Investigating strain localisation in clay using mica markers and X-ray computed tomography

Mass movements in clay deposits result in damage to infrastructures and buildings with significant social, economic, and environmental consequences. These processes are characterised by strain localisation, a complex process to investigate experimentally and model. Strain localisation in clays is particularly worrisome and possess huge destructive capabilities because clay is characterised by low shear strength. Conventional laboratory tests are essentially a post-mortem destructive analysis of localised deformations, and do not account for the fundamental physics of soil behaviours. Hence the development of 4-dimensional (4-D) non-destructive imaging approaches to study soil mechanical behaviour. However, due to the small size of clay particles compared to achievable X-ray computed tomography (X-ray CT) resolution, it has not been possible to directly evaluate particle scale clay micromechanics non-destructively using 4-D imaging techniques. This thesis presents a novel technique involving the use of plate-shaped (“platy”) muscovite mica marker for the evaluation of the initiation and propagation of strain localisation in kaolin. First, an investigation was carried out to understand the suitability of the use of mica particle markers for the study of clay by carrying out both chemical and mechanical characterization of mica. Subsequently, sample preparation techniques were experimented to understand the appropriate sampling approach with least microstructure disturbance. Furthermore, a novel miniature triaxial cell instrumented with a high capacity tensiometer and a novel platy particle matching algorithm were developed for the study of mica marker particle kinematics within kaolin. Kinematic analysis (displacement and rotation) of mica particle markers within the kaolin sample was then carried out. The results presented in this thesis demonstrated that (i) The particle configuration of silt sized muscovite samples consistently varied (dispersive and non-dispersive) with pore-water chemistry, regardless of whether the samples being tested were suspension sediments or compacted samples. (ii) By adding both silt sized muscovite or sand sized muscovite to kaolin for up to 30% sand sized muscovite or silt sized muscovite, the compressive behaviour is still clay-dominated. Similarly, the addition of mica (up to 30%) to kaolin does not significantly affect its hydraulic conductivity of kaolin. However, the shear strength characteristics of kaolin may significantly change by the addition of about 2.5-30% of silt sized muscovite or sand in the low normal stress (<100 kPa) but not at higher stress regime. (iii) PLATYMATCH (algorithm developed in this thesis) can effectively match platy particles in consecutive sample scans when adequately registered and the particles adequately segmented. (iv) A conceptual model of the initiation and propagation of strain localisation in kaolin was developed. The findings of this thesis implies that there is the potential to use platy mica particle marker images for early (pre-peak shear strength) detection of the initiation and propagation of strain localisation in kaolin and this may possibly be useful in enhancing available constitutive models such as the double scale constitutive model for improved clay behaviour prediction.Mass movements in clay deposits result in damage to infrastructures and buildings with significant social, economic, and environmental consequences. These processes are characterised by strain localisation, a complex process to investigate experimentally and model. Strain localisation in clays is particularly worrisome and possess huge destructive capabilities because clay is characterised by low shear strength. Conventional laboratory tests are essentially a post-mortem destructive analysis of localised deformations, and do not account for the fundamental physics of soil behaviours. Hence the development of 4-dimensional (4-D) non-destructive imaging approaches to study soil mechanical behaviour. However, due to the small size of clay particles compared to achievable X-ray computed tomography (X-ray CT) resolution, it has not been possible to directly evaluate particle scale clay micromechanics non-destructively using 4-D imaging techniques. This thesis presents a novel technique involving the use of plate-shaped (“platy”) muscovite mica marker for the evaluation of the initiation and propagation of strain localisation in kaolin. First, an investigation was carried out to understand the suitability of the use of mica particle markers for the study of clay by carrying out both chemical and mechanical characterization of mica. Subsequently, sample preparation techniques were experimented to understand the appropriate sampling approach with least microstructure disturbance. Furthermore, a novel miniature triaxial cell instrumented with a high capacity tensiometer and a novel platy particle matching algorithm were developed for the study of mica marker particle kinematics within kaolin. Kinematic analysis (displacement and rotation) of mica particle markers within the kaolin sample was then carried out. The results presented in this thesis demonstrated that (i) The particle configuration of silt sized muscovite samples consistently varied (dispersive and non-dispersive) with pore-water chemistry, regardless of whether the samples being tested were suspension sediments or compacted samples. (ii) By adding both silt sized muscovite or sand sized muscovite to kaolin for up to 30% sand sized muscovite or silt sized muscovite, the compressive behaviour is still clay-dominated. Similarly, the addition of mica (up to 30%) to kaolin does not significantly affect its hydraulic conductivity of kaolin. However, the shear strength characteristics of kaolin may significantly change by the addition of about 2.5-30% of silt sized muscovite or sand in the low normal stress (<100 kPa) but not at higher stress regime. (iii) PLATYMATCH (algorithm developed in this thesis) can effectively match platy particles in consecutive sample scans when adequately registered and the particles adequately segmented. (iv) A conceptual model of the initiation and propagation of strain localisation in kaolin was developed. The findings of this thesis implies that there is the potential to use platy mica particle marker images for early (pre-peak shear strength) detection of the initiation and propagation of strain localisation in kaolin and this may possibly be useful in enhancing available constitutive models such as the double scale constitutive model for improved clay behaviour prediction

Estimating and understanding motion : from diagnostic to robotic surgery

Estimating and understanding motion from an image sequence is a central topic in computer vision. The high interest in this topic is because we are living in a world where many events that occur in the environment are dynamic. This makes motion estimation and understanding a natural component and a key factor in a widespread of applications including object recognition , 3D shape reconstruction, autonomous navigation and medica! diagnosis. Particularly, we focus on the medical domain in which understanding the human body for clinical purposes requires retrieving the organs' complex motion patterns, which is in general a hard problem when using only image data. In this thesis, we cope with this problem by posing the question - How to achieve a realistic motion estimation to offer a better clinical understanding? We focus this thesis on answering this question by using a variational formulation as a basis to understand one of the most complex motions in the human's body, the heart motion, through three different applications: (i) cardiac motion estimation for diagnostic, (ii) force estimation and (iii) motion prediction, both for robotic surgery. Firstly, we focus on a central topic in cardiac imaging that is the estimation of the cardiac motion. The main aim is to offer objective and understandable measures to physicians for helping them in the diagnostic of cardiovascular diseases. We employ ultrafast ultrasound data and tools for imaging motion drawn from diverse areas such as low-rank analysis and variational deformation to perform a realistic cardiac motion estimation. The significance is that by taking low-rank data with carefully chosen penalization, synergies in this complex variational problem can be created. We demonstrate how our proposed solution deals with complex deformations through careful numerical experiments using realistic and simulated data. We then move from diagnostic to robotic surgeries where surgeons perform delicate procedures remotely through robotic manipulators without directly interacting with the patients. As a result, they lack force feedback, which is an important primary sense for increasing surgeon-patient transparency and avoiding injuries and high mental workload. To solve this problem, we follow the conservation principies of continuum mechanics in which it is clear that the change in shape of an elastic object is directly proportional to the force applied. Thus, we create a variational framework to acquire the deformation that the tissues undergo due to an applied force. Then, this information is used in a learning system to find the nonlinear relationship between the given data and the applied force. We carried out experiments with in-vivo and ex-vivo data and combined statistical, graphical and perceptual analyses to demonstrate the strength of our solution. Finally, we explore robotic cardiac surgery, which allows carrying out complex procedures including Off-Pump Coronary Artery Bypass Grafting (OPCABG). This procedure avoids the associated complications of using Cardiopulmonary Bypass (CPB) since the heart is not arrested while performing the surgery on a beating heart. Thus, surgeons have to deal with a dynamic target that compromisetheir dexterity and the surgery's precision. To compensate the heart motion, we propase a solution composed of three elements: an energy function to estimate the 3D heart motion, a specular highlight detection strategy and a prediction approach for increasing the robustness of the solution. We conduct evaluation of our solution using phantom and realistic datasets. We conclude the thesis by reporting our findings on these three applications and highlight the dependency between motion estimation and motion understanding at any dynamic event, particularly in clinical scenarios.L’estimació i comprensió del moviment dins d’una seqüència d’imatges és un tema central en la visió per ordinador, el que genera un gran interès perquè vivim en un entorn ple d’esdeveniments dinàmics. Per aquest motiu és considerat com un component natural i factor clau dins d’un ampli ventall d’aplicacions, el qual inclou el reconeixement d’objectes, la reconstrucció de formes tridimensionals, la navegació autònoma i el diagnòstic de malalties. En particular, ens situem en l’àmbit mèdic en el qual la comprensió del cos humà, amb finalitats clíniques, requereix l’obtenció de patrons complexos de moviment dels òrgans. Aquesta és, en general, una tasca difícil quan s’utilitzen només dades de tipus visual. En aquesta tesi afrontem el problema plantejant-nos la pregunta - Com es pot aconseguir una estimació realista del moviment amb l’objectiu d’oferir una millor comprensió clínica? La tesi se centra en la resposta mitjançant l’ús d’una formulació variacional com a base per entendre un dels moviments més complexos del cos humà, el del cor, a través de tres aplicacions: (i) estimació del moviment cardíac per al diagnòstic, (ii) estimació de forces i (iii) predicció del moviment, orientant-se les dues últimes en cirurgia robòtica. En primer lloc, ens centrem en un tema principal en la imatge cardíaca, que és l’estimació del moviment cardíac. L’objectiu principal és oferir als metges mesures objectives i comprensibles per ajudar-los en el diagnòstic de les malalties cardiovasculars. Fem servir dades d’ultrasons ultraràpids i eines per al moviment d’imatges procedents de diverses àrees, com ara l’anàlisi de baix rang i la deformació variacional, per fer una estimació realista del moviment cardíac. La importància rau en que, en prendre les dades de baix rang amb una penalització acurada, es poden crear sinergies en aquest problema variacional complex. Mitjançant acurats experiments numèrics, amb dades realístiques i simulades, hem demostrat com les nostres propostes solucionen deformacions complexes. Després passem del diagnòstic a la cirurgia robòtica, on els cirurgians realitzen procediments delicats remotament, a través de manipuladors robòtics, sense interactuar directament amb els pacients. Com a conseqüència, no tenen la percepció de la força com a resposta, que és un sentit primari important per augmentar la transparència entre el cirurgià i el pacient, per evitar lesions i per reduir la càrrega de treball mental. Resolem aquest problema seguint els principis de conservació de la mecànica del medi continu, en els quals està clar que el canvi en la forma d’un objecte elàstic és directament proporcional a la força aplicada. Per això hem creat un marc variacional que adquireix la deformació que pateixen els teixits per l’aplicació d’una força. Aquesta informació s’utilitza en un sistema d’aprenentatge, per trobar la relació no lineal entre les dades donades i la força aplicada. Hem dut a terme experiments amb dades in-vivo i ex-vivo i hem combinat l’anàlisi estadístic, gràfic i de percepció que demostren la robustesa de la nostra solució. Finalment, explorem la cirurgia cardíaca robòtica, la qual cosa permet realitzar procediments complexos, incloent la cirurgia coronària sense bomba (off-pump coronary artery bypass grafting o OPCAB). Aquest procediment evita les complicacions associades a l’ús de circulació extracorpòria (Cardiopulmonary Bypass o CPB), ja que el cor no s’atura mentre es realitza la cirurgia. Això comporta que els cirurgians han de tractar amb un objectiu dinàmic que compromet la seva destresa i la precisió de la cirurgia. Per compensar el moviment del cor, proposem una solució composta de tres elements: un funcional d’energia per estimar el moviment tridimensional del cor, una estratègia de detecció de les reflexions especulars i una aproximació basada en mètodes de predicció, per tal d’augmentar la robustesa de la solució. L’avaluació de la nostra solució s’ha dut a terme mitjançant conjunts de dades sintètiques i realistes. La tesi conclou informant dels nostres resultats en aquestes tres aplicacions i posant de relleu la dependència entre l’estimació i la comprensió del moviment en qualsevol esdeveniment dinàmic, especialment en escenaris clínics.Postprint (published version

The use of potential field and seismological data to analyze the structure of the lithosphere beneath southern Africa

The Bouguer gravity anomaly of southern Africa ranges from low values of approximately -200 mGal in the Kaapvaal Craton to over 50 mGal in the Lebombo near the Mozambique border. Three major contributions to the long wavelength Bouguer gravity along a profile extending from Cape Town, South Africa to near Masvingo, Zimbabwe have been investigated: (1) large scale crustal features such as the Bushveld Complex and the Limpopo Belt (2) variations in the depth to the Mohorovičić discontinuity (Moho) and (3) upper mantle seismic velocity perturbations. Crustal thickness determinations from receiver function analysis from 82 sites were used to forward model the gravitational response due to Moho depth variations. These crustal thickness variations display little correlation with surface topography. The variations in the sharpness of phase weighted stacks of the receiver function results suggests regional varying density contrasts at the Moho correlating with geological terrain or crustal thickness. A change in crustal thickness of over 10 km results in a gravity anomaly amplitude at surface of ~120 mGal for a density contrast of 300 kg/m3 at the transition between the Namaqua Natal Mobile belt and the Kaapvaal craton, which is not observed in the measured gravity data. It appears probable that the ~350 km wide Bushveld Complex is connected laterally at depth within the crust. However, this dense layered mafic intrusion has a limited long wavelength gravity signal as it is compensated by significantly thickened underlying crust