The Mexico City soft clay deposits are very compressible with low undrained shear strength, and they are subject to subsidence and dynamic amplification phenomena. Because of its low strength, sedimentation process, historical loads and subsidence, these clay deposits show semi vertical deep cracks. During tunneling, or excavation of shafts, these activities generate stress changes on the soil located at the front and around the tunnel or around shafts, so the existing cracks could reopen again. An adverse effect on the stability of the tunnel caused by this reopening of cracks is that it eliminates the necessity confining pressure for proper work in the tunnel lining, especially when it is formed by concrete slabs. This paper presents a historical case where this particular soil cracking was discovered during tunneling and shaft construction in the very soft clay deposits of Mexico City, especially when a water duct channel was located very near or when the tunnel was built under it. Numerical analysis of this phenomenon is presented using the finite element method. The results confirm that soil cracks are the main factor that produces tunnel support instability. In conclusion, some countermeasures are proposed to avoid and/or confront these situations of cracking soil in actual tunneling scenarios.
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