Tunnels are built with a life expectancy of over 100 years, which means that standards for tunnel construction must be high, in particular those involving sealing and waterproofing systems. Modern means of communication incorporating road as well as rail networks require tunnels to be completely dry. Because the seal protects and secures the construction, saving maintenance costs and repairs throughout the tunnel’s life, such systems are an essential component of tunnel structure.
During the last decade, different waterproofing materials have been studied and tested in tunnel applications, mainly in Switzerland and Germany. These materials are based on polyolephines compounds (PE and PP). Their main difference from plasicised polyvinylchloride (PVC-P) lies in the absence of plasticiser in the compound. This plasiciser provides a better chemical resistance and a longer life, at least from a theoretical point of view.
Unfortunately, the application of these polyolephines has presented various problems, especially in difficult circumstances. For example, in tunnels with water under pressure, the problem has mainly involved the stiffness of the material and the difficulties of hand welding. In addition, preparation of the excavated surface had to be improved because of the polyolephines’ poor ability to adapt to irregular surfaces and their low resistance to static puncturing. PVC-P membranes can be applied on irregular surfaces if they have no sharp edges.
The waterproofing system in a tunnel is composed of a combination of many factors. The characteristics of the membranes are very important but they cannot be considered separately from other aspects.
These factors are: preparation of the excavation support by regulating the shotcrete; drainage layer; protection layer (geotextile); waterproofing membrane; application system (fixings and welding of mem branes); particular points (passing through fixings for steel reinforcement or through pipes).
Welding the membrane
Arguably, the most important aspect of the waterproofing operation is the welding of the membranes, which has to be carried out very carefully to maintain the continuity of the waterproofing system; any failure in the welding will cause leakage in the tunnel, with catastrophic results if water under pressure is present.
Bearing in mind the difference between welding in situ – normally a very hostile environment – and in the laboratory, PVC-P membranes still represent the optimal choice. They combine smoothness; high weldability; long life (a 2mm thick membrane has an estimated life of over 100 years); excellent mechanical characteristics; and proven application experience.
Underground construction is an ideal environment for waterproofing applications using PVC plasticised membranes. The behaviour of the synthetic membranes in PVC-P, together with other materials and ‘modified polyolephines’, demonstrates important advantages. The smoothness of PVC-P membranes and the ease of welding are the most important characteristics. It is difficult to weld membranes containing polyolephine, particularly in wet and uncomfortable conditions, because of the its stiffness and narrow temperature parameters. Failure is particularly relevant when using hand held hot air welding equipment. In addition, during welding it is evident that there is a distortion of the joints in PE membranes which could cause them to tear under stress.
Harsh environment
Waterproofing membranes subjected to this harsh environment must provide a high resistance to compression. PVC-P membranes can be applied under significant load pressure. Comparative tests carried out according to DIN 16726 have shown that membranes made of modified PE are subject to permanent deformation which can cause loss of waterproofing function; this permanent deformation is higher than that for PVC-P. Tests were performed at 6 bar.
The elastic modulus of PVC-P is lower than that of polyolephine. The absence of ‘stress cracking’ makes PVC-P much more able to withstand the possible deformations which occur in underground construction. Figure 1 shows a typical load elongation diagram of a PVC-P and a modified PE membrane, analysing the trend of the load value up to an elongation of 300%, the load then increasing from 300% to 600/800%. This increase comes from the type of co-polymer used for polyethylene modification.
At first sight, it is possible to mistake this behaviour as an index of the capacity of the PE membranes to adjust to all situations such as surface irregularities or discontinuities; permanent deformations; static punching; or dynamic punching. After a more profound analysis of the load elongation diagram it is clear a break of 800% is not enough to assure the resistance to the above mentioned stresses.
With regard to the diagram area included between 0%-300% of elongation, it can be noted that the corresponding load is in the range of 5-6N/mm². This means that when constant load is applied, there is irreversible and progressive membrane elongation. In tunnel waterproofing, the presence of constant load is frequent, as well as surface irregularities and static punching.
The PVC-P membrane stress elongation diagram has a pseudo-elastic trend with an increase of applied load that is proportional to the elongation increase. This kind of behaviour is ideal for applications with a load that induces planar stresses on the membrane. In this situation, the membrane can support high strength with relatively low elongation.
The membrane thickness trend is analysed as a function of the elongation percentage of the membrane. In the case of the modified PE membrane, there is a gradual reduction of thickness from the low elongation percentage. This behaviour is shown in Fig 1 and is due to the yielding zone. In this zone, the applied load is constant and the elongation percentage increased.
The PVC-P membranes confirm their pseudo-elastic behaviour and do not register much thickness variation up to an elongation of 100%. In the range up to 50% of elongation, the behaviour is elastic and the thickness is constant. In the case of PE modified membrane, the elastic behaviour is restricted for elongations of 20-30%.
The great thickness reduction registered on the PE modified membranes will be reduced compared with the ones for PVC-P, resistance to static loads or, in any case, resistance to all those situations where loads cause significant membrane deformation.
Another important attribute is the flexibility of the formulation of the various PVC-P membranes. It is possible to produce special membrane for almost any kind of application: resistance to oils and bitumen; Contact with potable water; fire resistance (DIN B1 or B2); resistance to bacteria.
PVC membranes for tunnelling
Because of the flexibility of PVC-P compounds, different kinds of this material have been developed for waterproofing tunnels. Today there are two main systems for PVC membranes. The first is known as ‘signal layer’ membrane and the second is based on the application of ‘transparent’ membranes.
The signal layer membrane is a heavy duty designed PVC-P compound with very high mechanical characteristics combined with an optical system of damage control. This kind of material is normally manufactured by a co-extrusion or calandering process, where it is possible to create a thin, light coloured layer of material (maximum one third of the full thickness on a dark contrast base. This provides easy recognition of any accidental damage during application or other working phases on the exposed material. This material was developed, and is used primarily, in Austria, Switzerland and Germany.
In 1979, SEMALY started a research project on the waterproofing system used on lines A and B of the Lyons Metro using traditional opaque PVC membranes. The aim of the research was to determine the percentage of hand welding defects taken at random along the tunnel. Forty-two per cent of the defects discovered were attributed to the lack of welding between overlapping layers (a simple superficial thermo-gluing process was encountered), with an average peeling strength resistance of only 2.5kg/cm against the normal 6-8kg/cm of good hand welding. Thirteen per cent of the defects were due to excessive welding temperatures that formed carbonised areas with an average peeling strength resistance of 2.5kg/cm.
These kinds of defects cannot be recognised by non-destructive controls on opaque membranes but they could easily have been detected and even avoided by using a transparent PVC membrane. This quick and easy means of visually inspecting the weld is of considerable assistance both to the welder, who can continuously check his own work, and to the contractor during inspection and quality control.
“For tunnels, where the weld is highly linear without crossing of joints, transparent membranes should be used with a double weld, enabling a test channel of 8-10mm to be produced, which is then tested by pressurising a coloured liquid to a maximum of 0.25MPa and stabilised for 90 seconds. This test is very quick, as a 28m long weld can be filled in 30 seconds. This testing procedure used in connection with a PVC-P translucent membrane enables the waterproofing to be tested both from the inside and outside of the waterproofing structure.”¹
The high number of destructive tests carried out on transparent PVC welding have confirmed a direct correlation between the visual aspect and the quality of the welding itself. The transparency of the membrane also guarantees that the formulation is pure.
In France, the use of transparent PVC membranes is regulated and is the principle obligation in Section 67 of the General Technical Clauses Document, Part 3, concerning the waterproofing of underground works, and positive results have been achieved in the last 10-15 years. At present, waterproofing works are under way at a new extension of Line 3 of the Milan metro, where a waterproofing system based on a 2mm thick transparent PVC-P membrane has been specified, with a 1.5mm thick protective layer in PVC and a PVC waterstop.
FLAG started producing a transparent membrane, FLAGON BT, in 1985. Since then, several millions of square metres of this material have been applied in Europe and around the world.
Related Files
Figure 2: thickness variation
Figure 1: tensile stress
