The paper aims at clarifying the up-to-now misappreciated role taken by pore water on the design of deep tunnels in saturated porous media. Numerical simulations with a non-dimensional FEM-model emphasise the influence of pore water characteristics (in situ pressure and compressibility) and boundary conditions (lining permeability) on the short and long term underground-structure equilibrium states. These results cannot be achieved by means of monophasic total stress analyses.

Dieser Beitrag untersucht den Einfluss des Porenwassers auf die Dimensionierung von tiefen Tunnelbauwerken in wassergesattigtem Untergrund. Numerische Simulationen mittels eines dimensionslosen FEM-Modells betonen den Einfluss der Porenwassereigenschaften (in-situ Druck und Kompressibilitat) und der hydraulischen Randbedingungen (Tunnelausbaudurchlassigkeit) auf Kurz- und Langzeitverhalten des Untergrund-Ausbau Gleichgewichts. Diese Ergebnisse können nicht ermittelt werden anhand von Einphasen- und Totalspannungsmodellen.

L'article s'interesse au rôle meconnu de l'eau interstitielle dans le dimensionnement de tunnels profonds creuses dans des milieux poreux satures. Des calculs adimensionnels par elements finis soulignent l'influence des caracteristiques de l'eau interstitielle (pression in situ et compressibilite) ainsi que des conditions aux frontières (permeabilite du revêtement) sur les equilibres court et long terme de l'ouvrage. Ces resultats ne peuvent pas être obtenus par des calculs en contraintes totales.

Design of deep galleries

In low permeable porous media, the excavation of galleries leads to both mechanical and hydraulic disturbances in their surroundings, namely a de-stressed damaged zone and a significant drop in pore water pressures. This phenomenon of instantaneous pore pressure decrease has already been observed in centrifuge tests (Mair, 1979) and around openings in low permeable formations (e.g. Opalinus stone, London clay). It is brought about by hydro-mechanical coupling in the saturated media.

Furthermore, during and after the excavation stage, fluid flow occurs, causing time-dependent changes for both the structure and the medium: variation of the total pressure acting on the lining and redistribution of stresses and displacements in the medium (Labiouse & Garber 2001).

To date, the design of underground structures is generally carried out by means of total stress analyses of monophasic media. Although their domain of use should be restricted to dry or compact rock masses, their application has often been extended to saturated media. Now, for several reasons, it may be stated that finite element calculations and analytical solutions available for monophasic media could become inappropriate to calculate the stability of galleries driven in saturated porous media. Indeed, these methods:

  • consider the solid skeleton and the pore fluid as one phase and do not differentiate between the respective contributions of these two components.

  • do not account for the in situ pore water pressure.

  • do not yield information concerning the decrease in pore pressure generated during the excavation of tunnels as a result of hydro-mechanical coupling. When this is the case (Randolph & Wroth 1979, Mair & al 1992), the pore fluid is considered to be incompressible and the decrease in pore pressure is calculated based on the variation in mean total stress.

  • do not provide any information about the fluid flow.

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