Design and optimisation of the lining of a tunnel in the presence of expansive clay levels

This paper describes the study made of the Trasvasur tunnels (Canary Islands, Spain), which were excavated in rocky formations of volcanic origin presenting metric levels of expansive clays. The building of these tunnels had to be abandoned 30 years ago, owing to the problems caused by the expansivity of the ground. It was subsequently decided to resume the project, making joint use of geotechnical investigation campaigns, convergence measurements and numerical simulations, thereby contributing towards the optimisation of the cross-section of the tunnelled area, support and lining. The work done has shown that when building tunnels in the presence of expansive clays it is advisable to use circular or similar cross-sections. It has also been found to be advisable to seal off the excavation as quickly as possible to prevent alteration due to the decompression of the expansive clay levels and their absorbing water from the tunnel itself.