Keywords: tunnel, instrumentation, settlements, numerical study, constitutive models.

Construction of shallow tunnels in urban zones induces ground settlements that can cause differential displacement of nearby structures. In this paper, a numerical study of the static performance of a tunnel built in very soft clay, which exhibits high compressibility and low shear strength, is presented. The data gathered from the instrumentation of a 4 m diameter sewer tunnel was used in this study. This tunnel is part of the sewer system that serves the southern region of Mexico City, and was built twenty-seven years ago in the so-called Lake zone, known by its difficult subsoil conditions, using the slurry shield tunnelling technique. Ground movements induced by the shield tunnelling throughout the soft clay were monitored by a number of deep and sallow settlement points and inclinometers. A cone penetration test was conducted at the site to obtain a distribution of the undrained shear strength with depth. In addition, the soil properties were also established from typical triaxial compression, triaxial extension and consolidation tests. A parametric study was conducted using several constitutive laws, including Mohr-Coulomb, Drucker-Prager, modified Cam clay, and hyperbolic models, to asses their prediction capabilities. Two and three-dimensional finite difference models of the tunnel were developed. The vertical and horizontal displacements due to tunnelling were computed and compared with the data obtained from settlement points and inclinometers. The computed soil movements were in good agreement with the measured response. From the numerical study, insight was gained regarding the static performance of this type of underground structure. An attempt was made to simulate as close as possible the excavation sequence, including potential over excavation (the void between the ground and the shield), gapping (the tailpiece void between soil and liner), and the grouting process behind the shield tail. Due to lack of information, the weight of the shield was not included in the simulation; however its effects in the final displacements field appear to be secondary.

The results gathered from the numerical analyses show that even very simple constitutive laws can provided reasonable good results when proper laboratory tests are available to obtain the model parameters according with the stress paths (i.e. triaxial extension and compression tests, and compressibility curves for undisturbed and remoulded clay), that occur during tunnel excavation. In fact, the settlements trends computed with the Mohr-Coulomb and modified Cam clay constitutive laws considered were very similar. However, the three-dimensional finite difference model, coupled with the modified Cam clay provided the best estimation of the displacement field. The discrepancies observed between the predictions and the data gathered during the instrumentation can be attributed to the soil heterogeneities, possible deficiencies on the construction process, and surface load irregularity due to near by structures.

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