ICE - Institution of Civil Engineers - Virtual Library

A new theoretical model describing the total pressure gradient and torque in a tunnelling machine screw conveyor is proposed. The theory develops new equations relating the pressure gradient and torque to the screw geometry, the shear stresses acting in the screw conveyor, and the material flow. The equations are expressed in dimensionless form, allowing application to screw conveyors of different scales. The theory is validated against measurements from laboratory model screw conveyor tests with clay soils with varying properties and operating conditions. The theory successfully describes key features of the observed mechanics of the model conveyor operation, and accurately predicts the test measurements. The proposed theory provides insight into the fundamental mechanics of screw conveyor operations, and can be used in the design of tunnelling machine screw conveyors. The theory can also be applied to other forms of screw conveyors and extruders operating with plastic materials.

Motivated by the need to explain the large displacement and rotation that numerous caisson-type quay walls suffered in the port of Kobe during the devastating 1995 earthquake, a detailed numerical analysis is presented for the response of such a wall from Rokko Island. Utilising the Pastor–Zienkiewicz elastoplastic constitutive model, an effective stress dynamic analysis is performed using as input the accelerogram recorded 32 m below the ground surface in the nearby Port Island. The evolution during shaking of lateral displacements, plastic strains and pore water pressures sheds some light on the complex interplay of several simultaneously occurring phenomena: the development of oscillatory inertia forces on the wall, in phase or out of phase with the backfill soil and water pressures; the simple-shear seismic deformation of the soil and the ensuing initial development of positive excess pore water pressures in the backfill and the foundation soil; the extensional deformation developing in the ‘active wedge’ behind the wall, with the ensuing generation of negative excess pore water pressures; and the continuous dissipation and redistribution of water pressures. The conventional generalised Mononobe–Okabe theory is also reviewed, and extensive comparisons are made with the numerically computed effective and water pressures against the wall. Finally, a surprising role of the relative density of rubble behind the wall is highlighted.

Comprehensive tests were performed to obtain the stress–strain and stiffness characteristics of a fine uniform sand in a hollow cylinder apparatus under monotonic and cyclic torsional loading. Unstable response with a sudden reduction in shear strength after peak was observed under undrained monotonic loading. After a quasi-steady state the specimens showed a phase transformation and a tendency to dilation. Drained torsional hollow cylinder tests showed an equivalent characteristic state marking the transition between contractant and dilatant behaviour and the same line at phase transformation as their undrained counterparts. The instability line, defined under monotonic loading, appears to form a boundary to the behaviour of sand under cyclic loading. Unstable behaviour is introduced when the effective stress path followed by the specimen during cyclic loading reaches the instability line at a shear strain of approximately 0·5%. The first loading cycle is at variance with the following cycles in terms of the stress–dilatancy relationship in drained or the excess pore water pressure generation in undrained torsional hollow cylinder tests. The liquefaction, stiffness and damping characteristics of the sand have been defined.

This paper proposes a new isotropic yield and failure criterion for geomaterials under general stress conditions. The new criterion is a function of the three stress invariants and the paper shows the well known criteria by Matsuoka & Nakai (1974) and Lade & Duncan (1975) are included in the new formulation. In order to include different features characterising soil and rock behaviour, the obtained function is successively modified to account for tensile strength and curvature in meridian planes. The influence of each parameter is discussed and finally the comparison between the new criterion and some experimental data on soils and rocks is shown. Finally, the domain of the new criterion is analysed in order to show if the yield function takes negative values also outside the shown elastic domain.

A new constitutive model is developed for the compressibility behaviour of soft clay sediments at low effective stress level. Central to this model is the realisation that at low effective stress levels the void ratio–effective stress paths of soil elements are non-unique, and dependent very much on their state. Beyond a threshold stress level, these relationships become unique. These observations have been used to develop a state-dependent constitutive model for the compressibility behaviour of soft sediments. Methods to obtain the input parameters of the model are discussed. The proposed model is used to solve finite-strain one-dimensional consolidation with pertinent initial and boundary conditions. The analyses performed using a finite difference computer code predict better the observed phenomena associated with self-weight consolidation, especially during its early stages. The comparison of the predictions shows that the present model can capture the decrease in void ratio patterns, as observed in experiments at low effective stresses, and which cannot be modelled using existing constitutive relationships.

Tunnelling in urban areas can damage the existing buildings and underground services, as a consequence of ground movements induced by excavation. In this paper the potential of a vertical diaphragm wall to modify the displacement field induced by tunnel excavation is investigated. A series of centrifuge tests were performed in which a diaphragm wall was embedded to one side of a tunnel. The location of the wall, its length, thickness and the roughness of the wall–soil interface were varied. In addition, a series of numerical analyses were carried out to supplement the experimental data. It was observed that the field of displacements was modified in different ways, depending on the combination of the above factors, and the sensitivity of the problem to the variation of each factor is discussed. An efficiency parameter is defined that expresses the ability of the wall to modify the reference (no wall) profile of settlements.