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Foam conditioning is a widely adopted technique in earth pressure balance (EPB) shield tunneling for the purpose of reducing sand permeability and preventing water spewing. The permeability of foam-conditioned sands differs from that of natural sands due to the presence of foam bubbles. This study investigated the effect of water pressure on the permeability of foam-conditioned sands using novel laboratory permeability tests. The water pressure, for the first time, is decoupled with the hydraulic gradient, owing to a newly developed permeameter with the controllable downstream hydraulic pressure in the laboratory. The results show that the permeability is significantly affected by the water pressure, and the effect is also predominantly dependent upon the foam injection ratio. The initial hydraulic conductivity increases with the increasing water pressure, while the initial stable period duration decreases. The water-plugging structure formed by foam bubbles and sand particles is prone to be damaged under high water pressure due to the shrinkage of foam bubbles. This means that the existing permeability tests with low water pressure underestimate the permeability of foam-conditioned sands. The underlying mechanism of water pressure in modifying the permeability of foam-conditioned sands is also examined from a particle-scale perspective. [ABSTRACT FROM AUTHOR]

The stabilities and associated collapse mechanisms of deep buried unlined elliptical tunnels in cohesive-frictional (c-[phi]) soils with the action of soil weight are investigated by the 'upper-bound finite element method with rigid translatory moving elements' (UBFEM-RTME). The soil masses are assumed to obey the Mohr-Coulomb yield criterion and a nonassociated flow rule. Upper-bound stability coefficients ([[gamma].sub.cr] D/c, where [[gamma].sub.cr] is critical unit weight; D is tunnel height; c is cohesion) are deduced for different values of friction angles ([phi]), dilatancy coefficients ([psi]/[phi] where [psi] is dilation angle), and dimensionless spans (B/D, where B is span). The obtained collapse mechanisms do not extend to the ground surface and are primarily composed of a series of mutually movable rigid blocks. The [[gamma].sub.cr] D/c values increase while the collapse zones decrease with an increasing [phi] and [psi]/[phi] and a decreasing B/D. Key words: deep buried tunnel, upper-bound finite element method, stability coefficient, collapse mechanism, rigid blocks. Les stabilites et les mecanismes d'effondrement associes des tunnels elliptiques non doubles profonds enfouis dans des sols c-[phi] avec l'action du poids de sol sont etudies par ' methode elements finis superieurs avec elements mobiles translatoires rigides (UBFEM-RTME) '. Les masses de sols sont censees obeir aux criteres de rendement de Mohr-Coulomb et une regle d'ecoulement non associe. Les coefficients de stabilite limite superieure ([[gamma].sub.cr]D/c, oU [[gamma].sub.cr] est le poids unitaire critique; D est la hauteur du tunnel; c est la cohesion) sont deduites pour differentes valeurs d'angles de frottement de coefficients de dilatance [psi]/[phi] ou [psi] est l'angle de dilatation) et de portees sans dimension (B/D, ou B est la portee). Les mecanismes d'effondrement obtenus ne s'etendent pas a la surface du sol et sont principalement composes par une serie de blocs rigides amovibles mutuels. Les valeurs [[gamma].sub.cr]D/c augmentent lorsque les zones d'effondrement diminuent avec une croissante [phi] et [psi]/[phi] et une baisse de B/D. [Traduit par la Redaction] Mots-cles : tunnel enfoui profondement, methode des elements finis a limite superieure, coefficient de stabilite, mecanisme d'effondrement, blocs rigides., Introduction The stability of a single tunnel has attracted considerable attention from scholars. With usage of limit analysis in conjunction with the associated flow rule, stability charts have been established [...]

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