By Peter Barrett & Marcus Jablonka
The Gotthard Base Tunnel runs beneath the Alps as part of a new north-south railway in Switzerland. At 56.8 km, it is the longest tunnel in the world, surpassing the Seikan Tunnel in Japan. The total is 152 km, when service tunnels and other shafts are included. With an expected service life of 100 years, and no major repairs being necessary for at least 50 years, this can be a very challenging environment for drainboards.
Drainboards are frequently used as tunnel liners, in both cut-and-cover and bored tunnels. They provide effective relief of hydrostatic water pressure while enhancing the performance of waterproof membranes. In the Gotthard Base Tunnel, the demands on these systems are very high and so are expectations for long-term performance.
However, the durability of drainboards can be affected by many factors. To provide adequate performance, drainage products must maintain their full integrity over the entire design life of the structure. While product specifications often avoid any reference to durability and long-term performance properties, aging processes clearly affect these polymeric materials. Durability and aging resistance of drainage membranes, like any polymeric products, are of primary concern.
The Gotthard Base Tunnel is a double-shell tunnel with a combination of waterproofing system and drainage layer between the shotcrete outer shell and the concrete inner shell. This design continuously drains seepage water away to protect the concrete shell against hydrostatic pressure, and to transfer high loads onto the concrete support structure.
At the base of the Gotthard Base Tunnel, where the mountain cover is up to 7,500 ft high, geothermal effects can generate rock temperatures of up to 45°C. Seepage water can also reach temperatures of up to 45°C. The water is mostly alkaline, but may also be acidic in some areas.
The function of the drainage layer is to provide permanent relief of hydrostatic water pressure, while the waterproof liner prevents any ingress of water into the tunnel. Such systems are subject to high mechanical, physical, chemical and sometimes biological operational demands, both during the installation period as well as later in-situ during the service life. In order to provide adequate performance, drainage products must maintain their full integrity over the entire design life of the structure.
There are currently no test standards with suitable criteria for such high loads and requirements. Previously available drainage materials had not been designed for nor tested under comparable conditions. A comprehensive product evaluation had to be developed and performed.
The test program comprised existing test methods that were augmented by additional procedures, such as aging resistance in oxygen-enriched water at elevated temperatures, compression creep tests between rough surfaces, tests under combined lateral loads and horizontal shear, as well as installation tests including the construction of the concrete support shell.
During the aging part of the material evaluation, the drainboard specimens were exposed to a number of different conditions for 24 months:
- Water circulated at 23°C, at 45°C and at 70°C
- Alkaline water (saturated calcium-hydroxide solution) circulated at 50°C
- Acidic water (0.5% solution of sulphuric acid) circulated at 50°C
- Oxygen-enriched water circulated at 70°C and 3 bar pressure
- Environments with aerobic and anaerobic micro-organisms
Specimens were tested for visual appearance, mass changes, in-plane water transmissivity and mechanical properties five times during the test period. Additionally, thermoanalytical tests were applied after 3, 6, 12 and 24 months to sufficiently characterize the aging behavior of the products. Performance under compression creep load was evaluated as well.
The drainage materials generally reacted to acidic and alkaline exposure. Some polyamide and polyester based products decomposed in acidic water and got brittle in hot water (70°C) within 6 months. Polyethylene-based drainboard specimens with a weight of 1,200 g/m2 and a maximum compressive strength of 950 kN/m2 – specifically formulated and stabilized to withstand the harsh environment – were relatively unaffected by these conditions, they did show noticeable effects of aging in hot environments when submitted to compression creep load. In spite of large deformations of drainboards, they still proved well suited as the water inlet pressure during in-plane water transmissivity tests under lateral loads remained consistently low.
Minor visual changes such as loss of glance were detected on polyethylene-based drainboards after 24 months exposure to all conditions. Only small changes were detected after exposure of the specimens to micro-organisms under aerobic or anaerobic soil conditions.
The mass loss of 0.8% for polyethylene drainboards after 24 months of aging in water at 23°C was very low. After 24 months exposure to 70°C water, the polyethylene-based drainboard yielded a modest mass increase. After exposure to alkaline or acidic conditions, the specimens also showed modest mass changes.
The in-plane water transmissivity values of the drainboard specimens measured at a lateral load of 200 kN/m2 after exposure were generally not different from those measured in the as-received state.
All exposure conditions as shown in Table 1 below could be met with a specially formulated polyethylene-based drainboard.
The Gotthard Base Tunnel was a very demanding project with extreme conditions. The extensive testing program showed that standard drainage systems could not meet the Swiss AlpTransit 100-year design life criterion. Typical polymer degradation and aging processes would be accelerated by additional, more punishing physical and chemical mechanisms.
A special polyethylene-based drainboard was manufactured specifically for the Gotthard Base Tunnel project. Using stringent test procedures, samples were aged over a period of 24 months. During the aging period the specimens were submerged in acidic and alkaline solutions at 50°C and in oxygen-enriched water at 70°C and then tested again. All required product specifications needed to confine the aging properties could be met.
Peter Barrett is Product Manager and Marcus Jablonka is Vice President-R&D and Production at Cosella-Dorken Products Inc.