Papers by James Hambleton
Computers and Geotechnics, 2016
Computers and Geotechnics
Computer Methods and Recent Advances in Geomechanics, 2014
Flexible rockfall protection barriers are used ubiquitously to safeguard people and infrastructur... more Flexible rockfall protection barriers are used ubiquitously to safeguard people and infrastructure against falling rock fragments along weak or fractured slopes. Performance of these barriers is often quantified in terms of the level of impact (kinetic) energy that can be withstood, referred to here as the "critical energy." As pointed out in recent papers, however, there is no single representative value of critical energy for a given barrier. Instead, critical energy decreases as the block size decreases, a phenomenon referred to as the "bullet effect." With a view towards explaining and predicting the bullet effect, the paper presents a simple analytical model for perforation of a flexible barrier caused by normal impact. The model rests on a two-dimensional idealization of the full three-dimensional impact problem, as well as a balance of energy between the initial kinetic energy of the block and the energy absorbed by the barrier. The model predicts a strong dependence of the critical energy on the block size, and the predicted trend agrees well with data available in the literature. Ramifications for practical applications and possible future refinements to the model are also discussed.
Anchors are widely used in foundation systems for structures requiring uplift resistance. As demo... more Anchors are widely used in foundation systems for structures requiring uplift resistance. As demonstrated by numerous theoretical and experimental studies on the subject, uncertainty remains as to both the theoretical uplift capacity of anchors in idealised soils and the suitability of the various modelling assumptions in capturing the responses observed during tests. This study, which deals exclusively with the theoretical uplift capacity, presents newly obtained predictions of uplift capacities and the corresponding collapse mechanisms for inclined strip anchors in sand. The analysis is based on the upper bound (kinematic) method of limit analysis and the so-called block set mechanism, in which a collapse mechanism consisting of sliding rigid blocks is optimised with respect to interior angles and edges of the blocks. It is demonstrated that the method provides lower (better) estimates of uplift capacity in some cases compared to previous upper bound calculations. Also, variations in the predicted collapse mechanism with changes in embedment and inclination are assessed in detail.
The paper presents a novel approach for lower bound limit analysis based on the Control Volume Fi... more The paper presents a novel approach for lower bound limit analysis based on the Control Volume Finite Element Method. The central concept of the solution procedure is a force balance on control volumes corresponding to nodes, where tractions on control volume faces are evaluated using linear interpolation of the unknown stress components at nodal points. An optimization routine incorporating second-order cone programming is subsequently employed to fi nd the stress fi eld that maximizes applied load subject to constraints imposed by equilibrium, boundary conditions, and the yield condition, thereby fi nding a load that is closest to the true collapse load. The formulation is for plain strain (i.e. two dimensions) and material obeying the Mohr-Coulomb yield condition. The proposed approach is applied to the benchmark problem of a uniform strip load applied to a half space, and very good agreement between analytical and numerical results is found.
Wollongong. With a forecast investment of over 250 billion dollars in Australia's energy and tran... more Wollongong. With a forecast investment of over 250 billion dollars in Australia's energy and transport infrastructure over the next five years, there is an unprecedented need to design and build this infrastructure as cheaply and safely as possible. In light of the size of investment involved, even small percentage savings resulting from scientific research will lead to huge returns in absolute dollar terms. Through advanced laboratory testing, physical modelling, full-scale field testing and cutting-edge computational simulations, the CGSE is providing engineers with new science-based tools for designing safer and cheaper energy and transport infrastructure such as roads, railways, port facilities, tunnels, dams, pipelines, mining operations and offshore oil and gas facilities. The CGSE has four geotechnical science themes, each of which is linked to advanced computational modelling, state-of-the-art physical modelling and laboratory testing,
Numerical methods have the potential to assist in prediction of subsidence, particularly in new m... more Numerical methods have the potential to assist in prediction of subsidence, particularly in new mining environments such as multi-seam mining. However, numerical methods have often been criticised for the predictions of subsidence above single-seam longwalls not matching field measurements well. To achieve a realistic prediction, a sound understanding of the mechanical laws governing the deformation of the subsurface strata is required. Within the framework of the finite element method (FEM), this study compares predicted subsidence profiles, as well as stress distributions in the sub-surface strata, for a selection of the different constitutive laws currently used by practising engineers to represent coal measure strata. Further, a strain-stiffening caved goaf material is introduced into the simulations to allow the vertical stresses along the longwall floor to return to the original overburden load. Both single seam and multi-seam longwall panels are considered. A key finding is that the best agreement between predictions and field observations of subsidence is when the coal measure strata are represented as an elastic material with closely spaced frictionless horizontal interfaces, representing bedding planes. With this model the vertical stress also returns to the overburden stress in the caved goaf material within the first seam, prior to extraction of the second seam. The results show that more sophisticated and numerically taxing constitutive laws do not necessarily lead to more accurate predictions of subsidence when compared to field measurements. The advantages and limitations of using each particular constitutive law considered in the study are presented.
In order to effectively predict the roof collapse of underground openings using continuum models,... more In order to effectively predict the roof collapse of underground openings using continuum models, it is imperative that a realistic failure criterion is used to represent the rock mass. However, for coal mining operations there is still uncertainty as to the appropriate failure criterion that should be used for the coal measure strata. This study compares stability numbers and collapse mechanisms for a rectangular underground opening predicted based on three commonly used failure criteria: the standard Mohr-Coulomb, Mohr-Coulomb with a tension cutoff, and Hoek-Brown criteria. Three methods of analysis are considered: an analytical upper bound method, an upper and lower bound finite element formulation, and the displacement finite element method. The theoretical results are also compared with field measurements of subsidence above longwall coal mining. For the standard Mohr-Coulomb failure criterion, the friction angle governs the shape of the collapse mechanism, and the so-called critical width (the cavity width above which failures extends to the ground surface) ascertained from field observations in the New South Wales coalfields corresponds to a friction angle of approximately 30 degrees. On the other hand, predictions obtained based on the Hoek-Brown failure criterion consistently overestimate the critical width. The results also show that tensile failure governs the stability of the cavity and shape of the collapse mechanism when a tension cutoff is introduced for the Mohr-Coulomb yield criterion.
Helical anchors, which are mostly used to resist uplift, are deep foundations installed by rotati... more Helical anchors, which are mostly used to resist uplift, are deep foundations installed by rotation into the ground. Despite the central role of the installation process, especially with respect to the effect of soil disturbance, relatively little is known about the forces and deformations occurring during installation. An exception is the field verification technique known as torque-capacity correlation, which attempts to relate installation torque directly to uplift capacity. However, there are open questions regarding this approach, since not all significant parameters, such as installation vertical force and helix pitch, are taken into consideration. This could be one of the main reasons behind the wide range of torque-correlation factors reported in the literature. This study presents a three-dimensional numerical analysis of the installation process for helical anchors in clay. The results are synthesised into convenient yield envelopes that predict the relationship between installation torque and normal force as functions of helix pitch, roughness, and thickness. The application of the findings to torque-capacity correlation is also discussed.
Installation forces play a central role in the design and performance of helical piles, especiall... more Installation forces play a central role in the design and performance of helical piles, especially since the installation torque is often used as an indicator of the pile's ultimate capacity. This paper presents an analytical model for predicting the installation torque for single-helix piles in clay. As an extension of a recent study by the authors, the proposed model considers not only the forces occurring on the helical plates but also the shear stresses generated along the shaft, both of which impact the installation forces. The model yields a straightforward expression that relates installation torque to the undrained shear strength of the soil, embedment depth, helix diameter and pitch, shaft diameter, crowd (axial) force, and adhesion coefficient along the shaft. The influence of these factors on the installation torque, as well as the "capacity-to-torque ratio" used to infer capacity from the installation, is assessed through a sensitivity analysis. Some level of validation is provided through a comparison with empirical capacity-to-torque ratios, and the sensitivity analysis reveals factors that are neglected in empirical models but nevertheless have a significant influence.
In spite of the development of more sophisticated constitutive models for soil, the Mohr-Coulomb ... more 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.
Rock Mechanics and Rock Engineering, 2012
The so-called ''bullet effect'' refers to the perforation of a rockfall protection mesh by impact... more The so-called ''bullet effect'' refers to the perforation of a rockfall protection mesh by impact of a small block, which has a kinetic energy lower than the design value, where the design value is determined through tests with relatively large blocks. Despite playing a key role in the overall performance of a flexible rockfall barrier, this phenomenon is still poorly understood at present. An innovative approach for quantitatively characterizing this effect based on dimensional analysis is proposed in this paper. The analysis rests on a hypothesis that the relevant variables in the impact problem can be combined into three strongly correlated dimensionless parameters. The relationship between these dimensionless parameters (i.e., the scaling relationship) is subsequently investigated and validated by means of data generated with a finite element model. The validation process shows that the dimensionless parameters are apt and that the proposed scaling relationship characterizes the bullet effect with a reasonable level of accuracy. An example from the literature involving numerical simulation of a full rock barrier is considered, and satisfactory agreement between the calculated performance of the barrier and that predicted by the established scaling relationship is observed.
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Papers by James Hambleton