http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12347 In spite of the development of more sophisticated constitutive models for soil, the Mohr-Coulomb yield criterion remains a popular choice for geotechnical analysis due to its simplicity and ease of use by practising engineers. The implementation of the criterion in finite element programs, however, presents some numerical difficulties due to the gradient discontinuities which occur at both the edges and the tip of the hexagonal yield surface pyramid. Furthermore, some implicit techniques utilising consistent tangent stiffness formulations are unable to achieve full quadratic convergence as the yield criteria is not C2 continuous. This paper extends the previous work of Abbo and Sloan (1995) through the introduction of C2 continuous rounding of the Mohr-Coulomb yield surface in the octahedral plane. This approximation, when combined with the hyperbolic approximation in the meridional plane (Abbo and Sloan, 1995), describes a yield surface that is C2 continuous at all stress states. The new smooth yield surface can be made to approximate the Mohr–Coulomb yield function as closely as required by adjusting only two parameters, and is suitable for consistent tangent stiffness formulations. 2013-01-08T03:21:59.079Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8807 The ultimate bearing capacity and the failure mechanism of cohesive-frictional soils with inclusion of shallow tunnels have been theoretically and numerically investigated assuming plain strain conditions. Despite the importance of the stability of foundations above such tunnels/openings, research on this subject has been very scarce. At present, no generally accepted design or analysis method is available to evaluate the ultimate bearing capacity of cohesive-frictional soils with shallow tunnel inclusions. In this study a continuous loading is applied to the ground surface, and the smooth and rough interface conditions between the loading and soils are modelled. For a series of tunnel geometries, shapes and material properties, rigorous upper and lower bound solutions for the ultimate bearing capacity of considered soil mass are obtained by applying recently developed numerical limit analysis techniques [3], [5]. For practical suitability the results are presented in the form of dimensionless stability charts, with the actual bearing capacities being closely bracketed from above and below. As an additional check and also a handy practical means, the upper bound rigid block mechanisms for circular tunnels have been developed and the obtained values of collapse loads were compared with the results of numerical limit analysis. 2011-09-02T04:00:06.068Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8806 This paper considers the undrained stability of a circular tunnel in soft ground where the strength increases linearly with depth. Solutions are obtained which enable designers to compute the tunnel pressure that is needed to maintain stability of an unsupported heading during the construction process. These solutions take account of the ground surcharge, the tunnel geometry, the soil unit weight, a non-homogeneous undrained shear strength, and are expressed in dimensionless form. The numerical results have been obtained using finite element formulations of the upper and lower bound theorems of limit analysis. By obtaining upper and lower bound estimates on the tunnel pressure, the actual tunnel pressure required to maintain stability can be bracketed from above and below. This provides an inbuilt error indicator for the accuracy of the solutions. Some upper bounds are also presented from the use of simple rigid block mechanisms. These mechanisms are shown to give accurate upper bounds in some cases. Where possible, the theoretical predictions are compared against experimental data obtained from centrifuge and laboratory tests. 2011-09-01T05:20:03.678Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:6118 The design of tunnels for roads and railways often utilise separate tunnels to carry traffic in opposite directions. In this paper numerical limit analysis and semi-analytical rigid block techniques are used to investigate the effect of inter-shaft distance on the stability of two square tunnels constructed side by side. The tunnels are modelled under conditions of plain strain assuming that the tunnels are infinitely long. Bounds on the stability of the tunnels are obtained using finite element limit analysis, the numerical formulation of which is based upon the upper and lower bounds theorems of classical plasticity. Solutions are obtained using advanced conic programming schemes to solve the resulting optimisation problems. Upper and lower bound estimates on the stability of the tunnels are obtained for a range of geometries. These bounds, which bracket the true collapse load from above and below, are found to differ by at most 5%. Results from this study are summarised in stability charts for use by practising engineers. 2010-05-13T01:00:01.840Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:6207 Yield surfaces for unsaturated soils are inevitably non-convex if the size of the yield surface has to increase with increasing suction. An expanding yield surface with increasing suction is crucial for modelling the volume collapse due to wetting. The non-convexity always exists at the transition between saturated and unsaturated states, irrespective of the stress variables used in the model. Some recent models for unsaturated soils also possess a stress path dependent hardening law. The non-convexity and stress-path dependency of the constitutive model make the implementation into finite element codes very challenging. This paper discusses aspects of stress integration schemes for non-convex and stress-path dependent models for unsaturated soils. 2010-05-10T02:00:11.831Z ]]>