The interpretation of measurements from the cone penetration test is still predominately based on empirical correlations, which can be attributed to the lack of understanding of penetration mechanisms, that involve severe stress-strain and shear dilatancy close to the probe. Even so, it remains one of the most widely used in-situ tools for site characterisation, and several methods for displacement pile design have been developed using CPT data. This research investigates the response of penetrometers and the behaviour of layered soils during installation of probes using geotechnical centrifuge modelling and cavity expansion analysis. Two series of centrifuge tests were performed in stratum configurations of silica sand in a half-cylindrical axisymmetric model, allowing the observation of the induced soil deformation through a Perspex window. The variations of penetration resistance and soil deformation with penetration depth, soil density, stress level and soil layering are examined from the results of the centrifuge tests. The quantified soil displacements and the resulting strains in the axisymmetric model have provided an effective approach for investigation of penetration mechanisms with soil element trajectories, strain paths and rotations of principal strain rate. The effects of layering on both resistance and soil deformation are shown with dependence of the relative soil properties and profiles. The results presented also serve as a base for applications of cavity expansion solutions, back analyses and further studies. Analytical solutions for cavity expansion in two concentrically arranged regions of soil are developed using a non-associated Mohr-Coulomb yield criterion for large strain analysis of both spherical and cylindrical cavities. The solutions are validated against finite element simulations and a detailed parametric study of the layered effects on the pressure-expansion curves is performed. To apply the proposed solutions to penetration problems, a simplified combination approach is suggested to eliminate the discrepancy between concentric layering and horizontal layering. The analytical study of penetration in two-layered and multi-layered soils is therefore achieved, with comparisons to elastic solutions and numerical simulations provided. The back analyses based on the resistance and soil deformation emphasise the influences of small-strain stiffness, soil-probe interface friction angle, and relative density/state parameter. The correlation between the cone tip resistance and the pile bearing capacity is also discussed, and the scale effects are examined through the ground surface effect and the layering effect by the developed cavity expansion solutions. The penetration mechanisms are summarised from the aspects of soil stress-strain history, particle breakage, soil patterns, and penetration in layered soils. The layered effects emphasised in this research indicate that the penetration resistance is strongly dependent on the soil properties within the influence zones above and below the probe tip, and also related to the in-situ stress gradient along the penetration path. It is also suggested that correlations from calibration chamber tests using uniform soil and a constant stress field may not be suitable for direct interpretation of CPT data. Finally, the averaging technique for pile design is suggested based on the transition curve of tip resistance in layered soils with consideration of the scale effects.

Cone penetration testing (CPT) is one of the most versatile devices for in situ soil testing. With minimal disturbance to the ground, it provides information about soil classification and geotechnical parameters. Several researchers have used different numerical techniques such as strain path methods and finite element methods to study CPT problems. The Discrete Element Method (DEM) is a useful alternative tool for studying cone penetration problems because of its ability to provide micro mechanical insight into the behaviour of granular materials and cone penetration resistance. This study uses three-dimensional DEM to simulate the cone penetration testing of granular materials in a calibration chamber. Due to the geometric symmetry of this study a 90 degree segment of the calibration chamber and the cone penetrometer was initially considered followed by a 30 degree segment to allow for the simulation of smaller particle sizes and to reduce computational time. This research proposes a new particle refinement method, similar to the mesh refinement of finite-element modelling, in the sense that a large number of small particles were brought into contact with the cone tip, while the large particles were distanced further away from the cone, to reduce computational time effectively. Using a radius expansion method for sample preparation and assigning a constant mass to each particle in the sample was found to reduce computational time significantly with little influence on tip resistance. The effects of initial sample conditions and particle friction coefficient were found to have an important influence on the tip resistance. In addition, prohibiting particle rotation was found to increase tip resistance significantly compared to when the particles were permitted to rotate freely. Particle shape in this study was simulated by replacing the spheres with simple two-ball clumps and was found to have an important effect on the tip resistance. DEM simulations of biaxial tests were conducted to investigate the effect of initial sample conditions, particle shape and particle friction coefficient on the stress-strain behaviour of granular materials. All the above mentioned parameters were found to have a significant effect on the stress-strain behaviour of granular materials. Biaxial test simulations were also conducted to obtain basic granular material properties to derive analytical CPT solutions from continuum mechanics principles. Some of the DEM simulation results were found to be in good agreement with the analytical solutions that used a combined cylindrical-spherical cavity expansion method. Particle crushing was simulated during the cone penetration tests by replacing a broken particle with two new equi-sized smaller particles with mass conserved. The results showed considerable reduction in the tip resistance for the crushing model compared to the non-crushing model and this reduction increased as the confining stress increased.

Non-coaxiality of the principal stress direction and principal strain increment direction has been observed in both numerical modelling and experimental studies. The importance of non-coaxiality has been widely recognised in the geomechnical engineering. Without considering the non-coaxiality in the design may lead to an unsafe soil structure. Therefore, it is essential to understand the non-coaxial soil behaviour better and take it into account in the numerical modelling. A new Hollow Cylinder Apparatus in Nottingham Centre of Geomechanics (NCG) has been employed in this study. A series of preliminary tests have been carried out to validate the reliability and repeatability of the testing results. Three series of tests, including 24 tests on Portaway sand and 2 tests on Leighton Buzzard sand, were conducted to study the non-coaxial soil behaviour of granular materials. The three stress paths followed were monotonic loading along fixed principal stress direction, pure rotation of the principal stress axes with constant deviator stress and combined rotation of principal stress axes with increasing deviator stress. Portaway sand was chosen because it has been used in NCG to investigate granular soil behaviour. Therefore, stress-strain behaviour including non-coaxial behaviour can be observed and used by the other researchers in NCG to develop or verify numerical models. The evidence of non-coaxiality has been obtained from the tests. In general, the non-coaxiality is relatively small in monotonic loading tests, but is more significant in the pure rotation tests and combined loading tests. The degree of non-coaxiality is affected by the density of the specimen, the stress path followed, the stress level and the material particle properties.

Soil stabilisation is a useful civil engineering technique that enables the insitu material to be used as part of an engineered structure. Stabilised layers are used in road foundation; working platforms and for slope stabilisation and sea defences. Chemical stabilisation involves the use of a hydraulic binder (and sometimes additional pozzolans). Commonly, quicklime (CaO) or slaked-lime (Ca(OH)2) is used. On mixing into the ground, this reacts with the aluminosilicates of the clay fraction, reducing its overall water content and plasticity. Further additions increase the insitu pH. Above pH 10.4, the aluminosilicates become soluble in the pore solution. They are then able to form a range of insoluble mineral hydrates which constitute a cementitious matrix. This results in both an increase in mechanical strength and a decrease in dimensional stability. If the insitu material contains sulfur bearing mineralogies, these can react with the hydraulic binder and the aluminosilicates to form expansive minerals. If this occurs after the initial setting and hardening of the stabilised layer has occurred, it can lead to severe dimensional instability and mechanical weakening. This is termed sulfate heave and the principal agent of this heave is a hydrous calcium sulfoaluminate hydrate, ettringite (AFt). The fundamental processes of ettringite formation and associated expansion are little understood in stabilised soils. This research used a range of artificial sulfate bearing, lime stabilised blended soil samples subject to two immersion tests used for material suitability assessment in the UK. The physicochemical response (in terms of dimensional heave and mechanical weakening) was assessed as a function of soil composition and the environmental conditions imposed by the two immersion tests. The fundamental microstructure and phase composition was characterised using a range of analytical techniques (XRD, SEM-EDX, dTGA). The relationship between the observed macro-physical properties and underlying chemical environment and microstructure was explored. Key findings include that the mechanism of ettringite formation and expansion was found to be governed by the fundamental structure of the bulk clay. This explained the greater swell response of the kaolin based soils compared to those of the montmorillonite. The SEM-EDX analysis identified a primitive, Ca-rich, AFt phase termed ‘ball ettringite’, in stabilised soils. This has only relatively recently been reported in studies of cement mortars. Also, small amounts of sulfate in the bulk soil actually increase soil strength. It was suggested that the preferential formation of monosulfate (AFm) plays an important role in this mechanism. The introduction of water to the pore solution is key to the formation of ettringite. This was evidenced by X-Ray CT of the damage caused to soil specimens on immersion, as well as low angle XRD studies of the principal AFt peak. Based on the limited testing undertaken one of the immersion tests (European accelerated volumetic swell test, EN13286-49), appears to be more onerous than the other (UK CBR linear swell test, BS1924-2).

It is becoming increasingly necessary to construct on land that was previously considered inappropriate for construction, such as soft clay. The properties of soft clay make it highly compressible and low in shear strength, meaning that bearing capacity failure and excessive settlement are of concern. Piled embankments are a ground improvement technique that can provide a solution for this problem. Piled embankments have the ability to transfer the greater part of the embankment load and any surcharge to more competent material at greater depth due to the 'arching' concept. Consequently, the soft foundation soil has little direct impact on the performance of the embankment. The concept of 'arching' of granular soil over an area where there is partial loss of support from underlying strata has long been recognised in the study of soil mechanics (e. g. Terzaghi, 1943). However, a number of competing theories exist to quantify this behaviour in piled embankments. In addition, the use of geotextile reinforcement in piled embankments placed above the pile caps in principle provides a number of technical as well as economical benefits. As the embankment fill is placed, tension is created in the reinforcement and it is the vertical component of this tension that transfers the embankment load onto the piles and reduces the load carried by the soft clay hence transferring the load of the embankment on to the piles. Differential settlement can be a problem for piled embankments of low height. Significant differential settlement can cause undesirable effects on any structures constructed on the embankment. 'Arching' limits the amount of differential settlement in embankments and the use of geotextile geogrid can also potentially have additional benefits. This thesis presents a series of centrifuge tests examining the performance of unreinforced and reinforced piled embankments constructed over soft subsoil in terms of stress acting on the subsoil, and differential movement at the surface of the embankment. A large range of embankment heights are considered, and the results for stress on the subsoil are compared with existing predictive methods, allowing generic conclusions to be drawn regarding the predictions of various methods. The effect of a 'working platform' below pile cap level and thus directly loading the subsoil is also considered, and used to examine the concept of a 'Ground Reaction Curve' (Iglesia et al, 1999) for arching in the embankment. In principle this can be used to consider compatibility of displacements at the base of the embankment, and thus improve design simultaneously considering the effect of arching in the embankment and underlying support from the subsoil and layers of reinforcement acting in tension.

The advancement of a slender object into soil (to depths of 10s of metres) is of fundamental interest to Geotechnical Engineers. Considerable advances have recently been made in beginning to understand some detailed aspects of the fundamental behaviour for penetration in sand using physical models. In this project, a 180° axisymmetric model is developed, which allows viewing of soil movement as a circular penetrometer advances into the soil. The model was tested in a geotechnical centrifuge with digital photographic techniques used to track soil movement. A series of 10 centrifuge tests is reported. The soil displacement during pile installation was measured. The soil strain paths during installation were also calculated from the measured soil displacements. The stress along the pile shaft was also measured by strain gauges during the pile installation. The displacements show reasonable correspondence with circular (cylindrical) cavity expansion. The amount of displacement generally increases with penetration. After about 8 to 10 diameters of penetration, the amount of movement does not vary significantly with depth. After the probe passes there is little systematic movement. The magnitude of displacements drops quickly as the radial position increases. The influence of re-driving, soil density, gravity levels and probe tip shape was examined. Results reveal that displacements are much less in the re-driving test. The centrifuge acceleration has some influence on the displacement and strains. It is found that there is no significant deviation in displacement and strains for different soil density and probe tip shape.

Most sedimentary deposits are inherently anisotropic due to their natural deposition in horizontal layers. This inherent anisotropy highlights the fact that the response of soils to loading is depending on both stress magnitude and its direction. Most of the field problems in geotechnical engineering are three-dimensional, and a soil is more likely to subject an anisotropic stress state (σ1 ≠ σ2 ≠ σ3), together with rotation of the principal axes. It is therefore essential that the soil behaviour under such realistic and general loading conditions is to be well understood, so that engineers can devise appropriate geotechnical design and analysis in practical situations. The Small-Strain Hollow Cylinder Apparatus (SS-HCA), developed by GDS Instruments Ltd. has been used to study drained anisotropic behaviour of sand under generalized stress conditions. In particular, the anisotropic stress-strain-strength characteristics, volume change behaviour, non-coaxiality and combined effects of α and b are studied. Three testing programs composed of two main types of stress paths (e.g. monotonic loading with different inclinations of the major principal stress and cyclic rotation of principal stress axes) were conducted. Inherently anisotropic behaviour of sands is clearly illustrated by deformation response that is strongly dependent on the loading direction in the monotonic shear tests. For a given loading direction, the mechanical response of sands is affected by the material density, the particle properties and the loading history. Non-coincidence of principal directions of stress and strain increment is observed and shear band inclinations in hollow cylindrical specimens follow the theoretical predictions. Results also clearly show the effects of intermediate principal stress on the deformation response of sand. This is seen in variation of stress-strain response and peak friction angle with differing b-values. A significant plastic deformation is induced during rotational shear despite the magnitudes of principal stresses remaining constant. Volumetric strain during rotational shear is mainly contractive and the amount of the volumetric strain increases with the increase in the stress ratio. Most of the contractive volumetric strain occurred during the first 20 cycles and its accumulation rate tended to decrease as the number of cycles increases. When principal stress rotation continues, the sand samples appear to be stabilized and the strain trajectory in the deviatoric plane approaches an ellipse. The test results also demonstrate that the mechanical behaviour of sand under rotational shear is generally non-coaxial, and the stress ratio has a significant effect on the non-coaxiality. The larger the stress ratio, the lower degree of non-coaxiality is induced. It was also observed that parameter b is not a negligible factor for the sand deformation during rotational shear, but has significant impact. The larger the b-value, the more the volumetric strain is accumulated.

The Earth's crust is deforming continuously due to plate tectonics. Deformation at plate boundaries causes volcanoes and most destructive earthquakes. Monitoring such deformation is essential to gaining an insight into the mechanisms of plate tectonics. Deformation analysis is one of the most important aspects of geodetic research. Space geodesy, with which long baselines can be measured to millimetre accuracy, plays an important role in determining crustal deformation parameters, since deformation in general, means a change in geometric configuration. The main deformation monitoring problem is to determine the spatial relationship of a set of object points relative to a number of reference points. Ideally reference and object point observations are made at regular intervals. After mathematical adjustment of each epoch's observations, which includes 'data snooping', a displacement vector is obtained by simply differencing the estimated coordinates at consecutive epochs. The use of thismethod also however, increases the noise level. In this thesis, the author proposes a deformation analysis technique which mainly uses a Kalman Filter. However, Kalman filter estimation may not be optimal if local movement occurs between observation epochs. To overcome this kind of deficiency, two sub-optimal filters have been proposed: Fading Memory Filter and Adaptive Kalman Filter for a System with Unknown Measurement Bias. These two filtering techniques have been used in this research and tested on real/simulated data based on the EASTMED project. In addition to this, data from the EUREF Permanent GPS Network, and from the UK Tide Gauge Monitoring C Project are also processed and the result presented.

A novel, non-coaxial soil model is developed in the context of perfect plasticity for the plane strain condition whilst incorporating initial soil strength anisotropy. The anisotropic yield criterion is developed by generalising the conventional isotropic Mohr-Coulomb yield criterion to account for the effects of initial soil strength anisotropy described by the variation of internal friction angles at different principal stress directions. The model is implemented into the commercial finite element(FE)software ABAQUS via the user defined material subroutine(UMAT). The proposed model is used to predict material non-coaxiality in simple shear tests. The non-coincidence of the directions of principal stresses and plastic strain rates can be reproduced. A faster rate of approaching coaxiality is observed when soil yield anisotropy is presented when compared to the model with an isotropic yield criterion. A semi-analytical solution of the bearing capacity for a smooth strip footing resting on an anisotropic, weightless, cohesive-frictional soil is developed based on the slip line method. A good match of the bearing capacity can be obtained between numerical and semi-analytical results. The results show that the vertical load at plastic collapse of a strip footing resting on an anisotropic soil is lower than that on an isotropic soil. The settlement prior to collapse is larger when the non-coaxial assumption is involved; however, no significant impacts can be observed on the ultimate failure load. In addition, the non-coaxial soil model is applied to investigate tunnelling induced displacement. The results are compared with the results from the centrifuge tests performed by Zhou (2015). For equal volume loss, the normalised settlement trough can be improved by adopting the soil anisotropic parameter β as compared to the experimental results. The maximum settlement is larger in light of larger non-coaxial coefficient for the same degree of the stress reduction. The cross-section of the anisotropic yield criterion developed is a rotational ellipse. Other types of the ellipse are possible. In addition, for simplicity we only consider the effect of initial anisotropy without considering induced anisotropy, and only the simple case of perfect plasticity is investigated. It is suggested that in order to capture the soil behaviours under more complex stress paths, the non-linear and anisotropic elasticity should be associated with the current model, and the development of hardening/softening rules is worth investigating.

Thesis (Ph.D.) -- University of Texas at Arlington, 2008.