Keywords: risk analysis, fine-grained soil, performance, tunnelling, fragile failure, reliability, finite difference models, FLAC-3D.

Conventional tunnelling, as defined by the International Tunnel Association, in cemented fine-grained materials can lead to localized ground instability, associated with low safety factors at the face, unsupported tunnel crown, and walls that may, in turn, preclude catastrophic failures. This paper presents a two-step novel approach proposed to conduct the performance evaluation of tunnels built using conventional tunnelling, which is aimed at unifying the revision of both state limits of failure and service. Initially, the relation between strength and deformation parameters is established through statistical analyses of the soil properties, and then, the bivariable point estimate method, coupled with tri-dimensional finite difference models is used to compute factors of safety, probabilities of failure, reliability indexes, and tunnel settlements at critical areas of the soil mass surrounding the tunnel. The case study corresponds to a critical section of one of the five tunnels found in a strategic 15 km long urban highway that was recently built in the north east area of Mexico City. Three-dimensional finite difference models were developed with the program. From the numerical studies, insight was gained regarding the expected behaviour of tunnels excavated using conventional tunnelling in brittle materials.

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