throughout the world and it has been proven to work.
For contractors or tunnel builders, there is also the benefit of reducing uncertainty. The time, expense and disruption from chasing leaks around a tunnel that should be dry, requiring expensive repair, should no longer be a concern when the waterproofing can be done correctly first time.
Consequently, there is now no reason for clients to accept a leaking tunnel; poor quality environments for tunnel users, long-term running costs issues, such as pumping and disposal of water, and early degradation by the action of water ingress and its associated damage should be a thing of the past.
Design opportunities
At present, the tunnel industry around the world is looking carefully at the issue of effective waterproofing, not only because of increasing requirements for water-tightness, but also because of a realisation that a fully-bonded sprayed waterproofing in tunnels of an SCL/SEM/NATM design offers some poignant design opportunities. This has much greater implications for tunnelling projects, in terms of reducing cost and time.
Sheet systems
In traditional SCL tunnel construction, regardless of how much sprayed concrete is applied as the primary lining, from a structural perspective, this concrete is ignored. The full structural load is supported by the final or secondary lining. The traditional build would, therefore, be sprayed concrete onto the excavated surface, followed by installation of a sheet membrane. The sheet membrane is tacked to various points and is not fully bonded to the primary concrete. The choice for final lining construction is then limited by the nature of the waterproofing membrane as it is exceptionally difficult to get sprayed concrete to bond to a sheet membrane system, because the membrane does not have a continuous bond to the primary lining. Thus the sprayed concrete, therefore, tends to rebound off the membrane surface. This effect can be reduced by the use of lattice girders and reinforcing steel mesh to help support the sprayed concrete during application. However, this tends to reduce the quality of the final lining as achieving adequate compaction of sprayed concrete through a network of steel reinforcement is difficult, resulting in voids and failure to passivate the steel against corrosion from ground water when it is not adequately encapsulated.
The construction method currently favoured for final linings tends to be traditionally reinforced cast-in-situ concrete. This is much slower than spraying concrete and, therefore, potentially much more expensive. In long tunnels of consistent cross-section casting, the use of shutters can be cost-effective. However, in complex geometry situations, such as metro stations, where interconnecting tunnels and passages have widely varying cross-sections, shuttering becomes increasingly complex and expensive. At the same time, waterproofing requirements are usually most onerous in these areas. For instance, London Underground Limited (LUL) and Crossrail of the UK are currently asking for completely dry tunnels in their specifications.
Sprayed systems
The great design benefit of a spray-applied waterproofing membrane is that the final lining can be installed using permanent sprayed concrete instead of cast in-situ. With fibre reinforcement, traditional lattice girders and rebar are no longer required, increasing build speed and reducing cost.
Colin Eddie from Morgan Sindall Underground Professional Services says: “Depending on the design of the tunnel, cost savings of up to 50 per cent are achievable with a sprayed solution, when considering the waterproofing and final lining taken together.”
“Reduced cost, faster build speed and higher-quality waterproofing performance are a powerful argument in favour of sprayed waterproofing membranes for tunnelling. However, the most significant advance that a fully-bonded sprayed membrane enables is use of the ‘composite effect’ between the primary and secondary sprayed concrete layers.”
Composite effect
In construction that includes sprayed waterproofing, the primary and secondary concrete layers are both fully and intimately bonded to the membrane.
Consequently, unlike when using sheet systems, both the concrete layers are acting together and therefore the primary and secondary linings contribute to the load-bearing capability of the tunnel.
Research carried out by Morgan Sindall, both in its underground professional services division and supported by further work at Warwick University to test this theory have shown that two concentric rings of sprayed concrete, bonded together by Stirling Lloyd’s Integritank HF tunnel waterproofing membrane, behave in the same way as a monolithic ring of the same dimensions.
Whilst earlier work has suggested that a mechanical key between the concentric rings is required by way of an uneven interface, the Warwick University work actually shows that this is not the case, and even with a smooth interface the full effect is achieved.
This is where the major benefit for future tunnel design lies and one that will have a profound effect on the industry. If part of the primary lining can be considered to contribute in structural calculations then the ultimate application could mean that tunnels can be built with a lower overall lining thickness. This means reduced excavation, reduced volume of concrete required and a reduction in the associated transport and installation costs.
There is also a significant environmental benefit in reduced waste and reduced carbon generation, in addition to the commercial benefits of building lower-cost tunnels in a shorter time-frame.
So the move to sprayed waterproofing that is occurring around the world in tunnelling has far wider implications than ‘only’ achieving the previously seemingly impossible dream of dry tunnels. It also produces significant environmental and commercial benefits. Better performance, greater longevity, reduced environmental impact and lower cost; in a world reeling under financial constraints, this could not have come at a better time.
