Coordinated sealing concepts for watertight tunnels
Watertight tunnels with a closed design are usually equipped with a sealing system of plastic geomembranes (PGM) or are planned as a watertight concrete structure. Before and after the installation of the inner concrete formwork, leaks can always occur, which, with the right systems, can subsequently be sealed in a targeted fashion or avoided from the outset. This post particularly refers to tunnels with a waterproofing system made of PGM. The solutions and measures described here may also be of interest in other seal concepts in tunnel construction.
Dipl.-Ing. Marc Meissner, M.BC., Arbeitskreis Tunnelabdichtung e.V./Association for Tunnel Waterproofing, Bückeburg/Germany
Andreas Leckeband, Meister-Kunststoffprofile GmbH, Witten/Germany
Dipl.-Ing. Sebastian Schwaiger, Müller+Hereth, Ingenieurbüro für Tunnel- und Felsbau GmbH, Freilassing/Germany
To enable the full functionality of a tunnel construction for at least 100 years, it makes sense to use different sealing systems that are suited to one another or complement each other. A sophisticated sealing concept permanently prevents the ingress of partially aggressive mountain waters, protecting the inner shell and, thereby, the load-bearing structure as well. Leaks frequently occur in the PGM as soon as during the construction phase or shortly thereafter. There are various possible causes.
Structure of the plastic geomembrane
The basic layer structure of a sealing system with PGM is illustrated in Image 1:
During the concreting of the inner shell, the PGM is pressed against the seal support made of shotcrete (1). For the sake of protection, a mechanically bonded non-woven fabric (2) with a basis weight of at least 900 g/sqm is used as a mountain-side protective geofabric. In pressure-sealed tunnels, PGM that is at least 3 mm thick is used (3). They are air-fitted with a bright signal layer in order to allow damage to be located quickly and to improve the lighting conditions in the tunnel during the laying work. Since the PGM is the main sealing element, the selection and installation have to occur with the utmost attention. After the laying process, the plastic geomembranes are welded together with welding machines. The seams are executed as testable double seams with test channels, which are then inspected for tightness by means of an air pressure test. In order to obtain a better seal, machine-run double seams should be performed in intersection and niche areas, if possible.
A tunnel can either be sealed all around by a plastic geomembrane (PGM) or designed from the outset as a waterproof concrete structure . Plasticised polyvinyl chloride (PVC-P) or plasticiser-free polyolefins are used for the material. A careful examination of the sealing material should be performed with regard to long-term stability and safety with respect to the environment. In order to be able to exclude a possible subsequent contamination of mountain waters, the recommendations of the Bundesinstituts für Risikobewertung [Federal Institute for Risk Assessment (BfR)] should be respected . Specific properties of all materials, particularly for German tunnelling, are defined in the ZTV-ING TL/TP .
With a smooth exterior of the in-situ concrete inner lining, plastic geomembranes resist the additional strains caused by external water pressure without any problems. Investigations on leaky tunnels have shown, however, that the outside of the in-situ concrete inner lining cannot always be made thoroughly smooth. This can be due to the concrete mix or the processing. This can happen, for example, when the inner shell is not sufficient and is concreted in full volume. Due to high reinforcement content, large inner shell thickness, poorly processable concrete, interruptions during concreting or any air bubbles present in the concrete, this risk is increased.
Since the outer side of the in-situ concrete cannot be inspected, harmful defects cannot be detected. The water pressure that is pending after installation places stress upon the PGM and presses it against the rough outer surface of the inner shell or even on exposed rebar. As a result, damage can occur. An examination of the outside of the in-situ concrete inner lining is not possible after the fact.
Top injection by injection hoses
One important measure is the closure of the ridge, which is caused by either the settling of the concrete after the concreting process or the incomplete filling of the tunnel block. As a supplement or variant of the system prescribed by the ZTV-ING, the depositing of the inner shell blocks can occur with cement injection. Imperfections are filled via grout supports and an injection hose installed in the ridge area. The basic idea in such a case is to ensure a fully concreted inner shell before the mountain-side water pressure presses the PGM on the outside of the inner shell.
When choosing the injection cement and injection hose system, attention should be paid to reliable processability, so that all the areas of the ridge gap as well as cavities and concreting shadows can be grouted safely. A combination of F9200 injection cement and the VPRESS injection hose has been tested, for example, by the Gesellschaft für Materialforschung und Prüfungsanstalt für das Bauwesen [Society for Materials Research and Testing Institute for the Construction Industry (MFPA)] in Leipzig, and the injectability of a 30 m long section has been demonstrated. This system prevents possible damage that may arise when the water pressure presses the PGM on uneven surfaces in the ridge area or, in the worst case, to exposed rebar.
Joint tape allows leaks to be sealed off
Joint tapes are mostly used for external construction joints. With hand welding devices, they are welded directly onto the completely installed plastic geomembrane and are therefore made of the same plastic as is the PGM. In the case of seals that contain water pressure, at least 0.60 m wide 6-piece exterior joint tape (Figure 2) has to be used. The joint tape is to be welded on both edges to the previously laid and grouted PGM with a joint seam of at least 30 mm wide. The connection has to be so strong that a subsequent separation of the joint tape from the seal is excluded. The welding of the joint tape with the PGM should in any case be subordinated to the welding of the plastic geomembranes to each other and must not interrupt the sealing level of the PGM or lead to increased construction seams of the PGM.
Compared to previous implementation methods, joint tape with additional grout tubes provides a significant advantage. The large joint tape width provides sufficient space for installing the front shuttering between the bridges. In addition, a tight integration of the joint tapes in the inner shell concrete can be ensured by the possibility of injecting.
Its main function is to divide the sealing plane PGM in delimited fields. This allows the leakage in the case of a lacking seal to be limited and to be addressed in a targeted fashion. Of particular importance in such a case is the segmentation or sealing off by external construction joint tapes in the block joint area. After the concreting of the inner shell, the joint tapes are activated, i.e. subsequently concreted by means of cement suspension. Thereafter, the corresponding localised block can be sealed across the entire surface. In this process, acrylate gels are used, mainly due to their high viscosity. The injection hoses can be grouted repeatedly with grouting cement and acrylate gel.
In this way, leaks can be limited through an escape of water. In the case of leaks in the geomembrane, the ability should be created with this system of sealing off the respective faulty sealing area through grouting, e.g. with resin or acrylate gel.
The composition of a sealing concept depends on the requirements of environmental protection, the expected chemical nature of the mountain water, hydrology and geology, the rated water pressure and the subsequent usage requirements for the structure.
To allow a durable, fully functional tunnel structure, it makes sense to methodically adapt various sealing and injection systems in relation to each other. Through the application of a flexible sealing concept, occurring leaks or damage can be minimised or resolved in an economically sensible scope.