Typically waterproofing of conventionally excavated tunnels has been done by installing prefabricated sheet membranes between the primary temporary support lining and the secondary permanent inner lining. The membrane acts as a separation or sliding layer between the linings, which is often desirable.
The geotextile installed behind the sheet membrane provides the required drainage and protection of the system. The design of the tunnel lining is strongly influenced and limited by the use of waterproofing sheet membranes since primary and secondary linings are separated from each other, forming a double-shell lining system. In that case, the secondary lining must be designed for all permanent loads, although the sprayed concrete primary lining continues resists ground and water loads for a long time after construction.
Significant progress has been made in sprayed concrete technology over the last two decades, with advanced admixtures, as well as the improved application technology using spraying robots. The development of new admixtures like superplasticizers based on poly-carboxylates, alkali-free accelerators and silica fume, enables designers to use permanent sprayed concrete linings (SCL) increasingly for long-term service life. When designed and applied correctly, sprayed concrete can be considered as permanent. Conventional waterproofing systems with sheet membranes don’t allow the use of SCL as a final lining. With the introduction of spray-applied waterproofing membranes, the tunnel lining can consist of a total sprayed system: sprayed concrete as the primary support lining, a spray-applied waterproofing system and a spray concrete secondary and final lining. The use of spray-applied membranes has seen increased use over the last decade in some countries, and demonstrated functionality and cost-effectiveness in several situations.
This paper describes the use of the sprayed waterproofing membrane MasterSeal 345, an EVA polymer-based product, and the practical experience of its use on the Crossrail project in London.
The sprayed waterproofing membrane is polymer based and has been specially developed for underground structures. The membrane is applied between primary and secondary concrete linings and develops high bond strength on both sides of the concrete (doubled-bonded).
It is applied onto a concrete substrate usually with an average of 3 mm in one pass, and covered later on by a secondary concrete lining or a protective non-structural concrete layer, depending on the design requirements.
Due to its double-bonded function, it provides the tunnel lining system with unique mechanical properties and waterproofing features. The bond is unaltered by the concrete placement technique, be it sprayed or cast in place, or by the presence of fibers. The use of this membrane is particularly advantageous in geometrically complex areas such as in lay-by niches, cross passages, turn-outs and crossover caverns.
In such cases, the installation of conventional sheet waterproofing membranes is difficult and locating of possible leaks and repairs are challenging. The use of a sprayable waterproofing membrane could be a solution. Additional advantages to owners, contractors and designers result from:
- Flexibility in the design and application
- Fast application and easy logistics
- Flexible work programming/sequencing
- Significant program and cost savings
- Reduced long-term maintenance costs
The waterproofing concept with the sprayed waterproofing membrane is based on both impermeability and bonding properties of the membrane. Due to the high bonding of the membrane to the primary lining, any groundwater inflow through a crack in the primary lining stops as it reaches the membrane, and cannot migrate along the interface between membrane and concrete. Potential groundwater paths can be eliminated, and the risk of water ingress into the tunnel is considerably mitigated.
Additionally, the same strong adherence between the membrane and the secondary lining provides a second barrier against groundwater ingress into the tunnel. Thus, to reach the tunnel a groundwater inflow has to pass through three aligned failure zones, i.e. a crack in the primary lining, a defect in the membrane, and a crack in the secondary lining. An eventual leak would be localized and could be easily treated with injection measures.
“Our goal at BASF is to promote a different design method for tunnel linings using the composite effect of the membrane, which actually glues together two concretes as a waterproofing product,” said Frank Clement, Global Technical Manager-Membranes for BASF’s Underground Construction division. “This will allow designers to reduce the total thickness of the lining which results in: smaller excavation diameter; less handling of spoil, including transport and disposal; more efficient tunneling as a result of faster advance, less labor and time savings; and a more ecofriendly way of building a tunnel. Design models have shown that a possible reduction of the tunnel lining of 20 to 30% is feasible. A simulation for a new build tunnel showed a savings of 20% of the consumed concrete volume resulting in an increased efficiency.”
Spray-applied membranes were introduced late 1990s and the first successful application was in 2000 on the Machadino project in Brazil. More recently, they were utilized in the landmark Crossrail project in London.
Experiences: Crossrail Project
Crossrail is among the most significant infrastructure projects ever undertaken in the United Kingdom. This new metro line will improve the way people travel around the capital by optimized journey times and connections. The new railway will be a high-frequency, high-capacity service to 40 stations linking Reading and Heathrow in the west, to Shenfield and Abbey Wood in the east via 21 km of new twin-bore tunnels under central London.
Flexibility during the construction of the stations, elevator and ventilation shafts was a key to the success of the use of the spray-applied membrane due to the complexity of the project.
Timing and logistics were crucial and coordination between the different contractors was essential and any disruption could cause delays for the project. The waterproofing system to be used should respond to this flexibility. The spray-applied waterproofing system chosen by the contractors is applied by simple, readily available dry spraying equipment that provides the required flexibility. The system uses compressed air for conveying the material. Water is added at the nozzle, creating a creamy consistency of the membrane. The equipment is suitable to be used even in small tunnel diameters.
This application is a simple stop-and-go application reducing any losses since no material will stay in the lines when spraying has to be stopped. Additionally, the contractor can resume application at any time by overlapping the already applied membrane. Based on the polymer chemistry, newly sprayed membrane will bond to previously applied membrane, resulting in a continuous waterproofing membrane, even after several months.
Due to the complexity of the project, BASF was asked to confirm that not only a new layer of polymer would bond to already cured membrane, but also concrete would bond to the membrane even when it was exposed for several months to the tunnel environment. To confirm this, microscopic investigations were performed on how the mechanism of the bond between the membrane and concrete was created. The bond between the first concrete layer and the membrane is based on a mechanical fixation and interlocking. During the curing of the membrane, the polymers penetrate into all the pores and irregularities of the concrete substrate, creating the required bonding of around 1.2 MPa after 28 days.
With a microscope the polymer interlocking onto the concrete pores can be seen. The second bond to the concrete applied on the membrane is a different mechanism. During the curing of the concrete, cement crystals from the hydration process are penetrating into the upper layer of the membrane. This provides the required second bond, resulting in a double-bonded membrane. Since these mechanisms are independent of the age of the primary lining or the age of the spray-applied membrane, a certain flexibility in the construction sequence is possible. Internal tests have confirmed this. Membrane samples which were 1 year old were washed and fresh concrete was placed on top. Bond results confirmed the above investigation resulting in bond values of >1.2 MPa.
Because of the dry sprayed method of the membrane application, stop-and-go of the membrane application was possible without jeopardizing the final quality of the tunnel lining.
Compatibility with other Waterproofing Systems
The original design of the stations and tunnels required sprayed waterproofing all round (fully tanked). Since the curing process of the sprayed membrane is caused by water evaporation from the membrane, the surface and eventual water ingress has to be managed upfront. During the excavation process, the surface needs to be relatively dry since a waterproofing membrane cannot be sprayed on running or dripping water. At the Whitechapel and Liverpool Street station there was a significant water ingress and it was decided to adapt the waterproofing design.
The solution was a hybrid waterproofing system – a combination between the spray-applied waterproofing system and a PVC sheet membrane. The spray-applied membrane was applied in the crown and the sheet membrane in the invert. This hybrid system was able to handle the water ingress but an adaption on the lining thickness was needed. A special design for the interface between the spray-applied membrane and the PVC sheet membrane was developed together with the subcontractor for the sheet membrane which complied with the required 120-year design life.
This meant also that the overall thickness and the reinforcement of the lining had to be increased because a PVC sheet membrane doesn’t allow a composite action as with a double-bonded spray-applied membrane. Injection hoses were installed in case a contact grouting was needed after placing the cast in-situ invert.
Influence of Regulating Layer
In order to optimize the application of the spray-applied membrane, it is advised to use a regulating layer in order to smoothen the surface of the sprayed concrete by a fine mortar. In the project a 40 mm layer was specified. The aim of the regulating layer is:
- Improve the irregularity of the surface
- Reduce the consumption of the membrane
- Easier quality control
Although mortar was specified, different contractors didn’t use the same type of mortar. Different solutions were adopted from ready mix to dry sprayed bagged mortars. The application of the regulating layer required special attention. Using the correct spraying technique, a smooth surface could be achieved. In some cases the regulating layer did not result in the required surface improvement. Mortars also created some additional problems which were only visible after the application of the membrane.
These problems were caused by the ventilation and/or the porosity of the regulating layer. During construction, water can find its way through the primary concrete, entering the tunnel. The regulating layer has to have a similar or higher durability and properties as the sprayed concrete used during the excavation. If that is not the case, water can migrate over a bigger area showing a bigger damp area. These areas can vary from a few sq in. to several sq ft. In case of a more porous regulating layer, different experiences were made.
The tunnel ventilation was causing some problems in localizing water ingress. With a high air speed and low humidity sometimes damp spots were not noticeable since the water evaporated quickly into the dry air showing a dry regulating layer. When the application of the membrane started, the membrane sealed off the regulating layer and water was trapped inside the regulating layer which sometimes created a water blister behind the membrane. Although this did not create a major problem, still these areas had to be repaired using a simple injection technique. Packers were installed and some acrylic resin injected into the regulating layer/crack in order to seal it off. It was advised to inspect the tunnel lining for damp spots by lowering the ventilation speed so they would be easier to locate.
Porosity of the regulating layer was also an issue to be addressed. With a low performance and porous mortar the water could migrate inside the regulating layer, making it more difficult to locate the correct entry point of the water from the primary lining into the regulating layer. The standard procedure is to install an injection packer without the injection nipple so it could act as a release point for the water. Then the waterproofing membrane would be applied.
The packers act like a temporary drainage for the water and afterward for injection with an acrylic resin. In reality it was seen that after application of the membrane, the water was not released through the packer but was traveling farther inside the regulating layer and appeared in a different location. It proved again that water travels the path of least resistance and the regulating mortar has to be of a high quality in order to avoid these problems.After discussion with the contractor it was decided to inspect the concrete before the regulating layer was installed so the exact location of the water ingress could be determined. A long packer was installed, acting as a temporary drainage. The regulating mortar was applied and after hardening of the mortar, the area with the damp spot was sealed off by a fast-reacting membrane (MasterRoc TSL865). By using this fast-setting product the water was forced to leave the system through the packers. This membrane is based on the same polymer technology but doesn’t have the same properties such as flexibility and crack bridging. The fast-setting membrane always needs to be over-sprayed with the waterproofing membrane.
BASF’s Clement added: “Spray-applied membranes have a proven track record and experiences are gained with every jobsite globally. The membrane is part of a system: concrete properties, quality of the regulating layer and water treatment are all factors that contribute to the success of the system. If designed correctly the system can allow a reduction of the lining thickness but the full system has to be considered. This means the correct design of the primary and permanent sprayed concrete, the use of a high-performance regulating layer, the double-bonded waterproofing membrane and the sprayed or cast concrete inner lining. Any compromise in this respect can lead to problems and delay during the construction process.”
This article is an adaptation of the paper “Experiences with Spray-Applied Waterproofing Membranes,” written by Frank Clement of BASF Construction Chemicals and Karl Gunnar Holter, Department of Geology and Mineral Resources Engineering, NTNU (Norwegian University of Science and Technology).