A Mapei system has been used to protect Copthall Green Tunnel – an 880-metre tunnel being constructed by Skanska Costain STRABAG Joint Venture (SCS JV). The works are progressing well. Mapei waterproofing products and accessories were supplied by BlueBay Building Products Ltd.

Copthall Green Tunnel sits between the twin bored Northolt Tunnel and the Colne Valley Viaduct. It is one of five shallower green tunnels on the HS2 route; its ‘cut and cover’ design significantly reduced the movement of soil and approx. 1.21 million cubic metres of excavated material were kept on site for reuse within the tunnel surroundings. This ensured a greener and better visual integration of the railway within the surrounding landscape.

SCS’s role on the project was to construct a rigid concrete box structure within a 20m deep cutting, which ultimately formed the green tunnel for the HS2 trains. This was constructed using steel reinforcement and in-situ concrete to form the base, walls and roof of the structure. During the project the Kilnbridge team installed both the Mapeproof ALAP system and the Mapeproof AL 1200 AP system to the base and walls.

Throughout the project, Mapei’s UK and international teams worked closely with its partner providing compliance testing, design input and commercial and technical site support throughout all phases. Particular attention to detailing was vital, taking into account the requirements of buildability, piles, joints, continuity and compatibility at interfaces, changing environmental conditions and long design-life and maintenance periods.

The tunnel specification included Mapeproof AL AP System – a pre-applied, fully bonded, self-adhering, HDPE waterproofing membrane – selected as the primary external tanking waterproofing system to receive the poured concrete for the tunnel slab. Cold applied, and requiring no heat and/or naked flames, the membrane creates completely watertight seams, prior to the installation of steel reinforcement and poured concrete. No protective layer is required, other than temporary peel-off protection film which is removed before steel fixing. Once concrete is poured, the system is fully bonded and prevents water from migrating laterally between the foundation structure and the membrane. Post-applied self-adhering version – Mapeproof AL 1200 AP – was then lapped onto the pre-applied membrane and continued upwards for the cast tunnel and ventilation shaft walls. Mapei’s Polyfond Kit Drain – a robust, cuspated HDPE membrane laminated with a polypropylene needle-punched nonwoven geotextile – was also used to protect the membrane against backfilling work. The tunnel joints were further protected against potential water ingress with Idrostop Multi re-injectable hose system and IDROSTOP preformed hydrophilic acrylic polymer profile strips.

Mapeproof AL AP System provides a fast-track, easily handled, light and economical primary waterproofing system. The robust membrane is highly resistant to both puncturing and tearing. It can be applied underneath foundations, on formworks (pre-cast), against diaphragms, soldier pile walls, pile walls or other excavation structures.

To find out more about Mapei tunnel solutions, please visit: https://utt.mapei.com/en/home-page

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