A basic engineering decision to be made in designing a tunnel in soft clay is whether or not the tunnel can be excavated without internal support. The safety of constructing a shallow heading in soft clay can be assessed in terms of the fluid support pressure which may be required to maintain stability. This can be estimated by means of the lower and upper bound theorems of plasticity. The Paper considers three different shapes of shallow underground opening relevant to tunnelling and upper and lower bound stability solutions are derived for collapse under undrained conditions. Solutions are also derived for assessment of the risk of blow-out failure caused by excessively high fluid pressures. Conditions are considered under which local collapse can occur, independent of the cover above the tunnel. Une décision fondamentale doit être prise lorsque l'on veut réaliser un tunnel dans de l'argile molle; il s'agit, en effet, de décider si un support interne sera ou non nécessaire pour l'excavation du tunnel. La sécurité de la construction d'une galerie peu profonde dans de l'argile molle peut être évaluée en fonction de la pression du fluide support qui pourrait être nécessaire au maintien de la stabilité. Cette estimation peut se faire à l'aide des théorèmes à limites inférieure et supérieure. L'article envisage trois formes différentes d'ouverture souterraine peu profonde intéressant le creusement de tunnels, et des solutions de stabilité à limites inférieure et supérieure sont trouvées pour l'affaissement dans des conditions non drainées. Des solutions sont également trouvées en ce qui concerne l'évaluation du risque de rupture par éruption due aux pressions excessives du fluide. Les conditions dans lesquelles un affaissement local peut se produire sont envisagées, indépendamment de la couverture située audessus du tunnel.

A method is presented for estimating the maximum bending moment for continuous ͑or rigidly jointed͒ pipelines affected by tunnel-induced ground movement. The estimation can be made based on the knowledge of tunnel and pipeline geometries, the stiffness of soil and pipeline, and tunnel-induced ground deformation at the pipeline level. The method takes account of soil nonlinearity by an equivalent linear approach, in which the stiffness of the soil is evaluated based on an average deviatoric strain developed along the pipeline. The approach is conservative and promises that the bending moment is not underestimated. The validity of the method as an upper bound approximation is evaluated against centrifuge test results.

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This paper examines the effect that tunnel size, depth and volume loss have on greenfield soil displacements above tunnels in sandy ground. The results of a series of plane-strain centrifuge tests performed on tunnels in a dry silica sand are examined. The cover-to-diameter ratio, C/D, of the tunnels ranged from 1·3 to 4·4. Features of greenfield settlement trough shape, both surface and subsurface, are illustrated by examining soil displacement data obtained using an image-based deformation measurement technique. The effects of tunnel size, depth and volume loss are demonstrated, and the suitability of typical fitting curves is discussed. The complex volumetric behaviour of drained soil is illustrated by comparing tunnel volume loss with the volume loss experienced by the soil. A set of equations is developed that provide a method of evaluating the change of settlement trough shape with tunnel size, depth and volume loss.

An elastic continuum solution and a Winkler solution of the problem of tunnelling effects on existing pipelines are given. A comparison is made between an elastic continuum solution and a closed-form Winkler solution with Vesic subgrade modulus. Although applying the Vesic expression results in the same moments and displacements under external loading in a Winkler system and the elastic continuum, it is found that its use is not necessarily adequate for the problem of tunnelling effects on pipelines and may not be conservative owing to possible underestimation of bending moments. An alternative expression for the subgrade modulus is provided, resulting in similar maximum bending moments in the Winkler and elastic continuum systems.