The purpose of this study is to obtain an insight into the mechanisms by which a geosynthetic membrane influences the performance of a plane strain and an axisymmetric two-layer soil system, where the reinforcement is incorporated either into a layer of fill, or at the interface of a layer of fill overlying clay subgrade. New axisymmetric membrane and interface element formulations are developed and incorporated in to an existing large strain finite element code. A linear elastic model of behaviour is used for the membrane material and an elastic-perfectly frictional model, based on the Mohr-Coulomb yield function, is implemented for the interface. These new formulations both take account of large global displacement and rotation effects, although the interface element is constrained to small relative displacements, and are checked against small and large strain closed form test problems. The finite element equations are based on an Updated Lagrangian description of deformation. Plane strain finite element investigations into the significance of the resolution and relative size of the finite element mesh, and the differences between large and small strain analyses, are undertaken. For typical unreinforced and reinforced plane strain and axisymmetric two- layer soil systems a detailed analysis is presented of the soil displacements, strains, stresses, principal stress directions, mobilised fill friction angles and the stresses on the underside of the footing. A series of plane strain and some axisymmetric parametric studies of the various material properties is conducted, to assess the influences and relative importance of those variables to the performance of the two-layer soil system under monotonic loading. The study considers various reinforcement lengths and stiffnesses, fill depths and strengths, and different clay strengths. The mechanisms of reinforcement are identified through careful examination of the footing load-displacement response, the reinforcement tension and the stresses and displacements at the interfaces with the surrounding soil. A comparative study is also undertaken between the results obtained by the finite element model and those predicted by a plane strain and axisymmetric limit equilibrium design method. The effects of including a low friction membrane within an oil storage tank base, as secondary containment against oil leakage, are investigated by a series of axisymmetric finite element analyses.

A series of finite element predictions of the behaviour of a reinforced unpaved road consisting of a layer of fill compacted on top of a clay subgrade with rough, thin reinforcement placed at the interface, is described in this thesis. These numerical solutions are obtained using a large strain finite element formulation that is based on the displacement method, and are restricted to the case of plane strain, monotonic loading. Separate elements are used to model the soil and reinforcement. In the finite element formulation, an Eulerian description of deformation is adopted and the Jaumann stress rate is used in the soil constitutive equations. Elastic perfectly-plastic soil models are used which are based on the von Mises yield function for cohesive soil and the Matsuoka criterion for frictional material. Emphasis is placed on obtaining new closed form solutions to parts of calculations that are performed numerically in many existing finite element formulations. The solution algorithm is based on a "Modified Euler Scheme". The computer implementation of the formulation is checked against an extensive series of test problems with known closed form solutions. These include the analysis of finite deformation of a single element of material and the calculation of small strain collapse loads. Finite cavity expansion is also studied. This numerical formulation is used to perform back analyses of a series of reinforced unpaved road model tests. The reinforcement tensions, and the stresses at the interface with the surrounding soil, are calculated using the numerical model and discussed with a view to identifying the mechanisms of reinforcement. Two existing analytical design models of the reinforced unpaved road are described and critically reviewed in the light of the finite element results.

Offshore structures are used in a variety of applications ranging from the traditional oil and gas extraction facilities to emerging renewable energy concepts. These structures must be secured to the seabed in an efficient and cost effective manner. A novel approach is to use shallow inverted buckets as foundations, installed by suction, in place of the more usual piles. These foundations lead to cost savings through reduction in materials and in time required for installation. It is necessary to determine how these foundations perform under typical offshore loading conditions so that design calculations may be developed. This thesis presents experimental data from a comprehensive series of investigations aimed at determining the important mechanisms to consider in the design of these shallow foundations for dense sand. Initially the long term loading behaviour (e.g. wind and current) was investigated by conducting three degree of freedom loading {V:M/2R:H} tests on a foundation embedded in dry sand. The results were interpreted through existing work-hardening plasticity theories. The analysis of the data has suggested a number of improved modelling features. Cyclic and transient tests, representing wave loading, were carried out on a foundation embedded in an oil saturated sand. The novel feature of the cyclic loading was that a 'pseudo-random' load history (based on the 'NewWave' theory) was used to represent realistic loading paths. Of particular interest was the tensile load capacity of the foundation. The results observed suggested that for tensile loading serviceability requirements rather than capacity may govern design. Under combined-load cyclic conditions the results indicated that conventional plasticity theory would not provide a sufficient description of response. A new theory, termed 'continuous hyperplasticity' was used, reproducing the results with impressive accuracy. Surprisingly, under the conditions investigated, loading rate was found to have a negligible effect on response.

A laboratory experimental programme was carried out to investigate the behaviour of the piezocone in lightly over consolidated clay. The clay samples were prepared from reconstituted kaolin. The powdered form of this clay was mixed with water to form a slurry at a moisture content of approximately 120% under vacuum. The clay slurry was consolidated and then allowed to swell in chambers which were well instrumented. Penetration tests were carried out with two sizes of piezocones. The smaller piezocone had a cross sectional area of 1cm² with pore pressure measurement at the cone shoulder. The larger piezocone with a cross sectional area of 5cm² had pore pressure measurements at four locations. In addition to penetration testing, further information on the strength and consolidation characteristics of the test chamber samples was obtained form shear vane, restricted flow consolidation, flow pump permeability and consolidated undrained triaxial tests. The horizontal effective stress and the vertical stress ratio were found to influence the generated excess pore pressure at all four different locations on the piezocone shaft and the net tip resistance. This led to the establishment of direct correlations for the tip resistance factor Q with the coefficient for lateral earth pressure at rest Ko and the vertical effective stress ratio (OCR). Direct correlations for the excess pore pressure ratio at all four measuring positions with Ko and OCR were also established. Several methods of estimating over consolidation ratio were examined. One of the factors examined was Bq which was found to correlate poorly with OCR before tending asymptotically to a value of approximately 0.4. Other examples of factors which showed promising results as estimators of OCR were Bmi, the excess pore pressure ratio, and Q, the tip resistance factor. The excess pore pressures well behind the cone shoulder, which are partly generated by the shear stress in the area, showed promising correlations with the undrained shear strength. However, because correlations with pore pressure on the piezocone are dependent upon the position of the filter element, the particular results from this series may only be used for piezocones whose filter elements are at equivalent positions. The undrained strength ratio was also correlated with the tip resistance factor Q and the result was very promising for future interpretation of piezocone data.

The cone-pressuremeter is a new site investigation device which incorporates a pressuremeter behind a standard cone penetrometer tip. This dissertation is concerned with an understanding of the new device, and in particular the establishment of a detailed procedure to allow the test to be used to determine the engineering properties of cohesionless soils. A series of 34 calibration tests was performed, in which three cone-pressuremeter prototypes with cross-sectional areas corresponding to 15cm², 10cm² and 5cm² were used. The tests were carried out on cylindrical samples, enclosed in a chamber 1.0 m in diameter and 1.5 m in height. Stress controlled boundaries allowed independent control of vertical and horizontal stress in the range of 50 kPa to 300 kPa. A raining deposition technique was used to produce three sand densities, corresponding to loose, medium and dense samples. A programme of calibration was designed to provide an examination of the influence of relative density, stress level and stress ratio on cone-pressuremeter test data. Soil properties were related to the values of the limit pressure obtained from the pressuremeter test and the cone resistance values from the cone test. Empirical relationships were proposed for deriving density, friction angle and horizontal stress. Cone-pressuremeter tests were used for assessing directly the shear stiffness of the soil. A series of calibrations was needed to obtain the best possible estimation of the unload-reload shear modulus. Interpretation of the measured modulus was made by examining appropriate methods of calculating the modulus from unload-reload loops. Strain arm measurements and volume change measurements were compared. A method has been presented that allows the shear modulus values to be correlated to the relevant stress level acting around the pressuremeter during the test. The values of shear modulus obtained with the cone- pressuremeter were compatible with those obtained from the self-boring pressuremeter. An assessment of chamber size effects yielded useful information regarding the applicability of test correlations derived from calibration chambers to field problems. An additional study identified experimentally the influence of length to diameter ratio on the pressuremeter pressure-expansion curve.

This thesis is concerned with four different types of offshore foundations, namely gravity foundations, jack-up foundations, the mudmats for piled jacket structures and caisson foundations for jacket structures. In most applications, these can be idealised as circular rigid foundations. Unlike onshore foundations, offshore foundations are subjected to large horizontal and moment loads. This research used the finite element method to examine the elastic behaviour and stability of circular footings under combined loading. Due to the circular shape of the footings and the combination of vertical, horizontal and moment loads, three dimensional finite element analysis was used. In-depth analyses of the elastic behaviour of circular footings under combined loading (V,H,M) were performed. The vertical stiffness coefficient was investigated using two dimensional axi-symmetric analyses whereas three dimensional analyses were used to examine the other coefficients. Different features of offshore foundations such as footing embedment and cone angle were taken into consideration. Based on the numerical results, a set of empirical expressions for elastic stiffness coefficient was derived. For footing stability calculations, large horizontal or moment loads can cause the footing to lose contact with the soil, or cause the footing to slide relative to the soil. In finite element analyses, this loss of contact and sliding are modelled by interface elements. A new zero-thickness iso-parametric interface element was formulated for both two and three dimensional analyses. An exact close formed solution for integration of the stress-strain relationship (for the two dimensional interface element) was found. The element is then used to explore footing stability. It was shown that by using a yield criteria which allows the interface to behave as either frictional or cohesive interface, depending upon the normal stress, numerical stability is achieved. The footing stability was examined by establishing the bearing capacity envelope. The envelopes for footings on undrained clays were established for surface flat strip footings and for surface flat circular footings. The effects of soil strength varying with depth, cone angle and embedment on the bearing capacity envelope were also investigated.

The results of a series of over 120 model tests to study the end bearing capacity of piles in layered calcareous soils are described. The tests were carried out on samples enclosed in a cylindrical testing chamber, 450 mm diameter and 450 mm high, which allowed independent control of horizontal and vertical stress in the range 25 kPa to 500 kPa. The samples consisted of a loose, uncemented calcareous sand consisting predominantly of foraminifera and mollusc micro-organisms (D50 = 0.2 mm, calcium carbonate content 92%). Into this was built a layer of the same material artificially cemented by a gypsum plaster. The layer had similar properties to naturally cemented deposits, and layers with unconfirmed crushing strengths in the range 500 kPa to 4000 kPa have been prepared. All samples were tested dry. Closed end model piles of 16mm diameter were jacked at 0.1mm/s into the sample, and continuous profiles of end bearing capacity obtained during penetration. A parametric study has been carried out to examine the effects on the bearing capacity of stress level, K0, cemented layer thickness (0.5 pile diameters to 5.0 pile diameters) and layer strength. In addition, tests have been conducted with different pile geometry, multiple cemented layers, and using dynamic installation techniques. The study has identified ranges of parameters for which brittle failure of the cemented layer occurs (low stress levels and high layer strengths) and ranges where the failure is ductile (high stresses and low layer strengths). Characteristic patterns have been observed of the variation of end bearing with position as a layer is penetrated. Examination of the samples after testing has revealed details of failure mechanisms. Simple procedures are proposed for modelling the bearing capacity of such layered systems, and some implications of the results for design methods are discussed.

This research is a theoretical, experimental and numerical study of the one-dimensional deformation of suspensions. The study is focussed on the transition between sedimentation and consolidation, and creep during soil consolidation. In the literature, sedimentation, traditional large strain consolidation and creep are explored independently. The theory of sedimentation has been derived in parallel with the mathematical description of shock waves. The large strain consolidation theory of Gibson et al. (1981) has been adopted, and attention is given to the material properties of compressibility and permeability. Traditionally creep has been studied on thin samples, and a review is given to identify parallels with creep behaviour of the thick samples studied here. The experimental work was carried out in the laboratory using settling column tests. During the sedimentation stage, when the soil particles are fluid supported, shock waves were monitored and tracked by means of an X-ray absorption technique to allow for the calculation of experimental flux functions. Settling column experiments on different natural soils have been performed to study the consolidation behaviour by means of the measurement of pore water pressure and X-ray density measurements. An in-depth study of the development of effective stress has been performed to quantify the creep behaviour of the soils studied in a strain rate surface. The sedimentation equation is classified as a hyperbolic partial differential equation. In this kind of equation, discontinuities can propagate, and standard solution methods, eg finite differences, fail to give adequate results. For this reason codes have been developed using the finite volume method (FVM) to solve the sedimentation equation numerically. A standard numerical code has been developed for the solution of the large strain consolidation equation, while for the unified sedimentation-consolidation model the finite volume method (FVM) has been used. The shock waves monitored in the experiments are successfully predicted by the sedimentation model using experimentally derived flux functions. This study made it possible to formulate a physically and mathematically correct definition of the transition from sedimentation to consolidation. The strengths and weaknesses of the traditional large strain consolidation model have been identified by means of an international Class A prediction seminar. A new unified sedimentation-consolidation model is proposed using a flux function, a permeability relationship and a strain rate surface as material functions. Successful predictions of experiments have been performed, showing the transition from sedimentation to consolidation and the inclusion of creep.

Previous experimental studies of soil-reinforcement interaction have generally concentrated on the effect of reinforcement working in axial tension; this study looks at reinforcement working in bending and shear. The experimental programme was carried out in a large scale direct shear apparatus able to contain a cubic soil sample of side 1m. A previous study showed that the apparatus required improvements to its boundaries. Modifications to the apparatus resulted in a significant improvement in the performance of the apparatus. The data being comparable with those from direct shearboxes with similar symmetrical boundary conditions. The effect of reinforcement in shear and bending was studied by varying the reinforcement cross section reinforcement orientation, method of installation, and the relative soil- reinforcement stiffness and strength. All tests were carried out on a well graded and uniform quartz sand. The reinforcement was typically mild steel circular bar. Data from tests on instrumented reinforcement bars allowed the distribution of lateral loading to be observed. This led to the development of a mathematical model for predicting the shear force available from reinforcement in soil. A comparison of this model with the test data and from data in the literature revealed it to provide an accurate upper estimate of reinforcement shear force are much greater than those required for axial force. The conclusions in this dissertation address much of the ambiguity over the use of soil reinforcement in shear and bending for soil nailing and dowelling design.

The research described in this dissertation follows on from the study made by Jewell (1980)into the effects of tensile reinforcement on the mechanical behaviour of sand. For this study Jewell used the direct shear test with reinforcement placed about the central plane as shown in fig. 1.1. The direct shear test was chosen for the following reasons. (1) The reinforcement variables could be better controlled and examined in a unit cell test than in modular field studies of soil reinforcement systems. (2) The pattern of deformation is similar to that experienced by soil in which a rupture band develops, with the principal axes of stress, strain and strain increment free to rotate as is the case in model and field structures. (3) The overall shear strength of the sample is measured directly at the boundaries of the apparatus. The direct shear tests were monitored by boundary measurements and internal measurements using a radiographic technique. The findings are outlined below with reference made to relevant observations by other researchers. 1) The optimum orientation for a relatively flexible steel grid was found to be approximately along the direction of principal tensile strains in the unreinforced sand, see fig.1.2. This indicated that the reinforcement functioned by limiting tensile strains in the sand. McGown et al. (1978) obtained a similar result for plane strain cell tests on sand containing a single layer of flexible reinforcement. However in both studies the reinforcement was observed to waken the sand. Jewell recognized weakening to occur when the steel grid was placed along the direction of principal compressive strains in the unreinforced sand. This was attributed to a reduction in vertical effective stress. McGown et al. observed weakening of the sand when the reinforcement orientation approached the rupture band which developed in the sand alone. This was recognized to be the direction of zero-extension in the unreinforced sand. The weakening was linked to a lower bond between soil and reinforcement than soil alone. 2) Internal strains determined by Jewell showed the tensile reinforcement modified strains in the sand over a well defined zone, see fig.1.3. This resulted in a significant rotation of principal axes of strain increment, with the bond of major strains which developed across the centre of the box in the unreinforced sand being prohibited from forming. This agreed with boundary measurements, indicating the reinforcement functioned by limiting tensile strains in the sand. Consequently a less favourable mode of failure took place. The limit of rotation of principal axes of strain increment was understood to be the alignment of a direction of zero-extension in the sand with the reinforcement. These findings agree with the ideas expressed by Basset and Last (1978) on the mode of action of tensile reinforcement, which in particular was related to the effect of tensile reinforcement on the strain field in a reinforced earth wall as shown in fig.1.4. 3) For efficient use of tensile reinforcement it was demonstrated that the bond with sand should be as high as possible. This could be achieved by roughening the surface. Alternatively, the bond was improved by introducing openings or apertures in the reinforcement, changing the shape to a grid. It appeared that the bond for a suitably proportioned grid could be as high as for a fully roughened surface. 4) The longitudinal stiffness of tensile reinforcement was observed to affect the magnitude and rate of increase in strength in the direct shear tests. The rupture strain of tensile reinforcement relative to maximum tensile strains of the soil, under the same operational stress conditions, have also been observed to influence the reinforcing effect in terms of its limiting behaviour, i.e. whether brittle or ductile (McGown, et al. 1978). With regards to the performance of reinforced earth walls, Al-Hussanini and Perry (1976) observed that steel reinforced strips produced a stiffer and stronger structure than a more extensible fabric reinforcement, even though surface roughness was less. The importance of reinforcement tensile stiffness is recognized in limit equilibrium designs for tensile reinforced soil structures by limiting the available reinforcement force to the tensile strains that can develop in the soil (e.g. Jewell 1985). For highly structured non-woven and composite geotextiles, McGown et al. (1982) demonstrated that the stress-strain behaviour can be significantly affected by soil confinement. Testing wider strips in isolation was not found to replicate the effects of soil confinement. Another factor which needs to be considered when assessing the tensile property of a polymer reinforcement is creep. McGown et al. (1984) illustrated an appropriate method of interpreting creep data using isochronous curves, which enable long term laboratory test data to be extrapolated to the design life of the soil structure. 5) The strain and hence stress fields in the reinforced direct shear tests have been shown to be complex and non-uniform. However Jewell successfully modelled the variation of reinforcing effect for tensile reinforcement at different orientations by using a simple limit equilibrium analysis, see fig.1.5. The effect of the tensile reinforcement force was represented as: - an increase in the normal effective stress acting on the central plane of the box due to the normal component of the force and - a reduction in the applied shear stress due to the parallel component of the force to the central plane. Subsequently this analysis has been applied to limit equilibrium design methods for reinforcing soil retaining walls and embankments, Jewell et al. 1984, and Jewell 1982 respectively. 6) A reduction in the reinforcing effect for individual reinforcement due to the presence of other reinforcement was observed in the shear box. This loss of efficiency of individual reinforcement was termed interference. Interference between tensile reinforcement has also been studied by Guilloux et al. (1979) for the pull-out resistance from soil. However interference between reinforcement has yet to be introduced into a limit equilibrium design method.