The aim of this research is to characterise the collapse and failure mechanisms of lattice structures under quasi-static and dynamic loading conditions and to determine the mechanical properties and energy-absorbing characteristics of lattice structures with different relative densities and geometric conditons. Lattice structures based on two simple architectures were manufactured from 316L stainless steel using Selective Laser Melting (SLM). The compressive properties of structures based on a body centred cubic (BCC) and a similar structure with vertical pillars (BCC-Z) were literally investigated at quasi-static rates of strain.

This thesis uses an energy-based approach to develop new analytical solutions for the time-dependent creep response of deeply embedded cavities. The new models developed here can be used for the initial design of tunnels and for other applications such as underground storage caverns and problems outside the field of geomechanics. The objective of using this energy-based approach is to develop models that can provide a quick estimate of cavity closure and that can be applied to different design situations and material behaviour. For the first time a three-dimensional analytical solution has been developed for the time-dependent response of a cavity embedded in a viscoelastic medium. The cavity is excavated quasi-instantaneously from an infinite body with an initial isotropic stress field. The problem is three-dimensional due to the effect of a tunnel face. This new solution can predict the full interaction between the tunnel and the surrounding creeping rock and thus can be incorporated with field monitoring data in an expert system for tunnel design. The accuracy of this model is comparable with finite element analysis. A new class of thermodynamically consistent constitutive models have been developed, which couple viscoplasticity and damage, describing both the secondary and tertiary stages of creep behaviour. Models were derived for both frictionless and frictional materials within the framework of hyperplasticity. The frictional model provided a good fit to data obtained from the triaxial compression testing of sandstone, illustrating its capability of describing creeping rock. These new constitutive models were incorporated into the energy-based method for cavity analysis, using a two-dimensional plane strain cylindrical cavity for demonstration purposes. A parametric study was carried out and results were also compared with FE analysis. Findings show that the models successfully describe the secondary and tertiary stages of creep behaviour. These new solutions only require a simple text file as an input and need minimal skill to operate. The formation of an initial geometry or finite element mesh is not necessary. This is shown through the creation of a standalone program for the three-dimensional model. The new solutions can take into account a wide range of different material behaviour, both two-dimensional and three-dimensional problems and due to their thermodynamic consistency are able to simulate other time-dependent processes, such as relaxation. This shows the flexibility of this approach and its applicability to different geomechanics problems.

Testing of an artificially weakly bonded material has been carried out to help our understanding of the role of bonding in natural soils, such as tropical residual soils that usually bonded in nature and frequently exist in an unsaturated state. The use of artificial specimens allows reproducibility of bond strength, which is difficult to achieve with naturally bonded soils, such as residual soil. The artificial samples were prepared with void ratio of0.6 by mixing sand and kaolin before firing at 500°C for 5 hours. A new technique for producing looser samples was also developed using coriander seeds. These artificial bonded samples are reproducible and are not agedependent. The bond formed has many advantages over cement or other bonding agents that require a curing period and show a change in strength with time. In order to observe the interaction between sand and kaolin, the thin sections of the samples were prepared and investigated under a microscope. Microscopic studies showed that the fired kaolin forms bridges between the sand grains, as well as coating the grains. The loose sample showed a very small amount ofash residue left after firing at high temperature. The engineering behaviour of these artificially weakly bonded samples was investigated in triaxial tests. Destructured material (where the bonding was broken down) was also tested to provide a reference state for a comparison. Conventional drained and undrained triaxial compression tests were performed on artificially bonded and destructured samples in a saturated state. A series of constant water content tests were also carried out on artificially weakly bonded samples in an unsaturated state at various initial matric suctions. The saturated behaviour of destructured and bonded samples was studied in order to understand the influence of bond structure and provide a useful reference for the interpretation of the unsaturated tests. The destructured samples showed no significant peak in q/p' ratio in comparison with the bonded samples. All bonded and destructured samples sheared at lower stresses sustained higher values of stress ratio than those sheared at higher confining pressure. The effect of bonding was clearly seen when comparing the stress ratio values between bonded and destructured materials. The bonded samples achieved a higher stress ratio values if compared with . the destructured samples. However, with an increase in stress level, the bond strength decreases progressively at failure, therefore the peak in stress ratio dropped closer to that of the destructured material. A nonnalisation of the stress paths was perfonned in order to compare the results from different tests on bonded and destructured materials. The effect ofbonding was evident from the nonnalised stress paths. A series ofconstant water content tests were perfonned on unsaturated samples with suction ranging from zero up to 500kPa. The unsaturated samples were initially consolidated under mean net stresses of 50, 100 and 300kPa before shearing under constant water content conditions. The water retention curve (WRC) was detennined for a drying path. It showed a very steep change at suction between 2 - 4kPa before flattened when the degree of saturation, Sr dropped below 25%. During consolidation, samples experienced initial compression before levelled off with time. Samples with higher suction showed less compression in comparison to samples with lower suction. The matric suction for all samples, generally showed an initial decrease after applying a mean net stress, p-ua, but then levelled offwith time. The stress-strain curve for samples with zero and low suction sheared at lower stress showed a clear peak compared to samples sheared at higher stress which indicated more ductile behaviour. The volume change of the samples during shearing showed the influence ofmean net stress and suction. The peak strength surfaces plotted in q suction space were parallel and indicated some curvature. The curvature of the ultimate strength in shear strength - suction relationship showed that the contribution ofsuction to strength represented by friction angle, ~b was not constant. The initial and final bond yield surfaces were represented by a linear increase with mean net stress over the stress range investigated. As matric suctions increased higher, both bond yield surfaces dropped back close to yield surface for lower matric suctions. The critical state stress ratios Ma (mean net stress component) and Mb (matric suction component) were used to examine the critical state of the bonded material in unsaturated conditions. It was found that Ma is higher than the saturated critical state stress ratio, Ms (Le. cpa> cp), therefore, the assumption that cpa =<p' is not always valid. The changes in Ma and Mb can be related to three different phase of water retention behaviour. This pattern of behaviour is probably explained by the narrow range of pore size in the uniform bonded sand. A relationship between water content and matric suction was studied based on the WRC at each stage of suction equalisation, consolidation and shearing. The COLs or Continuously Disturbed Lines (representing the water content - suction relationship at ultimate state) were plotted. It was interesting to see that samples with initial low suction showed an increase in suction to reach the COL while samples with initial high suction showed a decrease to approach the COL. The decrease in suction during shearing for samples with high initial suction was accompanied by dilation behaviour in terms of volumetric strain. This behaviour is the opposite of what might be expected. In saturated soils, dilative behaviour would produce a drop in pore water pressure (increase in suction). This opposite behaviour expressed by unsaturated samples under shear emphasises the role of fabric in controlling the engineering behaviour of unsaturated soils.

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Even in simple, quasi-static cases, blast and its interaction with structures is a complex phenomenon. Long duration blast, dened for the purposes of this research as a blast event with a positive phase duration in excess of 100ms, increases this complexity due to not only the persistent eects of the static blast overpressure, but also the dynamic pressure associated with drag wind trailing the shock front. Brittle materials such as concrete and masonry produce large numbers of initial fragments, which when caught in the drag wind produce substantial debris distributions, presenting a number of potential hazards ranging from infrastructure obstruction to personal injury. This research investigates the eects of long duration blast loading of masonry structures, using a modular `Base Panel' approach to descirbe structures as a composition of simple panels. Five experimental blast trials were conducted, testing a total of 22 masonry structures against varying blast parameters. The results showed correlations between both the breakage and debris distributions with respect to panel geometry. Computational Fluid Dynamics and the Applied Element Method were used to model the experimental trials, with the results showing good agreement oering a promising modelling platform for future work. The results, both experimental and computational, demonstrate the `Base Panel' approach to be an eective tool for the prediction of masonry debris distribution.

The results of an investigation into the characteristics of catenary action and their effects on structures are reported here. There are five aspects presented in this thesis: a literature review on progressive collapse and beam-to-column connections; an exact analysis of catenary action of truss; nonlinear finite element analyses of different post-column removal performance as a function of the original and retrofitted beam-to-column connection geometries; comparison of different behaviours of different beam-to-column connection geometries; and the minimisation of the strengthening plate weight for the retrofitted structures. This thesis demonstrates that when a column is removed, the bending moment will decrease significantly as the catenary tensile force and the plastic deformation increase in the beams connected to the damaged column. This thesis also shows that when a column is destroyed by a blast, the failure strain will be reached at the simple beam-to-column joint. To enhance the survival capability of the steel framed structures subjected to terrorist blast, retrofitting schemes were proposed for strengthening the joints of tall steel framed structures, ensuring the full development of catenary action. This thesis simulates the post-column removal behaviour of the original and the strengthened structures by means of the ABAQUS finite element package. For this purpose, sophisticated two- and three-dimensional models of catenary action are developed. Through comparing different results of the original and the strengthened structures from a column removal, the advantages of the proposed retrofitting schemes have been demonstrated. This thesis investigates the relative advantages and shortcomings between two different retrofitting schemes and compares the proposed retrofitting schemes with other moment beam-to-column connections used for new construction. For the same resisting capacity of the retrofitted structure, the Vertical Plate Scheme will require much more material than the Flange Plate Scheme. But the Vertical Plate Scheme still has its advantages. The Vertical Plate Scheme does not require the removal of the floor slab and easier to conduct than the Flange Plate Scheme. Finally, it is demonstrated that the sizes of strengthening plates can greatly change the overall behaviour of the retrofitted structures; with the reduction of the strengthening plate sizes, the stress and strain will increase and the structure will experience elastic stage and elasto-plastic sequentially. This thesis minimises the thickness and the length of the strengthening plates by the exact analytic method for simple structures and the finite element method for complicated structures.