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Título

INFLUENCE OF ISOTROPIC COMPRESSION STIFFNESS IN SHALLOW TUNNEL DISPLACEMENTS

Resumo

Tunnel construction in highly compressible soils, such as porous clays, may induce large ground deformations, potentially damaging buildings and existing infrastructure. Typically, these soils exhibit significant volumetric contraction, leading to a complex distribution of displacements around the tunnel. Notable examples of tunnels in porous clays are the Metro tunnels in Brasilia and the Paraíso tunnel in São Paulo. Given the complexity of the problem, the use of numerical modeling with sophisticated constitutive models becomes imperative. The Hardening Soil Model, a well-known constitutive model for soils, accurately reproduces the behavior of various soil types. Its parameters can be easily derived from laboratory tests or estimated from in-situ testing. For this reason, this constitutive model is widely used in Geotechnical Engineering practice. In this paper, the Paraiso tunnel is analyzed using a 2D plane strain Finite Element Method (FEM) model using the tunnel volume loss control technique. The Hardening Soil Model was calibrated using compression and extension triaxial tests on undisturbed samples of the residual porous clays of São Paulo. A good match was found between the numerical model results and field data when the tunnel volume loss equals 1.25%. Then, a sensitivity analysis was conducted to assess the influence of the 1D-compression stiffness, Eoed, on the ground deformations observed. The Eoed parameter is related to the isotropic compression stiffness. The results demonstrate that larger displacements at the ground surface, compared to those at depth, occur when the 1D-compression stiffness (Eoed) is significantly smaller than the secant stiffness at 50% of failure, E50. This is a consequence of the pronounced increase in volumetric contractive strains near the ground surface above the tunnel as Eoed decreases. Observing larger settlements at the surface than at depth is an unusual behavior commonly seen in tunnels excavated in porous clays. Thus, the results presented in this paper suggest that the Hardening Soil Model can represent this unusual behavior by selecting an Eoed that is sufficiently smaller than E50.

Palavras-chave

Shallow tunnel, Hardening Soil Model, Numerical modeling, Compressible soils, Porous soils.

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