Innovative Tunnel Drainage

By Alan Bamforth

Tunnels are constructed for transport, hydro, sewers and cables by methods such as TBM (bored), drill and blast, road header or cut and cover with linings of concrete, segments or sprayed concrete (SCL/NATM). Most tunnels are intended to be waterproof, in that the construction prevents seepage of water into or out of the tunnel. Often the design also includes pressure relief of ground water behind the waterproofing.

Conventionally, the required waterproofing and pressure relief of the arch is achieved by a 700gsm non-woven geotextile layer behind a welded waterproof membrane. From there, the water is normally led into a crushed stone drainage layer and a longitudinal pipe trench to pump sumps at tunnel low points. There are known problems with this arrangement:.

  • The geotextile and perforated pipe are known to clog with precipitates in iron and calcium carbonate rich groundwater.
  • The 700gsm geotextile’s in-plane flow capacity is woefully inadequate at around 0.015l/m/s at 20kPa confining pressure (even less at higher confining pressures).
  • The crushed stone drainage layer is typically 300mm thick and requires a larger tunnel cross section.
  • Construction of the pipe trench is often a second operation on the critical path before waterproof lining can commence.

Innovation since 2000 has led to several alternative methods of tunnel drainage by making use of geosynthetics. ABG/ Halcrow/ Dumez pioneered a novel invert drainage layer first used on the Black Hills Tunnel for the MTRC in Hong Kong and subsequently in several tunnels around the world. Cavidrain Invert is a deep profile HDPE cuspated core that forms a drainage layer across the whole tunnel invert, replacing the crushed stone and the pipe trench. With a typical thickness of 40mm or 60mm, Cavidrain Invert reduces excavation and can be installed in conjunction with the waterproofing installation, leading to a quicker construction program. The concrete slab is cast into the Cavidrain Invert which then transfers the design pressure onto the formation. ABG manufactures a range of Cavidrain Invert products and will input to the technical design/detailing and also tailor the product to suit the project.

Cavidrain Invert Drainage geocomposite - Tunnel Drainage

Cavidrain Invert Drainage geocomposite

Tunnel wall water pressure relief is also innovating away from the conventional 700gsm geotextile, with the BTS specification indicating that the drainage function must be designed and that geosynthetic drainage layers may be considered. ABG manufactures Cavidrain Protector for this application. This is a cuspated geosynthetic in 400, 500 or 700gsm HDPE that has in-plane flow capacity of up to 2.5l/m/s at 250kPa long term confining pressure. Cavidrain Protector is installed in the same way as a geotextile, however, it provides greater than 100 times more in-plane flow capacity and a partial secondary liner function.

ABG Cavidrain Protector - Innovative tunnel drainage

ABG Cavidrain Protector

Even more innovative is ABG Cavidrain Liner which combines the waterproofing and water pressure relief functions in one product. Cavidrain Liner is an LLDPE, flexible, cuspated sheet that is welded using conventional machines, into a fully waterproof lining system. Benefits are the speed of installation of one layer instead of two and that LLDPE is difficult to ignite, resists flame spread and does not produce toxic flames.

Cavidrain Liner Tunnel Drainage composite

Cavidrain Liner Tunnel Drainage composite

Where cut and cover tunnels are constructed the waterproofing is usually applied externally and here, ABG Deckdrain provides a robust drainage and protection layer. Deckdrain drainage geocomposite has a cuspated core bonded to a geotextile filter compatible with the soil backfill. Accordingly the in-plane flow is tested with Soft Platens to EN ISO 12958 and ABG has pioneered the application of the Stepped Isothermal method (SIM) to the determination of the compressive creep of geocomposite drainage so as to demonstrate long term performance over a 120 year design life. Deckdrain has been granted a BBA Certificate for this application.

ABG Deckdrain Cut and Cover tunnel application

ABG Deckdrain Cut and Cover tunnel application

Though successfully and advantageously used in numerous prestigious projects, not all of these innovations are widely known or understood. Our technical team of ABG engineers is eager to assist you to maximise the benefits within your potential new projects.

Tunnel Projects using innovative geosynthetics

Tunnel Invert Drainage, Brisbane Australia – Cavidrain Invert

Tunnel Invert Drainage, Graz, Austria – Deckdrain

Tunnel Invert Drainage, Black Hills Tunnels, Hong Kong – Cavidrain Invert


Additional information