PhD Theses: 2013

The rising population in urban environments comes with an associated demand for increased public transport. Due to the level of surface congestion an often utilised solution is to construct rapid transit systems within tunnels. Any sub-surface construction will generate ground movements which have the potential to cause damage to existing surface and sub-surface structures. The risk of damage has driven the need for accurate predictions of tunnelling-induced settlements. The majority of the readily used prediction techniques are largely empirically based and concern single-tunnel greenfield arrangements. In general, rapid transport systems comprise a pair of tunnels constructed within relative close proximity. This is known as twin-tunnel construction. A series of plane strain centrifuge tests investigating twin tunnelling-induced settlements in overconsolidated clay was carried out. The main variables were the spacing between the tunnels, both horizontally and vertically, and the magnitude of volume loss. The tests were conducted at 100g where the cavities represented two 4 m-diameter tunnels at (usually) a depth of 10 m at prototype scale. The tests utilised novel apparatus designed during the research to enable the simulation of the construction processes by careful control of the volume loss in separate tunnels.

Contacts: Dr R. J. Goodey and Professor R. N. Taylor

This research concerns the influence of ribs on the ultimate capacity of a bored pile in overconsolidated clay. The investigation sought to explore the effectiveness of ribs at increasing the ultimate capacity of a pile, and furthermore to understand how this enhanced capacity is derived. Experimental data were obtained from a series of 23 centrifuge model tests undertaken at 50g. The performance of several rib designs and spacings was investigated. Of all the rib profiles tested the helical profile was shown to be the most effective. A plastic failure envelope for the base of the pile rib has been identified and has been used to provide a detailed design solution for the ultimate capacity of a concentrically ribbed pile. The design solution is simple and requires a summation of the constitutive contributions from each rib and from the base and shaft of the pile. A modification factor (Γ) has been proposed to allow for correction of the rib angle generated by the helix. This correction factor allows the same design solution to be used for all helically ribbed piles. This detailed design method has been used successfully to predict the ultimate capacity of any pile tested to within 8%.

Contacts: Dr A. M. McNamara and Professor R. N. Taylor

Centrifuge modelling was used to investigate the relative effectiveness of minipile groups in undrained clay with no inner piles (perimeter group), when compared to the more common grid configuration. The pile groups were subject to monotonic axial loading and variables included group geometry (group shape and pile spacing) and number of piles. Finite-element analysis allowed consideration of variables including length/diameter ratio and soil strength. The pile groups failed either as individual piles or as a block and it was demonstrated that grid groups were less efficient than perimeter groups as a result of the soil–pile behaviour and interaction.

Sponsors: EPSRC, Foggo Associates, Geotechnical Consulting Group, Balfour Beatty Ground Engineering, Keller Group, DFI and Isherwood Associates.

Contact: Professor N. Taylor

This study develops Bayesian approaches for probabilistic geotechnical site characterisation, which integrate systematically prior knowledge (i.e., site information available prior to the project) and project-specific test results to characterise probabilistically soil properties and underground stratigraphy (e.g., the number and thicknesses/boundaries of soil layers). The proposed Bayesian approaches address explicitly the inherent soil variability and accounts rationally for various uncertainties (e.g., measurement error, transformation uncertainty, and statistical uncertainty) arising during geotechnical site characterisation. They effectively tackle the difficulty in generating meaningful statistics from the usually limited number of soil property data obtained during site characterisation and provide proper probabilistic characterisation of soil properties and underground stratigraphy based on both prior knowledge and limited project-specific test results for probabilistic analysis and reliability-based design of geotechnical structures, such as probabilistic slope stability analysis. In this study, a Monte Carlo simulation (MCS)-based probabilistic slope stability analysis approach is developed, in which an advanced MCS method called ‘subset simulation' is applied to improve the efficiency and the resolution at relatively small probability levels. In addition, a probabilistic failure analysis approach is developed that makes use of failure samples generated in MCS to shed light into the relative contributions of various uncertainties to failure probability.

Sponsor: Research Grants Council of the Hong Kong Special Administrative Region, China

Contact: Dr Y. Wang

The majority of slow-moving landslides in Switzerland are rather large and the sliding layer is heterogeneous, resulting in rather low reliability of the soil parameter values derived from laboratory tests on soil samples obtained from site investigations. The most reliable approach to the long-term displacement and stability analysis of large creeping landslides is based on the back-calculation of soil parameters from observed displacements and water pressures in the sliding layer. Additionally, it is suggested to measure the in situ stiffness and pressure in the sliding mass. Two simplified analytical models are explored in an attempt to quantify evolution of landslide stabilised by a retaining wall, or by a natural barrier at the bottom of the sliding mass. In both models, the back-calculation method has been used to obtain of the soil parameters, avoiding high uncertainties associated with determination of the viscosity coefficients in laboratory tests. Additionally, the soil parameters have been compared with in situ measurements from field campaigns and laboratory tests. The proposed analytical models and field investigation techniques can be a powerful tool for the investigation of constrained creeping landslides and allow for improved assessment of landslide hazard and suggest measures for its mitigation.

Sponsors: ASTRA/VSS grant VSS 2005/502 ‘Landslide-Road-Interaction' and ASTRA/VSS grant VSS 2010/502 ‘Landslide-Road-Interaction: Applications'

Contacts: Professor A. M. Puzrin and Professor E. E. Alonso

Mechanised tunneling with earth pressure balance tunnel-boring machines (EPB-TBM) is frequently the chosen excavation method for underground construction in difficult soil conditions. The targeted injection of foams and polymers leads to enhanced mechanical properties of the face supporting soil paste and optimises the excavation. This thesis focuses on the mechanical characterisation of conditioned clay pastes and on the analysis of the interaction processes between the clay particles and the chemicals. For mechanical quantification new devices are introduced allowing the measurement of shear strength, adhesion and clogging potential of soft pastes at different boundary conditions (applied pressure and rate). The results show a general reduction of the measured parameters, however the magnitude is dependent on the clay, the chemical and the chemical concentration. Additionally the observed lubrication mechanism of the dispersing chemicals has a limited effect on stickiness reduction. Therefore, new chemicals (patented for the application in EPB tunneling) are introduced, which lead to a drastic stickiness reduction over a wide range of water contents for all types of tested clays. The results allow for an optimised application of commercial chemicals and build new knowledge for the development of future new products.

Sponsors: Swiss Confederations Innovation Promotion Agency (CTI, Grant No. 8554·2) and BASF Construction Chemicals

Contacts: Professor A. M. Puzrin and Professor G. T. Houlsby

Ports play a critical role in transportation infrastructure and the economy, but are vulnerable to seismic hazards. Downtime and reduced throughput from seismic damage in ports results in significant business interruption losses for port stakeholders. Current risk-management practices focus on the effect of seismic hazards within individual port structures; however, damage and downtime of these structures has a significant impact on the overall port system's ship-handling operations. Presented in this thesis are the concepts and methods developed for the seismic risk management of a port-wide system of berths. In particular, the thesis discusses the framework used to calculated port losses: the use of spatially correlated ground motion intensity measures to estimate damage to pile-supported marginal wharves and container cranes, repair costs and downtimes subsequently determined via repair models for both types of structures, and the impact on cargo-handling operations calculated via logistical models of the port system. Results are expressed in the form of loss exceedance curves that include both repair/replacement costs and business interruption losses. The thesis also discusses how the results from such an analysis might be used by port decision makers to make informed decisions in design, retrofit, operational and other seismic-risk-management options.

Sponsors: National Science Foundation – George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research

Contact: Dr G. Rix

Evaluations of stress history and the geostatic state of stress of soils are ascertained on the basis of field geophysical measurements that provide paired complementary types of shear waves. It is well established that multiple types of shear waves occur in the ground due to their directional and polarization properties. The shear wave velocity (V_s) provides the magnitude of small strain stiffness (G₀) which depends on effective stress, void ratio, stress history and other factors (cementation, age, saturation). This study examines a hierarchy of shear wave modes with different directions of propagation and particle motion from in situ geophysical tests (HH, VH, and HV) and laboratory bender element data. A special compiled database from well-documented worldwide sites is assembled where full profiles of stress state, stress history and several paired modes of V_s profiles have been obtained from crosshole tests, downhole tests and rotary crosshole. Reference profiles of the lateral stress coefficient (K₀) are available from direct in situ measurements (self-boring pressuremeter, hydrofracture and push-in spade cells). Stress history is documented in terms of yield stress ratio (YSR) from consolidation testing and careful engineering geology studies. A methodology is developed that relates both the YSR and K₀ to stiffness ratios obtained from directional shear wave velocities.

Sponsors: US Department of Energy, Savannah River Site

Contact: Dr P. W. Mayne

This research proposes a testing method to determine strength–deformation characteristics of unsaturated crusher-run materials used in base course layer. A series of laboratory element tests using multi-ring shear apparatus, which can take into account rotation of principal stress axis, was carried out to examine shear behaviour of base course materials under different degrees of saturation. This study focuses on mechanical response in terms of cyclic plastic deformation of unsaturated crusher-run materials under various cyclic loading conditions, which include the phenomenon of principal stress axis rotation (experienced by pavement elements under traffic load). The results show that the degree of saturation has an obvious effect on the shear and cyclic plastic deformation behaviour of crusher-run materials. It is also explained that cyclic plastic deformation under repeated axial and shear loading significantly enhances due to rotation of the principal stress axis when compared with cyclic plastic deformation under repeated axial loading regardless of type of material and degree of saturation. Finally, it is pointed out that results of cyclic plastic deformation obtained from multi-ring shear tests under the influence of principal stress axis rotation are much more realistic and these results can be incorporated in practical pavement design.

Contacts: Professor S. Miura and Professor T. Ishikawa

Triassic to Eocene marine mudrock deposits cover much of south-east England, exhibiting a wide range of micro- and macrostructures. An Imperial College research project run between 2007 and 2012 investigated their structure and mechanical properties through advanced laboratory and in situ testing (see also Hosseini Kamal (2012)). The author focused on establishing the stiffness and shear strength anisotropy of the Oxford, Kimmeridge and Gault clays, testing high quality block and rotary cored samples were tested in two hollow cylinder apparatuses (HCAs), which controlled independently the intermediate principal stress factor (b) and the orientation α of the major principal stress (σ₁) as well as q and p′. At least six tests was performed on each mudrock on specimens reconsolidated to their in situ K₀ stresses before being sheared undrained at various α values while controlling b to set values. The interpretation took account of comprehensive HCA studies on London Clay (Nishimura, 2006; Anh-Minh, 2007). Marked shear strength and stiffness anisotropy was noted whose degree varied between the deposits and depended on the constitutive framework employed. Macrostructure also affected shear strength anisotropy markedly. Good correlation was found between the mechanical the microstructural anisotropies, the latter being quantified in Wilkinson's associated PhD study (Wilkinson, 2011).

Sponsor: EPSRC

Contact: Professor R. Jardine

Large areas of southern UK are covered by Triassic to Eocene mudrocks whose depositional conditions and subsequent histories led to a range of natural structures. This study investigated and interlinked the mudrocks' structures and mechanical behaviours. Oxford, Kimmeridge and Gault Clays were sampled, tested and compared with London Clay data from recent PhD studies at Imperial College by Gasparre (2005), Nishimura (2006) and Minh (2007). High-quality block and rotary core samples were used by the author and Brosse (2012) in addition to reconstituted samples. The author employed triaxial apparatus equipped with bender elements and high-resolution displacement transducers, as well as high-pressure tests that established the strength envelopes of each soil, their undrained stiffnesses and drained elastic parameters. Oedometer were employed to investigate one-dimensional compression, complimented by ring shear and index testing on all mudrocks. Further microstructure analysis was performed by Wilkinson (2011). The study established a valuable database that highlighted highly and brittle anisotropic behaviour, while finding no systematic correlation between mechanical behaviour and geological age or depth of burial. The highly fissured macrostructure of the soils was noted and, for Gault Clay, the important effects of weathering and root action were investigated.

Sponsor: EPSRC

Contacts: Professor R. Jardine and Professor M. Coop

In the thesis, a soundness-determination method of boulder-type rockfall is proposed in a quantitative manner. Previously, this kind of evaluation was made based solely on site-engineers' experiences and skills. In this study, two quantitative indices were intruduced. One is addressed to the rockfall-risk degree by vibration measurement of boulders on slope. The other refers to the influence degree to railway track based on arrival probability of rockfall simulation. The applicability of the proposed method in use for the slope management practice is examined, by performing the basic experiments, eigenvalue analyses by three-dimensional finite-element modelling, on-site measurement, together with a number of case histories. The proposed method will enhance the safety of railway business against rockfall.

Sponsor: West Japan Railway Company

Contacts: Professor S. Shibuya, Professor I. Fukita and Professor H. Morikawa

This thesis covers the simulation of slowly creeping slopes with the existing viscose material models after Norton and after Vulliet–Hutter and with a new extension of an existing viscose material model. The first material model calculates the viscose strain rates as a function of the deviatoric stresses, the other two also take the hydrostatic pressure into account. For the evaluation of the material properties many laboratory experiments are evaluated, including creep tests in triaxial cells. The viscose material models are implemented as user material subroutines in the finite-element software Abaqus. A case study is calculated hydro-mechanically coupled with the material parameters of the laboratory experiments and the calculated creep velocities are compared to the measured creep velocities of the case study. The measured creep velocities show a linear creep trend which is superimposed by cyclic accelerations and decelerations. Those changes in creep velocity are most likely due to seasonal fluctuations of pore water pressure. The influence of the pore water pressure can be mapped with the viscose material model after Vulliet–Hutter and with the new, extended viscose material model. The extended viscose material model also estimates the relation between creep velocities due to different pore water pressures well.

Contacts: Professor W. Fellin and Professor E. Tentschert

The current flexible Australian pavement design approach is based on structural principles using layered elastic models considering standard axle loads to represent the entire traffic spectra. A more rational approach would be to consider directly the individual traffic axle loads and configurations. Another serious drawback in the current approach is that only one set of parameters is used to define material properties of the pavement layers. However, it is well established that pavement material properties significantly change with prevailing climate. For example, asphalt modulus is heavily dependent on the temperature and the modulus of unbound materials depends on the moisture content. This project examines methods to incorporate climatic conditions in pavement design, where the pavement deterioration is modelled continuously taking into account the actual traffic load spectra. It is expected that these studies can lead to more rational design methodologies for pavement design.

Contacts: Professor J. Kodikara and Professor A. Bouazza

In Singapore, two-thirds of the land area is underlain by residual soils. One distinct characteristic of residual soils is its unsaturated nature. This study aims to investigate shear stiffness and damping ratio properties of residual soils under cyclic simple shear loading in both saturated and unsaturated conditions. Two commercial NGI-type cyclic simple shear apparatuses were evaluated using a standard sand, Ottawa 20-30. The evaluation revealed that there was a rocking movement on the top platen experienced by both apparatuses. A correction factor based on actual horizontal displacement ratio was proposed to correct the shear modulus. In addition, a detailed correction procedure on friction energy was described for damping ratio determination. Unsaturated soil specimens for cyclic simple shear tests were prepared using the pressure plate apparatus. Independent matric suction measurement using a high suction tensiometer was performed during the equilibrium process. Corrected rates of gravimetric water content change of 0·15%/day and of 0·08%/day were suggested as criteria for equilibrium in the pressure plate test for drying and wetting processes, respectively. Unsaturated soil zones (transition and residual zones) can be distinguished from saturated soil zone (boundary effect zone) using the soil-water characteristic curve (SWCC). A simplified method to estimate the SWCC of fine- and coarse-grained soils was developed and compared with other studies. The comparison showed that the proposed method is simpler and performed better than existing one-point methods. Cyclic simple shear tests were conducted on undisturbed and reconstituted residual soil specimens under saturated and unsaturated conditions. The normalised G/G_max and damping ratio of residual soil specimens were compared with other studies. The comparison showed that the differences in G/G_max and damping ratio with shear strain of residual soil can be attributed to the different weathering conditions of the residual soils.

Sponsors: Nanyang Technological University Research scholarship (RGM 8/05) and PTRC-CEE/DSTA/2006·01 scholarship

Contact: Professor E.-C. Leong

The stabilities of shallow tunnel headings with and without forepole reinforcements were studied by the use of small scale centrifugal physical models and upper bound plasticity models. The physical models were tested at 100g in the National University of Singapore geotechnical centrifuge which modelled a prototype tunnel that has a diameter of 6·5 m and under the same depth of ground cover. Reconstituted kaolin clay, which was made stiff by overconsolidation, was used for modelling the ground. Zinc chloride solution, which was produced to have the same bulk density as the stiff clay, was used for modelling the tunnel support pressure. Several modes of collapses were identified from the model tests which were dependant on the length of tunnel heading protrusion and length of forepoles. A collapsing tunnel heading with long unlined length was shown to display large roof movements and relatively lesser movements at the face. The presence of forepoles was found to modify the collapse mechanisms. Moderate forepole lengths promote the formation of the face wedge. Whereas a tunnel heading reinforced with very long forepoles was found to be susceptible to void formation and migration. The mechanistic studies guided the formulation of upper bound plasticity models. The stabilising effects of forepoles were also included, which were modelled as beams that undergo plastic collapses and their interaction with the ground was through complete lateral shearing of beam sections. Predicted stability conditions and collapse mechanisms of tunnel headings according to these theoretical models were in reasonable agreements with observations from the physical models.

Contact: Professor L. F. Hou

In this study, the use of a modified geotextile tube (M-GT) has been introduced. The theoretical development for the prediction of tensile forces in the geotextile material has been adapted from existing theories used for geotextile containers and tubes. After identifying the key parameters and the lacking in the understanding in existing models have been identified. Furthermore, the M-GT has been proposed to be filled with using dredged soil mixed with a small portion of cement (named as cement mixed soil, CMS). Centrifuge modelling is used to model the installation process of a M-GT. A comprehensive testing program was performed to evaluate the tensile forces for a M-GT filled with sand and CMS. Based on the findings from the centrifuge modelling, recommendations and valuable inputs are given to the contractor for the implantation of M-GT at the actual site. Besides the conventional way of installing a M-GT using a split hopper barge, a novel way of installing the M-GT has been implemented at the actual field work by the contractor. As this construction project has very strict requirements on the final positioning of the M-GT and the settlement criteria, the M-GT has to be accurately placed. This resulted in the use of Type C M-GT, which involved the use of a specially fabricated installation barge (Nereis). The installation barge is able to lower the filled M-GT to be as close as the seabed and release the M-GT from the frame underwater. The novel use of such an installation barge has effectively eliminated the uncertainty of the placement accuracy and also effectively eliminated the installation process of Phase 3 (free falling) and Phase 4 (impacting onto the seabed).

Contact: Professor C. S. Hoe

Embankments and cuttings form an integral part of UK infrastructure. These structures are susceptible to variations in climate which observational correlations suggest can affect their stability. It is therefore important to investigate the impacts of future climate on slope stability. A modelling procedure has been developed to assess the effects of climate on slope deformation and rate of failure. Use is made of models to calculate pore pressure response to climate and derive the resultant mechanical response. Climate data were derived for present and future climates using a weather generator. Assessment of the impacts of climate change on a diagnostic embankment and an instrumented cutting in the Newbury area were performed. A back-analysis on an instrumented natural slope in Belfast was also undertaken. The results indicate that laboratory and field permeability measurement techniques are inadequate in deriving macroscopic permeability characteristics of clay slopes. Also, deformation magnitude is linked to annual maximum pore pressures so the occurrence of wetter years and increases in the frequency of their occurrence will considerably increase deformations and failure rates. Finally, the permeability of a slope determines its susceptibility to increased failure rates when subjected to a future climate scenario.

Contacts: Dr M. Rouainia and Professor S. Glendinning

Subsidence related to shallow abandoned colliery workings are considered a threat to safety. To constrain better the conditions leading to subsidence, a parametric study to investigate the features and properties significant in the causes of instability was undertaken within the modelling code FLAC3D. The effects on stability of variations in excavation geometry, variations of the in situ stresses in the overburden and variations in the material properties of the overlying rock mass were investigated. A case study describing the subsidence that occurred at Dolphingstone in Scotland along with a parametric study into the event is also included. Interface elements were used to model coal measures rocks as horizontally layered strata whereby discontinuous behaviour representative of the failure could be captured. The modelling of dipping discontinuity features was investigated using an anisotropic constitutive model. The results indicate that the friction angle of the rock mass is a key parameter in controlling the collapse geometry of overlying strata. Elevated pore water pressures are also shown to trigger instability. Empirical tools to estimate the height of collapse of the void are introduced along with a methodology for the modelling of collapse propagation in rock masses above voids.

Sponsor: Network Rail

Contacts: Dr C. T. Davie and Professor S. Glendinning

During installation of geomembranes, the combination of solar heating, a high coefficient of expansion and the stiffness of high-density polyethylene (HDPE) causes the geomembrane to expand and buckle, forming wrinkles. Up to 20–30% of the area of the geomembrane may be under hydraulically connected wrinkles, which could substantially increase leakage through the composite liner if there is a hole on or near a wrinkle in the connected network. A technique for aerial photography and photogrammetric correction was developed to quantify individual wrinkles and the length of the longest hydraulically connected wrinkle at each time for nine field cases. Solar radiation, air and geomembrane temperature was recorded as permitted by site conditions and instrumentation. The longest measured connected wrinkle was 5330 m on a 0·61 ha slope. For a 1·5 mm-thick geomembrane, the average wrinkle width over a geosynthetic clay liner was 0·20–0·23 m and 0·24–0·32 m over a compacted clay liner. The average wrinkle height was 0·06 m, and the tallest wrinkle measured was 0·18 m. The longest connected wrinkle length was <200 m when the sum of the wrinkle lengths was <580 m. Results suggest that limiting the time of day when cover soil is placed and/or reducing the area in which wrinkles can form may greatly reduce the length of connected wrinkles after covering.

Sponsors: Natural Sciences and Engineering Research Council of Canada (NSERC), Ontario Ministry of the Environment, Solmax International Inc., Terrafix Geosynthetics Inc., AECOM, AMEC Earth and Environmental, Golder Associates Ltd and CTT Group

Contacts: Professor R. K. Rowe, Professor R. W. I. Brachman and Professor W. A. Take

A numerical model (BioClog) is developed to examine changes in key municipal solid waste (MSW) leachate characteristics and the porosity of porous media (clogging) as the leachate passes through the drainage layer of a leachate collection system (LCS). The model considers multiple-species reactive leachate transport through porous media. It simulates biofilm growth and loss, deposition of suspended particles, and precipitation of minerals on the surface of porous media. It is used to examine the long-term performance of both the granular porous media and nonwoven geotextiles in LCSs. Modelling of laboratory mesocosm cells filled with gravel usually used in landfills and permeated by landfill leachate shows encouraging agreement between the observed and measured effluent chemical oxygen demand (COD) and calcium concentrations as well as the gravel porosity within the saturated drainage layers. Studies of early generation LCSs involving finger (French) drain systems show that the finger drains are not effective at controlling leachate mounding within the landfill and the calculated leachate mound thicknesses agree well with observed field data. A numerical examination of the recent generation of LCSs, comprised of the granular drainage blanket and perforated drainage pipes, shows that an increase in grain size increases the service life and that increasing the spacing between collection pipes (i.e., the drainage path) decreases the service life of LCSs. Filter-separator layers between the waste and granular drainage layers are shown to increase the service life of LCSs. The modelling results indicate that the calculated clog mass within the saturated drainage layer is dominated by the inorganic material and the calculated service life of LCSs is dependent on the leachate strength examined. Finally, a new practical model for estimating the service life of LCSs is developed and calibrated against the data from the BioClog model. The simplified model could be used by the practising engineers for estimating the service life and optimising the design of LCSs in MSW landfills.

Sponsors: Natural Sciences and Engineering Research Council and Queen's University, Canada

Contact: Professor R. K. Rowe

The use of clay-based material for backfilling purpose can be problematic due to build-up of lateral stresses on retaining structures, caused by post-compaction saturation. This thesis explores this aspect in detail using K₀ stress path testing of London Clay, Belfast Upper Boulder Clay, Glacial Till and Kaolin Clay. The samples were prepared at different initial conditions and were allowed to saturate by reducing the suction in stages, while maintaining constant overburden pressure. The results have shown that coefficient of earth pressure K₀ increased substantially and the magnitude of it influenced by level of compaction, compaction water content, clay mineralogy and the level of overburden pressure.

Sponsor: Brunei Government

Contact: Dr V. Sivakumar

The determination of soil stiffness at very small strains, as well as the decrease in stiffness with increasing shear strain amplitude, in laboratory and field tests is shown. Typical properties and empirical correlations of coarse-, fine-grained and organic soils are collected and significant differences in soil stiffness and stress-dependence at small strains compared to large strains are shown. The soil behaviour at small strains predictably the hypoplastic constitutive model with intergranular strains and the determination of the material parameters is discussed. For a realistic prediction of the soil behaviour due to cyclic loading a modification of the hypoplastic constitutive model is proposed, introducing an additional material parameter. It is shown how this additional parameter can be determined in cyclic laboratory tests and how the accumulation of strains in drained conditions and excess pore pressures built up in undrained conditions can be reproduced. A dynamic numerical analysis is performed for both one-dimensional and half-space conditions. Different constitutive models are applied and compared with analytical solutions. The results demonstrate requirements on numerical analysis of wave propagation, in particular with regards to time steps, element size, node spacing, size of the finite-element mesh and boundary conditions. Considering a dynamic loading of a railway embankment on soft marshy ground, the modified hypoplastic constitutive model is applied. It is demonstrated that the dynamic soil behaviour can be reproduced realistically. Numerical results show a good agreement with the measurements of vibrations and permanent displacements.

Contacts: Professor Dr.-Ing. habil. I. Herle and Professor Dr.-Ing. T. Neidhart

A stress-controlled erosion apparatus was developed to investigate the initiation and development of internal erosion under complex stress states and to study the mechanical response of soil to internal erosion. Extensive internal erosion tests were conducted on a gap-graded soil under complex stress states. The entire internal erosion process can be divided into three phases: initiation, development, and failure. Accordingly, three critical gradients named initiation, skeleton-deformation and failure hydraulic gradients can be defined. After loss of a significant amount of fine particles in the soil, the original dilative stress–strain behaviour changes to be contractive one and the shear strength decreases. Moreover, field jet index tests were conducted on two landslide dams formed during the 2008 Wenchuan earthquake to investigate the erodibility of fresh landslide deposits. Two empirical equations are developed for estimating the coefficient of erodibility and critical erosive shear stress of the fresh landslide deposits based on their basic soil properties. Finally, a physically-based breach model considering the variations in soil erodibility along depth is developed to simulate the overtopping breaching process of earth dams and landslide dams.

Sponsor: Research Grants Council of the Hong Kong SAR

Contact: Professor L. M. Zhang

Experimental evidence indicates that fabric anisotropy has a strong influence on the behaviour of geomaterials. This dissertation presents a theoretical study on the characterisation of anisotropic deformation and strength in soils and rocks. The study highlights the important role of fabric played in dictating the anisotropic behaviour of geomaterials. Based on a fabric tensor representing the anisotropic internal structure in either soils or rocks and an anisotropic parameter jointly defined by the fabric tensor and the stress tensor, systematic approaches have been developed to effectively evaluate the strength anisotropy in sand, clay, rock as well as fibre-reinforced sand. An advanced critical state plasticity model has been further developed for sand with full consideration of the effect of fabric and its evolution. The model has been implemented in displacement-driven finite elements and further applied to the prediction of shear band formation in inherently cross-anisotropic sand.

Sponsor: Research Grants Council of Hong Kong (RGC/GRF 622910)

Contact: Dr J. Zhao

An integrated investigation of the microstructure, compression behaviour, the soil-water characteristic curve (SWCC), shearing behaviour and critical state characteristics of coarse and widely graded soils with different coarse contents is conducted. The triaxial tests were conducted under low net confining stresses (5–15 kPa) to reflect the shallow-seated slope failures. As the coarse content increases, the soil microstructures change from fines-controlled to coarse-controlled with a transition state at a critical coarse content of approximately 70%. The high stiffness skeleton of the fines-controlled and coarse-controlled soils results in low compressibility and dilative behaviour. The transition soil exhibits high compressibility and strong contractive behaviour even under very low confining pressures due to the high compressible skeleton. With increasing coarse content, the pore structure of soil changes from unimodal pore structure to a dual-porosity structure. Accordingly, the SWCC changes from unimodal to bimodal. A physical model is developed for the bimodal SWCC. A general shear strength model is proposed for granular soils with a bimodal SWCC. The critical state characteristics of unsaturated coarse granular soils were investigated. An equivalent mean effective stress taking into account the suction stress is proposed to describe the instability of an unsaturated soil subject to wetting.

Sponsor: The Research Grants Council of the Hong Kong SAR (Grant No. 622210)

Contact: Professor L. M. Zhang

Adaptive finite-element methods provide a means for obtaining more reliable solutions by continuously adjusting the discretisation in time and space according to the current solution. In this thesis, the h-adaptive finite-element technique has been employed to solve some complex geotechnical problems involving material nonlinearity, large deformation, changing boundary conditions and time-dependent nonlinearity. To achieve this, the main features of the technique including advanced mesh generation algorithms, error estimation methods and a procedure for remapping of state variables will be discussed and developed in company of a robust analysis program. The performance of the h-adaptive finite-element method is then represented by considering its accuracy and efficiency method in solving some classical geomechanics problems such as the bearing capacity of footings, expansion of cavities, and the stability of slopes. In addition, this thesis has addressed the efficiency of alternative error estimation techniques for particular geotechnical applications involved with changing boundary conditions and inertia forces, such as static and dynamic penetration of an object into soil. This thesis also presents a new and innovative combined adaptive method based upon a combination of the arbitrary Lagrangian–Eulerian method and the h-adaptive finite-element method developed as a part of the thesis.

Contacts: Professor D. Sheng, Professor A. Abbo and Laureate Professor S. Sloan

Jack-up reinstallation response is relatively complex and in this study three controlling parameters were identified: the footprint's geometry, the footprint's soil properties and the structural properties of the jack-up unit. This study reports a comprehensive series of experimental investigation on each of these three parameters, first in isolation and finally in combination. The motivation is that with an improved knowledge of the role of these parameters, a prediction method can be developed and the effectiveness of mitigation measures can be assessed, ultimately reducing the failure rate of jack-up reinstallation. The first part of the study was conducted in the drum centrifuge and idealised footprint cavities were cut within a clay sample. This eliminated the effect of variation of undrained shear strength in a real footprint and the isolated effect of footprint geometry was investigated. In the second part of this study, a real-time hybrid testing method was developed to model the reinstallation of a full jack-up unit. This allowed the effect of structural properties to be evaluated for the first time. The critical element of the reinstalling jack-up leg was modelled physically and it was connected to a new actuator specifically designed for the vertical, horizontal and angular movements. The remainder of the jack-up unit was modelled numerically. The numerical model interacted with the physical model through a real-time control algorithm to model the response of the full jack-up unit.

Contacts: Winthrop Professor M. Cassidy and Professor C. Gaudin

This thesis is concerned with the on-bottom stability analysis of offshore pipelines under the action of wave and current loading. It details how hydrodynamic load modelling, pipe–soil interaction modelling and the coupling effect between the hydrodynamic load and the pipe–soil interaction can be properly considered. The motivation is to develop an integrated pipeline on-bottom stability analysis program and design methodology, and to use it to achieve a better understanding of hydrodynamic pipe– soil interaction. A hydrodynamic modelling program that generates a three-dimensional ocean surface, estimates the wave kinematics at the pipeline level and calculates the hydrodynamic loads on the pipeline was coded in FORTRAN. It has been named UWAHYDRO. Pipe–soil interaction is modelled using plasticity based techniques, again coded in FORTRAN in the UWAPIPE program. A unique pipeline on-bottom stability simulation program was developed by integrating UWAHYDRO and UWAPIPE with the commercial finite-element program ABAQUS. The developed modelling program can efficiently evaluate the movement of a long pipeline under storm conditions, as shown by a parametric study of 1250 m of pipeline under 1 h of storm characteristic of the Australian

North West Shelf region.

Contact: Winthrop Professor M. Cassidy and Professor Y. Tian

Arch action in geomaterials was investigated for both active and passive conditions. As an application of active arch action, the problem of a rigid retaining wall under active translation mode was investigated in a two-dimensional system of equilibrium. Exact stress solutions based on wedge-shaped hoppers theory were generalised and new equations were developed to estimate the vertical, lateral and shear stresses in the failure zone behind a retaining wall. The accuracy of the developed equations was validated through physical modelling. As an application of the passive arch action, the problem of undercut slopes was investigated where the arch action in the stable scarp in undercut slope and the corresponding failure mechanisms were clarified in this study. Theory of cohesive arching in hoppers was adopted to develop new equations for passive arching effect in undercut slopes and the developed equations were validated through physical modelling. A new surface mining method was proposed based on the developed theory which can be effectively undertaken with the classical cut-and-fill mining method in order to increase the working space with a lower risk of slope failure during operation. The proposed mining method was applied to the Mae-Moh lignite mine of Thailand as a case study.

Sponsors: Japan Society for the Promotion of Science (JSPS) and Electricity Generating Authority of Thailand (EGAT)

Contacts: Dr T. Pipatpongsa and Professor J. Takemura

A vacuum consolidation field test was successfully implemented for a 10 × 10 m area of raised bog, in order to investigate the performance of the method in pseudo-fibrous peat deposits and also to evaluate the viability of this technique for the construction and improvement of roads over peat. The efficiency of two different spacing of prefabricated drain was investigated for the 4 m-deep peat deposit. The test was instrumented and monitored for a period of 11 months. Two different vacuum-generating systems were also tested during the project. The field data were back-analysed using a range of finite-element models incorporated in PLAXIS. Acceptable results were obtained for the ground vertical displacements. The design parameters were determined by means of oedometer tests, indicating that the geotechnical properties of the peat obtained in conventional tests can be used in the design of vacuum consolidation projects.

Sponsors: National Roads Authority (Ireland), Ussher Fellowship from Trinity College, Dublin, Bord na Móna, Geotechnical Trust Fund award, and Universidad de Antioquia by granting the leave of absence to work on the project

Contacts: Dr E. R. Farrell and Professor B. C. O'Kelly

Shallow landslides in non-collapsible, physically weathered fine-grained soils may cause huge consequences – in terms of environmental and economic damages – due to their simultaneous occurrence over large areas. Indeed, the prediction of the areas potentially affected by these phenomena is a relevant issue for land use planning and/or design purposes. A multidisciplinary and multi-scale methodology able to fulfill this goal is proposed, tested and validated over an area in southern Italy affected by widespread shallow landslides that can be classified as earth slides-earth flows. In particular, moving from small (1:100 000) to large scale (1:5000), the methodology is based on geological (heuristic) criteria and it allows both the automatic detection of the areas potentially affected by these phenomena and, for each scale, the proper identification of the main factors leading to the landslide occurrence. On the other hand, moving from detailed (1:1000) to large scale the analyses are based on geotechnical (deterministic) procedures – that is, physically based (TRIGRS and TRIGRS-unsaturated) and limit-equilibrium methods – and they can allow landslide susceptibility zoning in a quantitative way. Efficiency and reliability of the proposed methodology are confirmed by its application to a well known case study in New Zealand using the data available in the scientific literature.

Contacts: Professor L. Cascini, Professor S. Di Nocera, Ing. G. Gullà and the late Professor G. Sorbino

This thesis presents an experimental investigation into the micromechanics of granular media. The novelty that this work brings is that during the campaign of triaxial compression tests (run on a series of three sands with different grain shapes, prepared dense and tested at different cell pressures) the specimens are imaged (in around 15 configurations) during loading using X-ray tomography. Specimens are relatively small (22 mm height, 11 mm diameter), allowing scanning at a sufficient resolution to identify all the grains (>50 000). Grains are identified in these three-dimensional images using a standard watershed algorithm, and techniques are developed to characterise the granular structure, grain-to-grain contacts, and to measure the full kinematics of all grains between imaged states. These measurements allow the micromechanisms responsible for the observed macroscopic behaviour to be understood. Important mechanisms such as the increasing rotational frustration of angular grains in a shear band are identified; this is used to explain the higher value of residual stress for these materials. Signs of localised deformation are seen to occur well before the peak in many samples, and complex patterns of rotating grains are noticed around the peak of each sample's response.

Contacts: J. Desrues, Professor G. Viggiani and Professor S. A. Hall

The behaviour of multiphase materials covers a wide range of phenomena of interest to both scientists and engineers. The mechanical properties of these materials originate from all component phases, their distribution and interaction. A new coupled hydromechanical model is presented in this work. It associates the discrete-element method (DEM) for the solid phase, and a pore-scale finite-volume (PFV) formulation of an incompressible flow problem. The emphasis of this model is on the description of the interaction between phases at the microscale. It has affordable computational costs, allowing the simulation of thousands of particles in three dimensions. Pore bodies and their connections are defined locally through a regular triangulation of the packings. The correspondence of the DEM–PFV model with the classic Biot's theory of poroelasticity is discussed. The model is validated through comparison of the numerical results with Terzaghi's analytical solution for the consolidation problem. An approach to analyse the hydrodynamics of a sea bed sediment subjected to waves is finally presented. The reproduction of the phenomenon of sediment liquefaction is analysed.

Sponsors: C2D2 program Hydrofond

Contacts: B. Chareyre and Professor E. Bathélémy

In the field, under specific external conditions, soils can turn into fluid – for example, during mud-flow or debris-flow initiation. This work deals, in a general way, with solid–fluid transitions in geomaterials behaviour and aims to develop a new constitutive model describing both solid and fluid phases and the transition in between. In order to describe this complex behaviour with a single numerical tool, the FEMLIP numerical method (finite-element method with Lagrangian integration points) is chosen since it can handle both history dependent behaviour, essential for solid description, and unlimited fluid strains. The main originality of the proposed transition model lies in two developments: the association of suitable elasto-plastic and viscous models and the use of a general criterion to detect the first solid–fluid transition at the unstable state – that is, the second-order work criterion. After implementation and validation of the elastoplastic formulation in the FEMLIP code, the second-order work criterion and a three-dimensional viscous yield stress are integrated. The solid–fluid transition model is then validated with homogeneous loading tests. Finally, the formulation is used to model Sarno and Quindici mud-flows (Italy, 1998), first with a simplified geometry and then with a realistic configuration, including a protection work

Contacts: Professor F. Dufour and Professor F. Darve

This thesis studies the influence of grain shape on the mechanical behaviour of granular materials, in particular its effect on load transfer around underground pipes, using experiments on two-dimensional granular materials and simulations with the discrete-element method (DEM). Particle shapes (convex polygons and concave clumps of discs) are generalised with a geometrical parameter alpha. Biaxial compression of granular assemblies (with DEM) reveals that mechanical and geometrical properties such as porosity, macroscopic friction and strain localisation depend both on alpha and on grain convexity.

The intergranular load transfer over a flexible pipe is studied both experimentally and numerically. The experiments show that the pipe has no significant impact on the macroscopic behaviour of the assembly. The numerical model agrees with the experiments and reveals that the arching effect in a granular assembly under shear exists above the pipe and is magnified with increasing the geometrical parameter alpha.

Contacts: Professor P. Villard and G. Combe

The modelling of damage in porous media plays a crucial role in several geomechanical problems. The objective of this thesis is to provide a reliable description of the influence of fluid pressure on damage evolution. The geomaterial considered consists of a saturated porous matrix including evolving fluid-filled cavities. Classic Biot's laws are used for the isotropic porous matrix and, by asymptotic homogenisation, a new macroscopic poroelastic model is built. The homogenised poroelastic parameters depend on mesoscale properties; a numerical characterisation shows the cavity-induced anisotropy of both the elasticity and the coupling tensors, consistent with experimental observations. As a second step, the cavity growth is taken into account and the energy release rate is characterised by means of a mesoscopic energy analysis of the whole body. A damage evolution law is then obtained by combining a mesoscopic cell energy analysis with the asymptotic homogenisation scheme mentioned above. Finally, some representative predictions of the obtained model are presented for both the cases of quasi-brittle and brittle damage with a Griffith-type energy criterion. It is shown that damage significantly increases for increasing imposed values of macroscopic fluid pressure. This response is qualitatively consistent with experimental results from drained tests.

Contact: Professor D. Caillerie, Professor C. Dascalu and Professor C. Callari

This thesis presents simplified and advanced probabilistic analyses of shallow foundations. In the simplified probabilistic analysis, the uncertain parameters are modelled by random variables. For an obliquely loaded footing, the numerical results based on the response surface method (RSM) and the collocation-based stochastic RSM (CSRSM) have allowed to identify the zones of failure mode predominance at both the ultimate and serviceability limit states. On the other hand, an efficient procedure was proposed to increase the number of the probabilistic outputs of the subset simulation (SS) approach with no additional time cost. In this procedure, the SS approach was combined with the CSRSM. In the advanced probabilistic analysis, the uncertain parameters are modelled by random fields to take into account the soil spatial variability. In such cases, Monte Carlo simulation (MCS) methodology is generally used in literature. Only the statistical moments were generally investigated in literature because of the great number of calls of the deterministic model required by this method. In this thesis, the subset simulation approach was first used as alternative to MCS methodology to compute the failure probability. This leads to a significant reduction in the number of calls of the deterministic model. Moreover, a more efficient approach called improved subset simulation (iSS) approach was developed to reduce again the number of calls of the deterministic model. The use of the iSS approach has reduced the number of calls of the deterministic model by about 50% with respect to the SS approach.

Contact: Professor A.-H. Soubra

Ground reinforcement anchors are a means of ensuring the stability of a wide range of structures. There are many ground anchorages installed worldwide in many different applications. Performance of an individual anchor depends on the tensile load that the anchor is carrying, and it is therefore necessary to have some way of measuring this over the anchor's lifespan. As it is impractical to assess this by measuring in situ failure loads, a need of assessing the loads within ground anchorages in a non-destructive manner after their installation is of great importance. GRANIT system (ground anchorage integrity testing) has been proven to be a reliable system to assess the integrity of rock anchorages. This research explores implementation of a non-destructive testing system at small scale size and full scale stress levels by means of a centrifuge modelling at the University of Dundee. Centrifuge modelling was undertaken to monitor and assess the dynamic response of soil anchorages installed in dry sand reinforcing a retaining wall in a 3 × 3 array. The practical importance of this research is that non-destructive testing may be usable to assess the soil anchors integrity to refine the relationship between both anchor load and geometrical characteristics with frequency response accomplished using centrifuge and numerical modelling.

Sponsor: NRP (North Research Partnership)

Contacts: Dr A. Ivanovic and Dr A. Brennan

Ground improvement work is crucial in enhancing the characteristics of weak soils commonly encountered in civil engineering, and one such technique commonly used is vibro-stone columns. An assessment of the effectiveness of such an approach is critical to determine whether the quality of the works meets the prescribed requirements. Conventional quality testing (laboratory and in situ) suffers limitations including: limited coverage (both area and depth), problems with sampling quality, are invasive and often destructive. However geophysical approaches offer a method by which improvement profiles can be measured in a cost effective way, overcoming these limitations. Of these seismic surface waves have proved the most useful to assess vibro-stone columns, however, to date much of the previous work conducted has focussed on field-based observations making detailed evaluation of this approach difficult. This study evaluates the application of surface waves in characterising the properties of laterally heterogeneous soil, specifically for using in the quality control of vibro-stone column. Three models were employed which began with a simple model and extended finally to complex model: (1) concrete mortar was used to establish the method, equipment and its system, (2) pilot test on a small scale soft kaolin to adopt a model vibro-stone column and (3) main test contained a configuration of vibro-stone column in soft Oxford clay. A generic scaled-down model of vibro-stone column(s) was constructed. Measurements were conducted using different arrays of column configuration, using sand to simulate stone material. This idealised set of laboratory conditions were used to provide guidelines for the interpretation of field measurements. The phase velocity obtained from the controlled tests showed close agreement to those reported in literature and with those generated through empirical correlations with vane shear test. The dispersive curve demonstrated an increased phase velocity with increasing wavelength for the measurements on the clay (between columns), and decreased phase velocity with increasing wavelength for the measurements on the column. More interestingly, the results showed that in the characterisation of lateral non-homogeneities, the phase velocity versus wavelength relationship varies on stone columns of different diameters and densities. This illustrated that the shear modulus profiles are influenced by the effective region that spans both the lateral and depth axes, and also demonstrated how the results can be influenced by the positioning of sensors with respect to the survey target. This research demonstrates how Rayleigh waves can be used for quality assurance when constructing vibro-stone columns.

Contact: Dr Ian Jefferson, Dr D. Chapman and Professor M. Culshaw

The application of chemical ground improvement using the electrokinetic stabilisation (EKS) method has the potential to treat soft highly compressibility soil, through the enhancement of strength and reduced compressibility of a wide range of soils. The aim of this study was to evaluate the use of EKS as an effective method to strengthen soft clay soils. A detailed laboratory programme of work was conducted using the initial base model developed by Liaki (2006). This study was conducted in two stages using laboratory scale models, using an inactive kaolinite clay. The test model using reusable electrokinetic geosythentic developed at the Newcastle University to apply a constant voltage gradient of 50 V/m across a soil sample approximately 400 mm. The first stage involved testing of a ‘pure' system with distilled water as the main pore electrolyte fluid supplied under zero hydraulic gradient conditions for periods of 3, 7 and 14 days. The second stage repeated test using calcium chloride and distilled water (CaCl-DW), DW and sodium silicate (DW-NaSiO) and CaCl-NaSiO, at the anode and cathode, respectively. Throughout both physical and chemical characteristics were measured. This enabled assessment of the design and configuration that could be used in the field together with a number of key limitations. The data gathered enabled a fuller understanding of the mechanisms contributing to the improvements achieved and how effective monitoring through the use of relatively simple tests – for example, pH and Atterberg limits – can be achieved. Specifically cation exchange was considered to be the main mechanism causing a significant increase in shear strength observed for the CaCl-DW system with the increase seen to be more uniform across the soil sample for a period up to 14 days. This was supported by results from the results of Atterberg limits, pH, electrical conductivity and chemical concentrations. Precipitation and cation exchange are the main contributors to the increase of shear strength for the CaCl-NaSiO system after 7 and 14 days' treatment. However, precipitation of calcium-silicate-hydrate gels has greater influence on the shear strength at the near proximity of cathode followed by cation exchange at the near proximity of anode and at the remainder of the soil sample.

Contacts: Dr I. Jefferson, Professor C. Rogers and Dr D. Boardman

A detailed examination of cone penetration testing in sands, clays, and intermediate soils (silty-sands, silts, clayey-sands, etc.) is presented. Effects of drainage condition on cone penetration testing in intermediate soils are evaluated through centrifuge experiments and numerical modelling of cone penetration as a cavity expansion process. A modified version of the MIT-S1 model is presented and its implementation into the commercial program FLAC (Itasca) is discussed and verified. Frameworks for estimating drained and undrained penetration resistances from cylindrical cavity expansion analyses with the modified MIT-S1 model are then presented and verified against available experimental data. These frameworks are subsequently used to evaluate a state-normalisation of cone penetration resistances in sands with fines. Clean sand corrections in terms of the state parameter are presented and the effects of initial state parameter and drainage condition on these correction factors are evaluated. Finally, liquefaction triggering curves of sands with fines are developed and compared to previously published relationships.

Sponsors: National Science Foundation (NSF) and United States Society on Dams (USSD)

Contacts: Professor J. T. DeJong, Professor R. W. Boulanger and Professor B. L. Kutter

Historical and recent earthquake events have proven that underground structures passing through liquefiable soil deposits are prone to floatation due to their lower unit weight as compared to the surrounding saturated soil. This research seeks to investigate the floatation mechanism of a rigid underground structure in liquefiable soil using the beam centrifuge at the University of Cambridge. An underground structure experiences an uplift force due to its buoyancy. Conversely, the overlying soil weight and the shear resistance developed in the soil inhibit the floatation. In the event of liquefaction, the soil loses most of its shear strength and the structure may float if there is a net uplift force. An additional force component is introduced to account for the excess pore pressure at the invert half of the circular structure. The force equilibrium is then considered to predict the uplift displacement of underground structures with a modified Newmark's approach. The prediction corresponds well with experimental uplift displacements in various conditions such as different peak earthquake acceleration, frequency and duration, different masses and diameters of structure, as well as different relative densities, types of sands and hydraulic conductivities of sand. The influence of the buried depth and diameter of the structure are also studied and depth-effect and diameter-effect ratios are developed. Similarly, the type of sand, hydraulic conductivity and relative density of the soil can affect the uplift response of the structure significantly. The influence of the soil-structure interaction on adjacent excess pore pressure was also investigated accompanied with particle image velocimetry analyses. With better understanding of the floatation mechanism, a novel remediation method is developed and has shown to be effective in mitigating the floatation of underground structures in liquefiable soil deposits.

Contact: Dr S. P. G Madabhushi

Heavily overconsolidated clays like London Clay present a challenge for constitutive models. Recent large scale geotechnical projects within the London Clay stratum prompted intensive research to fully characterise this soil type. These studies have highlighted several unique features of heavily overconsolidated clay that should be considered by comprehensive soil models, for example: stiffness anisotropy, strength anisotropy, destructuration and stress history effects. In this study, the BRICK (e.g. Simpson, 1992) and M3SKH models (Stallebrass, 1990; Grammatikopoulou, 2004) were enhanced to consider stiffness anisotropy. Anisotropy is included in BRICK through an evolving transformation of the strain space coordinate system, where as it is included in M3SKH by elastic anisotropy, rotated yield surfaces, and nonassociativity in the meridian plane. The performance of the new models is demonstrated through simulations of conventional and nonconventional laboratory tests. The importance of anisotropy for boundary value problems is demonstrated through two-dimensional/three-dimensional finite-element analyses of case histories, including construction of a three-pinned arch and embankment in Western London, excavation of the Heathrow Trial tunnel and construction of a deep excavation at King's Place with a single-layer prop system.

Sponsor: Arup

Contact: Professor K. Soga

Submarine landslides have significant impacts and consequences on offshore and coastal facilities. The unique characteristics of submarine landslides include large mass movements and long travel distances at very gentle slopes. This thesis is concerned with developing centrifuge scaling laws for submarine landslide flows through the study of modelling submarine landslide flows in a mini-drum centrifuge. A series of tests is conducted at different gravity fields in order to understand the scaling laws involved in the simulation of submarine landslide flows. The model slope is instrumented with miniature sensors for measurements of pore pressures at different locations beneath the landslide flow. A series of digital cameras are used to capture the landslide flow in flight. The depth averaged material point method (DAMPM) is used in the numerical simulations to deal with large. Parametric studies are performed to investigate the validity of the developed centrifuge scaling laws under the initial and boundary conditions given in the centrifuge tests. Both the results from the centrifuge tests and numerical simulations appear to follow the proposed centrifuge scaling laws.

Sponsors: Norwegian Geotechnical Institute

Contact: Professor K. Soga

The incorporation of reactive magnesia (MgO) in cementitious systems has recently received attention due to its significant potential in enhancing the mechanical and durability performance and sustainability credentials of those systems. This research focussed on the quantification of those impacts in cement-based pastes and concrete in terms of variables such as MgO source, content, cementitious blend composition, water/cement ratio, use of admixtures and curing and exposure conditions and linked them to microstructural observations. This work highlighted the enhanced performance, in terms of shrinkage reduction, strength enhancement and resistance to chemical attack that can be achieved with those blended systems.

Sponsors: Yunnan Construction Engineering Group, Yunnan Construction Concrete Co. Ltd, Kunming City Council, China, Cambridge Overseas Trust and David Ball Group

Contact: Dr A. Al-Tabbaa

Centrifuge experiments were conducted to investigate the mechanisms governing the settlement of shallow circular foundations on clay and saturated sand models. A poly(methyl methacrylate) window in the centrifuge package allowed digital images to be captured of a central cross-section, during and after footing loading. These were used to produce deformation mechanisms by particle image velocimetry for undrained penetration, consolidation due to transient flow and creep. An energy approach with the observed undrained mechanism was used to determine the load–settlement behaviour of circular shallow foundations on linear-elastic and nonlinear clays, with yield defined using the von Mises yield criterion.

Sponsors: Cambridge Australia Trust (Poynton Cambridge Australia Scholarship) and Overseas Research Students Awards Scheme

Contact: Professor M. D. Bolton

The project integrated deep soil mixing and artificial neural networks to develop correlations and predictions for the strength of cement-mixed soils with the many variables that affect it, focussing on the soil and binder properties and curing conditions. Extensive data, mainly for strength, from around 30 international projects, research and commercial, laboratory and field, was collated and analysed and Bayesian predictive artificial neural network models were developed. These correlations highlighted the significance of cement content, water to cement ratio, soil type and curing time and the results from the neural network analyses emulated the observed trends and confirmed their significance.

Sponsors: Cambridge Overseas Trust and Cambridge University Engineering Department

Contact: Dr A. Al-Tabbaa

In this thesis, a series of dynamic centrifuge experiments are discussed which aimed to establish how the distribution of axial loads along the length of a pile changes during a strong earthquake. In each test, a 2 × 2 pile group was installed such that its tips were embedded in a dense sand layer overlain by liquefiable soil. The tests examine the effects arising from the hydraulic conductivity in the bearing layer, the influence of axial pile cap support and differences in the behaviour of nominally jacked or bored piles under seismic loading. In tests representing bored piles, it is shown that where significant excess pore pressures are developed near the pile tips, then relative settlement between the soil surface and the pile group leads to very high proportions of the axial load transferring to the base of the pile cap. The very high stresses applied to the soil (and subsequently locked in) during the installation of jacked piles delays the build-up of excess pore pressures local to the piles, reducing the overall settlement of the pile group. However, in longer shaking events, the improvements in pile performance are lost and the behaviour of the jacked pile group becomes similar to the bored pile scenario.

Sponsor: Engineering and Physical Sciences Research Council (EPSRC)

Contact: Dr S. P. G. Madabhushi

Methane hydrate has been attracting international interest because of its potential to ensure energy security. However, to date, only one successful trial of gas production from hydrate-bearing sediments was reported and there are still large uncertainties over possible geotechnical problems associated with hydrate dissociation. This dissertation contributes to four advances in the field. First, a fully-coupled thermo-hydro-mechanical formulation is derived. Second, a constitutive model for hydrate-bearing sediments is established. Third, a mechanically-coupled sand production model is developed. Fourth, advantages of these models are thoroughly investigated by a series of analyses such as past and future field operation trials.

Sponsor: JOGMEC

Contact: Professor K. Soga

Reactive magnesia (MgO) cements have emerged as a potentially more sustainable and technically superior alternative to Portland cement due to their lower production temperature and ability to sequester significant quantities of carbon dioxide. Porous blocks containing MgO were found to achieve higher strength values than PC blocks due to the formation of various hydrated magnesium carbonates. This research work focuses on enhancing the sustainability of MgO-cement porous blocks by increasing the degree of carbonation through the investigation of different variables including cement and aggregate components, curing conditions, and inclusion of additives, all of which influence the carbonation process and mechanical performance.

Sponsors: Cambridge European Trust, David Ball Group and private funding

Contact: Dr A. Al-Tabbaa

Geotechnical design is the raison d'être of the geotechnical engineering profession. To design geo-structures, a solid understanding of the mechanisms of failure is essential. To most geotechnical engineers, failure still implies the attainment by soil of its entire available strength. What is often overlooked, not least in the development of codes of practice such as Eurocode 7, is that structural serviceability failures caused by small to moderate soil strains are far more common. Therefore, selection of representative soil stiffness values is the key to a meaningful geotechnical design calculation. A database of the stress–strain behaviour of clays and silts was compiled and analysed to construct general stress–strain curves for small strain and moderate strain responses and evaluate the uncertainty in their prediction. The mobilised strength design method offers a framework for ground displacement predictions based on moderate strain data. This was applied to the performance of bored piles, and verified using a published database of load-test information for London clay. The outcome is discussed in relation to soil mechanics analysis of pile capacity, and professional ideas of partial safety factors in codes of practice. It is argued that sustainability in geotechnical design requires objective and performance-based verifications, for which accurate assessments of nonlinear soil stiffness are essential.

Sponsors: Ove Arup & Partners and Cambridge Commonwealth Trust

Contact: Professor M. D. Bolton

Buried pipelines may be subject to monotonic uplift and cyclic ratcheting hazards if depth of the soil cover is insufficient. As the buckling load of a strut decreases with increasing out-of-straightness, not only the maximum available resistance from the soil cover but also the movement of the pipeline required to mobilise this are important for design. Full-scale and centrifuge testing results suggest that both can be predicted based on geometry and soil properties, and that a displacement-based approach should be adopted for safe design. The reliability of centrifuge testing in modelling pipeline upheavel behaviour is also discussed.

Sponsor: IGS from Trinity College, Cambridge

Contact: Dr S. Haig

The continuous helical displacement (CHD) pile is an auger displacement pile developed by Roger Bullivant Ltd in the UK. The CHD pile is installed in situ through the use of a drilling auger and has performance characteristics of both displacement and non-displacement piles It is known that the load capacity performance of the CHD pile significantly exceeds the current design predictions. Model CHD piles were created in pluviated test beds and compared to model displacement and non-displacement piles. It is shown that CHD piles have a similar ultimate capacity to displacement piles. Model instrumentation allowed load distribution throughout the pile length to be determined. The CHD design parameters are found to have a lower bearing capacity factor and higher earth pressure coefficients than current suggestions. Through the use of a model cone penetration test probe, the CHD pile was found to cause significant changes in soil relative density laterally around the pile shaft. The CHD pile is found to cause a densification of the in situ soil for all relative densities with the greatest increase occurring in loose sand. The ultimate capacity of the CHD pile is determined from load tests carried out on field CHD piles with the aid of capacity prediction methods for piles not loaded to ultimate capacity. The results from model testing were applied to field pile tests to allow the development of design parameters including appropriate pile diameter, bearing capacity factor N_q and the earth pressure coefficient k which are suitable for CHD piles.

Sponsors: Engineering and Physical Sciences Research Council and Roger Bullivant Limited

Contact: Dr M. Brown

The work explores, experimentally, the influence of evolving anisotropy on the stress-strain behaviour of unsaturated soils and proposes a new constitutive elasto-plastic model for unsaturated soils accounting for evolving anisotropy. A programme of controlled suction probing triaxial testing was performed on unsaturated and saturated samples of Speswhite kaolin prepared by two different methods of compaction: isotropic and anisotropic. Experimental results were interpreted in terms of mean net stress, deviator stress and suction as stress state variables and, alternatively, interpreted in terms of mean Bishop's stress, deviator stress and modified suction (a function of suction). The experimental results showed that fabric anisotropy can evolve during plastic straining and that no apparent influence of anisotropy on the critical state. The constant suction cross-sections of the yield surface can be represented by distorted ellipses in the deviator stress-mean net stress plane, intersecting the negative axis at the point of intersection of the corresponding critical state line, and, alternatively, can be represented in the deviator stress-mean Bishop's stress plane by distorted ellipses passing through the origin. A new constitutive model was formulated in terms of Bishop's stresses and modified suction. Model simulations showed significant improvement in the accuracy of the predicted soil behaviour.

Contacts: Professor D. Gallipoli and Professor S. Wheeler

Two laboratory tests using image-based methods to determine particle size distribution of soil were developed. The sediment imaging or ‘sedimaging' test determines size distribution of soil having particle diameters between 0·075 mm and 2 mm. The test utilises a statistical method based on wavelet transformation. The method requires images of relatively uniform particle sizes, thus sedimentation of a soil specimen through a column filled with water is implemented to segregate particles by size. An image of the sedimented soil is analysed incrementally by wavelet transformation of overlapping 128 × 128 pixels areas. A translucent segregation table (TST) test determines size distribution of soil having particle diameter larger than 2 mm. The test utilises a deterministic method facilitated by watershed segmentation. The method requires thresholded images, thus a translucent plate and a backlight table are implemented to provide a bright and uniform grayscale contrast to the soil particles. Particles are introduced at the top of the inclined TST and are allowed to pass beneath the series of bridges having decreasing underpass heights. Touching particles in an image of roughly segregated particles are segmented by the watershed segmentation. Both tests produce particle size distributions that compare well with results by sieving.

Sponsors: National Science Foundation and Michigan Department of Transportation

Contact: Professor R. D. Hryciw

The main objective of this thesis research is to develop simple and reliable techniques, models or approaches that can be used in geotechnical engineering practice to estimate soil suction and the mechanical properties of unsaturated soils. In the first part, simple techniques are proposed to estimate the suction values of as-compacted unsaturated fine-grained soils in the range 0–60 000 kPa using a pocket penetrometer and a conventional tensiometer. In the second part, approaches or semi-empirical models are proposed to estimate the variation of mechanical properties of unsaturated soils with respect to suction, which include: (a) bearing capacity of unsaturated fine-grained soils; (b) variation of bearing capacity of unsaturated fine-grained soils with respect to matric suction; (c) variation of initial tangent elastic modulus of unsaturated soils below shallow foundations with respect to matric suction; (d) variation of maximum shear modulus with respect to matric suction for unsaturated non-plastic sandy soils. In the third part, approaches are suggested to simulate the vertically applied stress versus settlement behaviour of shallow foundations in unsaturated coarse-grained soils assuming elastic-perfectly plastic behaviour. These methodologies are extended to simulate the stress versus settlement behaviour of both model footings and in situ plates in unsaturated coarse-grained soils. The models (or methodologies) proposed in this thesis research are promising and encouraging for modelling studies and practising engineers to estimate the variation of mechanical behaviour of unsaturated soils with respect to matric suction.

Contact: S. K. Vanapalli

Subsea on-bottom (partially buried) pipelines, which carry hydrocarbons across the seabed, are typically subject to axial compressive loading due to heating and internal pressurisation. Predictions of the response of pipelines to this axial loading tend to be made with the aid of finite-element analysis, in which the interaction between the pipeline and the seabed is accounted for by a macro-element, or lumped-parameter, model. In this thesis, a new macro-element model has been developed for on-bottom pipes on sandy (drained) seabeds. The model is of the strain-hardening plasticity class and its various components have been calibrated against data derived from both continuum-based numerical analyses and experimental (laboratory floor) testing. The key novelty of the model is its ability to predict the evolution of the loading imparted onto the pipeline during cycles of large-amplitude lateral displacement in which substantial changes in the local seabed topography occur (e.g. berm formation). As such, the model is appropriate for use in lateral buckling analyses, for which the peak lateral displacement at the crown of a buckle lobe is typically 10–20 times that of the pipe diameter. Results of structural analyses using the model are presented to show its predictive capabilities and practical use.

Sponsors: EPSRC and ConocoPhillips (U.K.) Limited

Contacts: Dr C. M. Martin, Professor G. T. Houlsby, Dr B. W. Byrne and Professor S. Bhattacharya

Until recently, there were two types of man-made rock slopes: civil engineering- and mining/quarrying-related. The factors differentiating these two slope types are relative life span, safety requirements and construction cost. For civil engineering slopes, these requirements are high, whereas for mining slopes they are usually lower. The tightening of UK/EU environmental legislation has created a third category of rock slope: the quarry faces produced by or remaining after the end of the extraction operation in areas of outstanding natural beauty and national parks. Such slopes have life spans and safety requirements similar to civil engineering slopes but with the construction cost of mining-engineered slopes. Due to the fact that a considerable amount of time may elapse between the end of the extraction operation and a quarry's potential or intended after-use, quarry operators want to know what will happen to the faces if they remain free of conventional support methods. Consequently, the aim of this project is to asses the impact of weathering agents on the stability of different types of carboniferous limestone quarry faces in the Peak District National Park, UK.

Two methodologies were followed. These comprised: first, in situ monitoring of the stability of selected quarry slopes and second, accelerated weathering tests on rock samples collected from the monitored slopes, and measurement of some of their mechanical and physical properties. Only cost-effective techniques that could be readily employed by the quarry industry were used. From comparison of the resultant data, the main conclusion is that, in contrast to the findings of published research into the rate of weathering of rock materials, carboniferous limestone rock faces at the sites studied do not display a linear decline in their mechanical and physical properties due to weathering processes. Rather, they either display no significant alteration or an alteration following a concave trend, with an initial decline and subsequent increase in their properties. This means that their stability does not deteriorate at a constant rate over time. The implications of this for the selection of appropriate restoration methods are discussed and some alternative strategies are proposed.

Contact: Dr J. Cripps

Pile heat exchangers are expected to make a significant contribution to meeting UK and EU renewable energy and carbon dioxide reduction targets. However, design for the thermal capacity of pile heat exchangers has to date been largely based on methods developed for borehole heat exchangers. Piles, however, have a different geometry and consequently their thermal behaviour is different in a number of important ways. Short-term pile behaviour is controlled by the size of the pile and the internal arrangement of heat transfer pipes, while long-term behaviour is controlled by the aspect (length to diameter) ratio of the pile. Temperature changes inside the pile are often characterised by a thermal resistance. Simple two-dimensional numerical modelling has been used to derive a new method for calculating pile resistance. It has also shown that for large-diameter piles a steady state resistance is inappropriate due to the length of time taken to reach steady state. This short-term transient behaviour also means that standard short-duration thermal response testing will return unreliable results for larger diameter piles. Consequently the use of thermal response tests is recommended to be restricted to piles of 300 mm diameter or less.

Sponsors: Mott MacDonald and EPSRC

Contact: Professor W. Powrie

Numerical algorithms to implement an advanced nonlinear constitutive model, S-CLAY1S, for natural soft clay into PLAXIS are studied in order to reduce the numerical instability. Simulations performed using several triaxial tests and a benchmark test indicate that the use of an automatic substepping modified Newton–Raphson (MNR) algorithm can eliminate or minimise the problem. Therefore, this algorithm is adopted to implement S-CLAY1S into PLAXIS as a user-defined soil model. The modified S-CLAY1S model was proposed by incorporating Lode angle dependency failure criterion. This has improved the model predictions in failure condition especially in extension. A hybrid implicit numerical algorithm for the Sekiguchi–Ohta (SO) inviscid/viscid models capable of robustly overcoming the vertex singularity is presented. The proposed hybrid algorithm consists of modified Newton–Raphson algorithm and Stolle's algorithm, used to implement the SO models into PLAXIS. Numerical simulations and benchmark test demonstrate the algorithm's accuracy and numerical stability even for large strains. The main part of this thesis is the development and implementation into finite-element (FE) code of a new constitutive model to represent the cyclic behaviour of natural soft clays. The S-CLAY1S model has been extended to the bubble surface plasticity in which smooth transitions between elastic and plastic behaviour can be modelled. The model developed in this research is named ‘B-SCLAY1S' and requires 13 parameters. Numerical examples show that the implementation into PLAXIS of the present model is successful. Finally, applications of the B-SCLAY1S model are made in two levels. First, simulations and comparison with laboratory tests including kaolin, Swiss lacustrine clay and Bothkennar clay test data. Second, the FE benchmark analysis of an embankment and a pile driving problem subjected to cyclic loading were presented. At both levels the B-SCLAY1S is shown to be superior to the S-CLAY1S models, mainly because it captures the small strain behaviour.

Sponsors: University of Strathclyde, Overseas Research Students Awards Scheme (ORSAS), Marie Curie Research Training Network on Advanced Modelling of Ground Improvement on Soft Soils (AMGISS: MRTN-CT-2004-512120) and Industry–Academia Partnerships and Pathways (IAPP) pr

Contacts: Professor M. Karstunen and Dr M. Kenny

Geomaterials can be often classified into two groups: virgin geomaterials such as soil and rock, and by-product materials such as mine tailings, coal fly/bottom ash, foundry sand, kiln dust, blast furnace/steel slag, reclaimed concrete and asphalt. Studies on these materials and their mixtures have been carried out extensively for geoengineering applications, including the characterisation of mechanical properties such as the strength, compressibility, compactivity and permeability, as well as mineralogical and geochemical properties. The goal of this study is to investigate the thermal and electrical properties of selected geomaterials and their mixtures for enhancement of knowledge and engineering applications. The thesis consists of three parts as follows. The first part presents the electrical conductivity measured on compacted kaolin clay using a circular two-electrode cell in conjunction with a specially designed compaction apparatus, which has the advantage of reducing errors owing to sample handling. The experimental results are analysed to observe the influencing factors on the soil electrical conductivity. The performance of existing analytical models for predicting the soil electrical conductivity is evaluated by calculating empirical constants in these models. The second part investigates the thermal and packing behaviours of mine tailings and tire crumbs mixtures in dry and wet states, which are important in engineering applications for utilizing recycled tire particles as lightweight fill materials with improved thermal insulation. The thermal and packing properties measurements of the mine tailings and tyre crumbs mixtures with different mixing ratios are presented, which are then analysed to establish the thermal properties relationships with respect to the mixing ratio of tyre crumbs, porosity, bulk density, water content, compactive effort and tire crumbs size. The multiple linear regression analysis shows that the thermal conductivity of the dry and wet mixtures can be estimated using a general model consisting of the factors affecting the thermal conductivity. The third part evaluates the thermal conductivity, compressive strength, elastic modulus and temperature changes of compacted mine tailings and fly ash mixtures during the curing period as functions of the fly ash ratio, molding water content and compaction energy. The microporosity structure of the fly ash treated mine tailings is also examined.

Sponsor: National Science and Engineering Research Council of Canada (NSERC)

Contact: Dr J. Q. Shang

This thesis covered the laboratory tests and numerical modelling. The behaviour of soft clays under cyclic loading with or without radial drainage was investigated. First, a series of undrained cyclic triaxial tests was conducted on specimens of reconstituted kaolinite at three cyclic stress ratios and four loading frequencies and their influence on the progressive shear failure and excessive plastic deformation were justified. An undrained cyclic model has been proposed based on the modified Cam-clay model to capture the soil behaviour under cyclic loading. Large scale cyclic triaxial tests were conducted. To allow for radial drainage during and after the cyclic tests, a single prefabricated vertical drain (PVD) was installed in the centre of the soil cylinder. The effectiveness of radial drainage in dissipating the excess pore pressure was examined. A radial consolidation model under cyclic loading has been proposed to capture the behaviour of soft clays subjected to cyclic loading when radial drainage is allowed during the loading period. This model was further verified against a field case history in Sandgate, between Maitland and Newcastle. It was indicated that the stability of the soft soil subgrade could be improved effectively by the installation of PVDs.

Sponsor: University of Wollongong

Contacts: Professor B. Indraratna, Dr X. Geng and Dr C. Rujikiatkamjorn

This research aims to study how the interface between ballast and geogrid copes with fouling by coal fines. The shear stress–displacement behaviour of fresh and fouled ballast, and ballast reinforced with geogrids was investigated through a series of large-scale direct shear tests where the levels of fouling ranged from 0% to 95% void contamination index (VCI). The results indicated that inclusion of geogrids increases the shear strength and apparent angle of shearing resistance, while only slightly reducing the vertical displacement of the composite geogrid-ballast system. A novel track process simulation apparatus was used to simulate realistic rail track conditions subjected to cyclic loading. The discrete-element method (DEM) was used to study the shear behaviour of fresh and fouled ballast in direct shear testing. The DEM simulation highlights the fact that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing of VCI. Furthermore, the distribution of contact force chains and particle displacement vectors clearly explains the formation of a shear band and the evolution of volumetric change during shearing.

Sponsor: Cooperative Research Centre for Rail Innovation (Australia)

Contacts: Professor B. Indraratna and Dr C. Rujikiatkamjorn

This thesis presents the studies relating to filter and filtration problems, namely time-dependent filtration processes of a given combination of base and filter soils, suffusion mechanism of filters and determination of the saturated hydraulic conductivity of filters. First, the time-dependent filtration process was studied by incorporating superior tools related to the pore network of filters and the transport of solid particles. The outcome of this model can be used to evaluate the effectiveness of filters through the analysis of the accumulation of base soils or the flow rate which indicate whether the layers within the filter are self-filtering or unstable. Second, a new method that includes the bimodal structure of suffusion soils was proposed. A procedure named ‘delimiting particle size' was introduced to consider the porosity of the finer and courser fractions of suffusion soils. The constriction concept was also applied to establish a criterion for assessing whether or not a given soil possesses the potential for suffusion. Finally, a new equation that enables the estimation of the saturated hydraulic conductivity based on the pore network of soil, represented by its constriction sizes was proposed. The predictions showed better agreement with the experimental results in comparison with existing approaches.

Sponsors: Vietnamese government

Contacts: Professor B. Indraratna and Dr C. Rujikiatkamjorn

The effectiveness of a permeable reactive barrier (PRB) to remediate contaminated groundwater from acid sulphate soil (ASS) on the Shoalhaven floodplain (Australia) was investigated. This study combined laboratory, field and numerical analyses to determine the feasibility and performance of a PRB utilising zero-cost recycled concrete. Long-term laboratory column experiments were carried out using synthetic and real groundwater from the study site. The column experiments investigated the acid neutralisation reactions occurring within the PRB and the precipitation of aluminium and iron from the acidic groundwater. Chemical armouring and physical clogging were also studied by evaluating the duration of buffering periods for maintaining neutral pH and also the changes in hydraulic conductivity and flow rate due to mineral precipitation. The novel use of three-dimensional image analysis allowed for the examination of the porous architecture, and the assessment of the performance of PRB in a quantifiable manner. Finally, the neutralisation mechanisms and geochemical evolution of groundwater along a flow path inside the PRB were modelled by MIN3P. The ability to make comparisons between the geochemically complex transport in the laboratory and field PRB demonstrates its use as analysis tool for investigating the performance of PRBs in ASS terrain.

Sponsors: Australian Government Endeavour Scholarship

Contacts: Professor B. Indraratna and Professor L. Nghiem

In this research, an investigation using the large-scale direct shear apparatus was carried out to study the ballast–geogrid interface shear behaviour and establish the effect of geogrid aperture size on the interface shear strength. A process simulation test (PST) apparatus was designed and the model track tests, instrumented with optical-fibre Bragg grating (FBG) sensors, were conducted to assess the influence of geogrid on the deformation and degradation of ballast. The large-scale direct shear tests reveal that the normalised geogrid aperture size (A/D₅₀) has a profound influence on the shear strength of the ballast–geogrid interfaces. The best geogrid aperture size to optimise the interface shear strength was determined to be 1·20D₅₀. The model track tests reveal that the geogrid successfully arrests the lateral strains in ballast, and that the ideal geogrid placement location to effectively stabilize the track is a function of A/D₅₀ ratio. The lateral strain profiles along the ballast depth were captured and the geogrid influence zone to assess the performance of geogrid-reinforced ballast was determined. The study further highlights the ability of FBG sensors to capture the internal deformations in ballast thereby encouraging their use in the monitoring of track stability under operating conditions.

Sponsor: Cooperative Research Centre for Rail Innovation (Australia)

Contacts: Professor B. Indraratna and Dr J. S. Vinod

The thesis is aimed at studying hydro-mechanical behaviour of ballast subject to fouling. A series of large scale hydraulic conductivity tests were conducted with different proportions of fouling to study the relationship between the extent of fouling and hydraulic conductivity. To investigate two-dimensional flow conditions which may prevail in reality, a numerical analysis was conducted using SeepW (2007a) to quantify the drainage capacity of ballast under different degrees of fouling. Subsequently, a quantitative classification for drainage in relation to the degree of fouling is presented. In order to establish the relationship between the extent of fouling and the associated strength–deformation properties, a series of large scale (300 mm diameter by 600 mm height) monotonic and cyclic triaxial tests was carried out for different levels of fouling for confining pressures in the range 10–60 kPa. Based on the laboratory findings, a novel empirical relationship between the peak deviator stress and void contamination index has been proposed to assist the practitioner in their preliminary track condition assessment. A constitutive model for clay fouled ballast is formulated using bounding surface framework under monotonic loading and drained condition. The model is validated with the large-scale triaxial experiments carried out in this research.

Sponsor: Cooperative Research Centre for Rail Innovation (Australia)

Contacts: Professor B. Indraratna, Dr C. Rujikiatkamjorn, Dr S. Nimbalkar and Dr M. Liu