Study of the unexpected collapse of the Ampurdán tunnel (Spain) using a finite elements model

This paper presents the case study of the Ampurdán tunnel that suffered an unexpected partial collapse during construction due to the weathering of the claystone groundmass after excavation and wetting by infiltration water. To overcome the problems encountered, a finite elements model was used to understand the behaviour of the tunnel and surrounding ground to determine the geotechnical properties that lead to failure, allowing engineers to choose suitable procedures for the construction of the tunnel. The parametric study performed simulated the deformations measured in situ and related to the tunnel collapse. The geotechnical parameters used for the weathered claystone, when compared with the intact portion, correspond to a wide range of reductions between 8 % in the apparent density and in the effective friction angle, up to 40 % in the effective cohesion and 56 % in the Young modulus.