Before deciding what waterproofing solution is desirable one has to know what the real problem is first. Similarly tunnel designers have to decide how the waterproofing measures can be integrated within the overall design of lining to meet the tunnel performance specification. Currently there are several accepted types of design involving sprayed or sheet membranes for waterproofing, but the opinions on how these can be achieved vary considerably. Whilst the newer concept of spraying a membrane on a tunnel surface to achieve waterproofing, or more often water-diversion to drains, sounds simple in practice, it is more complicated in concept and execution. Nevertheless it can still offer advantages over installing most sheet membranes if carried out properly.
The claimed advantages of sprayed waterproofing are becoming well known and include:
• much more tolerant of rough surfaces, with little need for ‘fleece’ layers to protect the lining from penetrating , sharp rock;
• equipment that is more mobile and less space-consuming than that for sheet membranes;
• not labour intensive;
• easier jointing, if necessary.
On the other hand, the advantages of the main competitor, sheet membranes, remain:
• cheaper materials;
• few airborne emissions during installation;
• established procedure.
Both methods are likely to require specialist installation, or at least until the correct procedures become more widely known. In addition to these two basis approaches, there are other methods of waterproofing that can aid or replace these methods providing the circumstances are right. These include wall drains or a drainage layer, injection grouting, less pervious concrete (with certain additives), well- sealed pre-cast lining and/or temporary dewatering.
Whatever the method chosen, the objective is to achieve the design specification for the effective life of the tunnel. It is sometimes revealed that there is no such thing as a ‘waterproof’ tunnel, or at least only in special circumstances. In most cases the objective is to divert water flow to where it can be managed while withstanding expected groundwater pressures.
The waterproofing membrane has to be considered as part of the whole lining, and its structural integrity to dealing with any ground movements and changes in groundwater pressure or potential flow. It is very difficult to achieve a 100 per cent impermeable layer in any case, especially when considered over the life of the tunnel. So the realistic waterproofing objective is to make water ingress totally manageable using a layer that is as impermeable as possible.
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By GlobalDataThe type of sprayed material used is another important consideration since different plastic resins have different properties is the areas of ageing, resistance to groundwater contaminants, bonding to other layers (if required) and safety.
One of the major issues in sprayed waterproofing is, ironically, how tolerant the material is of water. Despite manufacturers’ claims of tolerance to wet surfaces, it is the case that none can actually be applied to surfaces with running water, and few securely against any free water. As such, other preparatory water diversion or priming measures have to be taken, increasing the complexity of the whole procedure, although perhaps not as much as sheet membrane installation.
Waterproofing specialist contractor and supplier Stirling Lloyd’s business development director, Mike Harper, told ‘Tunnels & Tunnelling International’, “No sprayed membrane (regardless of what anyone else might say) will bond to a surface where water is running over it. It is not a problem as such, as long as there are clear ways of dealing with the different scenarios for water ingress.”
Masterseal
In the relatively short life of sprayed waterproofing, Meyco BASF’s Masterseal 345 material has become perhaps the best known. It is promoted as part of a continuous structure to prevent water migration along the concrete-membrane interfaces on either side of the membrane.
The claimed bonding aspect can be important in that it allows the primary (shotcrete) lining to be considered as part of the permanent support structure with the inner final concrete lining. This is sometimes termed a ‘single-shell’ composite lining, although a single-shell strictly has only one layer of concrete with no membrane. According to the engineer’s design, the primary lining may not be included in the permanent structure, but any future activity between the membrane and primary lining has to be taken into consideration.
Masterseal 345 is applied in the same way as dry-process shotcrete use a spraying unit like the Meyco Piccola. Such a unit must be fitted with a dust collection filter or similar dust collector. Although the product has not toxic components, the use of standard ppe (gloves, eye protectors and a mask) is recommended.
Meyco says that the concrete surface to which the lining is to be applied has to be thoroughly pre-wetted, and any contamination removed. Following application the membrane must not be exposed to extremes or great variations of temperature for five days.
As with other membranes the roughness of the surface to be treated has to be considered. Meyco says that if the roughness necessitates more than 6kg/m2 of Masterseal 345 a smoothing layer of cementitious mortar is recommended, having an maximum aggregate size of 4mm, thus reducing the necessary consumption of Masterseal 345.
Stirling Lloyd
Stirling Lloyd’s sprayed waterproofing products can be used in both cut-and-cover and immersed tube tunnel structures as well as excavated designs, but in such cases open-air application is often the case.
On wet surfaces requiring waterproofing, Mike Harper continued, “Damp concrete is not a problem for Integritank HF – its has a primer system specifically made for sealing and enhancing the strength of the bond of the membrane to the damp concrete. Where we draw the line is wet substrates (ie free liquid water on the surface). Such a surface needs pre-treatment to stem the water ingress. This is dealt with by a number of specialist products from Stirling Lloyd.”
Another potential bonding problem is between layers of sprayed membrane, such as between shifts, or after similar stoppages. Commenting on this Harpur said, “The chemistry of some materials does not lend itself to achieving a truly seamless application. Well-known examples would be polyurethane resins for example. These have many good properties, but on of their limitations is that their surface chemistry becomes ‘closed’ after approximately 72 hours. Where a truly seamless installation is the goal, then any such mechanism that reduces the effect of intimate chemical bonding between new and old material is undesirable. This is one of the reasons that materials based on polyurethanes and polyureas are unsuitable for tunnel waterproofing, in addition to their toxicity. We do not use them for tunnel waterproofing for this reason.”
Recent project references have included the Croydon cable tunnel, the 4DCF extension of the Thames Water London Ring Main to Honor Oak and various projects in the Emirates.
In the Honor Oak project Morgan Est employed Stirling Lloyd’s Integritank HF as a sprayed membrane sandwiched between primary and secondary sprayed concrete layers for the back-shunt of the TBM launch chamber. The tunnel has continuous water pressure both inside and outside, with the tunnel around 52m below ground level. Typically the water table is around 15m above the axis level in permeable chalk Stirling Lloyd Construction applied the material as specialist sub-contractor. First the primary lining was cleaned of dust and contaminants, and then primer applied by spray to the whole concrete surface being lined.
Integritank HF is supplied in two IBC caged tanks for ease of handling and eliminating site addition of water. This avoids any risk of material variation, as does the use of a computer-controlled, airless pump for mixing the two components. An access platform was required to enable close access to the substrate all around the tunnel profile, for accurate application and inspection.
More than one coat is colour-coded to ensure proper coverage. Two spray operatives at different levels coated the whole 360 deg of the profile in one pass. Spraying is done by operatives manually, not by robot. Commenting on robots, Mike Harpur points out, “A robot operator spraying remotely cannot tell if it’s ‘missed a bit’ and waterproofing is an exact science. A membrane is either waterproof or it isn’t. There is no such thing as 95 per cent.”
Trials
Phil Richardson of NCD (Natural Cement) has been engaged with Stirling Lloyd and leading UK tunnelling contractor Morgan EST in intensive trials of a wide range of spray-on waterproofing products to determine which of these works best, both in applications and longer term in situ. NCD products, including Natural Cement shotcrete, have even been used to treatment particularly difficult areas in the construction of the Hindhead Tunnel lining, that otherwise used mainly Meyco Masterseal 345. This shows that not only synthetic chemical materials have to be used for waterproofing.
Part of the de Neef range of waterproofing products is Aquatek Super XA that is applied as a cementitious slurry coating that can be used against water pressure. The company also offers Denepur 200. This is a polyurea-polyurethane hybrid hot-sprayed membrane applied in layers 2-3mm thick.
Phil Richardson highlights another potential problem with synthetic sprayed waterproofing. “The materials are expensive, and the layers applied have to be thin. Obviously, if a 2mm layer is required, applying 3mm in an attempt to ensure continuous cover increases the cost by 50 per cent. Consequently the development of peaks and troughs in the thin layer can be an issue. In comparison, the application of wet-process shotcrete helps to produce a smooth surface, but the layers are much thicker. Most sprayed waterproofing materials are fast setting, including polyurea, which is instantaneous, but still with variations. One supplier claims a 12-h cure time but actually 30h has been known.”
On drying time, Shotcrete Technologies claim a 30-s ‘tack time’ for its 2-component ST Waterseal, with a curing time of around 60 minutes. The product has no toxic vapours and is resistant to attack from most other chemicals. Once cured it has a tensile strength of 600 lbf/in.2, allowing elongation of 500 per cent.
Testing
As with sheet membranes, testing of sprayed waterproofing membrane is necessary to ensure adequate protection of the tunnel and lining, because even with all installation procedures correct, there may still be tiny defect, such as an ‘invisible’ pin-hole. Once installed the membrane is almost impossible to repair with resorting to traditional waterproofing techniques such as grouting.
Stirling Lloyd and other contractors use an electrical discharge-testing device known as a ‘holiday tester’. The testing procedure is based on the fact that the membrane is an electrical insulator so that a voltage applied will not result in a current if the membrane is continuous.
Waterproofing the Croydon Cable Tunnel using Stirling Lloyd material A custom-built platform give operators better access in the Thames Water Ring Main extension The Masterseal system Applying methacrylate resin waterproofing in Dubai Applying Masterseal memebrane in the UK’s new Hindhead Tunnel
