Safeguarding tunnels to achieve design lives of 100 years or more can be challenging as materials used must be durable enough to cope with conditions both above and below the ground.
Reinforced concrete is often used in tunnels and while a high-level guidance on service life and durability for road tunnels exists in Australia, tunnelling for transport structures is not covered by a single specific national design code. A combination of codes are required when defining durability of individual concrete elements used within underground structures.
Rob Kilgour, WSP Principal Engineer for Materials Technology, explores the balancing act required in defining requirements for tunnel material durability performance to help extend the life of tunnel assets.
He says, “Reinforced concrete is the key material used for lining of tunnel bores. These must be able to resist water ingress and potentially aggressive ground conditions along with the effects of the internal tunnel environment.
“Internal environments might include high concentrations of CO2 along with humidity and temperatures dependant on the volume of traffic. The buried environments typically encountered in projects across Australia include sand, sandstones, weathered clays, riverine alluvium, marine sediments and hard rock. These environments can expose concrete to harmful conditions and compromise design.
“Defining and measuring the durability performance requirements for concrete exposed to both these conditions requires balancing a number of conditions. The design engineer must assess various scenarios as the material needs to be resistant to aggressive conditions, but there may also be difficulty with access for future maintenance.
“While there are shortcomings in the various codes used as the basis for design of tunnels across Australia, design teams have access to a broad suite of design tools and performance testing and verification methods that can be leveraged to define and validate minimum durability performance requirements.
“Technologies such as SFRC provide design teams with the opportunity to optimise designs by significantly reducing the likelihood of reinforcement corrosion.”
Rob Kilgour, WSP Principal Engineer – Materials Technology, presented on the Durability of Concrete in Tunnels at Corrosion and Prevention 2018 in Adelaide on 12 November. The paper was co-authored by his wife, Udaya Sathiamurthy, WSP Senior Engineer – Materials Technology. For more information, read the abstract here.
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