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  Performance based design of tunnels built in cemented fine-grained stiff soils

  The analysis of tunnel excavations carried out in densely populated cities requires the use of tools that adequately model the soil behaviour, as well as different structures, geometries and construction processes, to estimate more accurately the settlements and deformations of the tunnel that could generate affectations in structures surrounding at the excavation. Modern tunnel design and construction in heavily populated urban areas requires a proper definition of State Limits of Failure, SLF, and Deformation, SLD, to ensure both the best solution from the economic standpoint, and to avoid costly failures during construction. Special attention must be taken in cemented fine-grained materials, such as those found in the northeast portion of Mexico City, due to the potential of fragile failure that these soils might exhibit. This paper presents a methodology novel approach aimed at unifying the revision of state limits of failure and service through Performance Based Design, PBD, where the risk associated with the determination of geotechnical parameters was evaluated. In practice, this method can be used to quantify tunnel risk, within a risk analysis framework. Initially, the parameter E was estimated for this study using the equation, which was determined by a multiple linear regression between mechanical strength parameters and the deformability parameter, E50, compiled from previous research works projects that were developed in the North West area of Mexico City. Then, the Bivariable Point Estimate Method, BPEM, coupled with tridimensional finite models is used to establish the factor of safety distribution, probability of failure, and tunnel settlements at the critical zones around the tunnel. Finally, the results gathered in this study were compared with the displacements obtained from an instrumentation campaign of a tunnel built in cemented fine-grained materials. The case study corresponds to a critical section of a tunnel located at the north west of Mexico City. Tridimensional finite difference models were developed with the program FLAC3D.

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