Tunnel waterproofing can be an extremely expensive exercise, especially when things don’t go to plan. One only has to look at the plethora of high-profile tunnels that have experienced problems, such as Boston’s Central Artery Tunnel in the US, the Burnley Tunnel in Australia and Sweden’s Hallandsås Tunnel, to appreciate some of the headaches and cost escalation that can be encountered.
Aptly described as the “nemesis of tunnelling engineers since the birth of the profession,” in a recent T&TNA Comment(1), groundwater inflow represents a unique set of challenges to the continued development of the underground construction industry.
Whether the aim is ‘water control’, in order to prevent water ingress affecting the function of a structure, or complete ‘water sealing’ (including pregrouting), to eliminate water from the structure and/or maintain groundwater levels, the list of tribulations past projects have run into is long and varied. In fact, those encountered to date include all types of tunnel construction, ground conditions and lining method.
TBM tunnels have suffered from seepage through segment grout holes, through cracks induced by jacking forces and misaligned joints(2). Hard rock tunnels have suffered from inadequate pre-grouting in areas of complex and extensive jointing, such as Norway’s highly-publicised Romeriksporten Tunnel(3). In cut and cover tunnels cracks and leaking joints are often the issue. Flexible sheet membranes under persistent water pressure have also accounted for a fair share of failures(4).
Trying to fix these problems can be difficult and unrewarding; the consequence being a huge negative impact on budget and schedule. The emphasis is, of course, to avoid these problems from the outset. The aim – to design and construct cost-effective water control or sealing solutions, which achieve their goals in spite of geological variation and minimise long-term operational costs.
Opening the floodgate to discussion
There are a large number of good-quality waterproofing products available, which provide tunnel engineers with the capacity to achieve any number of water sealing aims. However, it has been suggested(1) that this is part of the problem. There is almost too much choice, and little impartial, academic, guidance available for engineers to fall back on. In addition, few manufacturers provide full and detailed specifications for the successful application of their products.
Considering the scale of the issue and potential impact if problems are encountered, or if serious mistakes in choice or application are made, it is also remarkable how few training courses and conferences are organised to share experiences and disseminate current knowledge. Independent consultant Dr Olav Blindheim, who acted as an expert witness on both the Burnley and Romeriksporten tunnels, agrees: “There is definitely a continued need for exchange of information in this area. If nobody shares their problems, development will be very slow.”
Specification, life-cycles and holistic approaches
During a recent gathering of the British Tunnelling Society for a lecture on tunnel waterproofing and grouting products, Martin Smith of Rascor, cited the urgent need for more informed specification of waterproofing and grouting systems. Smith also stressed the benefit of thinking in terms of total life cycles, “this means taking a positive view of buying good quality waterproofing products, which might be more expensive to purchase but provide enhanced water resistance and fewer maintenance and repair costs during the tunnel lifetime.”
Knut Garshol, director of Degussa UGC Latin America, takes a similar view: “Personally, I think that research will certainly be helpful, but it will never alone solve the problem. There are far too many variables for that to be remotely likely. The most immediate positive improvement for new projects, in terms of ensuring that ground water control targets are met, regardless of local variation in ground conditions, would come from adequate project management. The owner and the designers would have to address the problem at the planning stage, on par with other functional and design requirements, to avoid water coming as a surprise during construction. Improvised and hasty “solutions” adopted during construction are seldom optimal. The owner must also be prepared to pay the price and when looking for lowest bidder this has to be linked to guarantees, experience, references and probability of delivering a success. Lifetime cost approach is clearly the right way to go.”
It is this life-cycle cost and the potential for more holistic approaches to lining and sealing techniques, that spurred Franzén and Celestino to discuss various methods of water control and sealing in a paper presented at the ITA-AITES conference in 2002(6). Using the Channel and Hallandsås tunnels as specific examples, the authors discuss a number of lining design methods and product application issues. They conclude that, in general, a number of design issues are not fully understood and therefore ‘safe’ solutions are often chosen. They suggest the answer is further international exchange of ideas, in relation to waterproofing specifications and design concepts, which they believe could lead to cheeper tunnels.
Within Franzén and Celestino’s paper the concept of utilising shotcrete to provide cost effective, watertight linings is highlighted as an example of possible future development (either by including a sealing agent, i.e. sprayed membrane, or by utilising modern shotcrete mix designs to achieve virtually watertight linings). The authors recall the findings of Pöttler & Klapperich (2001)(6), who call attention to the importance of, and the need for, further implementation of single shell tunnel projects. Adopted for trial sections or adit tunnels of the high speed German Railway, Pöttler & Klapperich gave examples of economy achieved with the single-shell approach in areas of limited groundwater pressure; suggesting savings in the range of 10%-15% when compared to conventional double-shell techniques.
“Single shell linings definitely provide an economical structural solution. However the challenge remains, particularly for road tunnels where good lighting and painted finishes are desirable, to totally exclude water ingress with its unsightly staining potential,” says Tony Rock, divisional director for Mott MacDonald. “In this situation, we are giving serious consideration to the use of sprayed membranes. Such membranes are particularly suitable at tunnel junctions, where a sheet membrane would require complex ‘tailoring’ i.e. cutting and welding. Nonetheless, as with any material constructed insitu, residual concerns about quality control and workmanship remain, particularly in ensuring adequate coverage and achieving the specified membrane thickness. With this in mind, we have recently developed a generic specification in conjunction with UGC International as a guide to good practice,” says Rock.
What do we do now?
In the short term, traditional waterproofing and grouting products continue to evolve in response to industry requirements. For example, the potential dangers of certain ‘chemical’ grouting agents to the environment were recently highlighted on the Hallandsås and Romeriksporten tunnels, placing the need for improved performance and application of cementitous grouts high on the agenda in Scandinavia(3).
Manufacturers also continue to enhance flexible sheet membrane systems, waterstops and seals and joint sealing systems(4), to simplify application and facilitate complex logistical requirements.
However, ultimately, it is clear that the industry as a whole needs to focus on sharing experiences (both good and bad) and to make more of the good quality materials and tools available, by ensuring a concerted and informed job-specific effort to satisfy desired long-term ground water targets is implemented from the outset of a project.
