Thesis (M.S.)--University of Wisconsin--Madison. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 66-67).

The design of shallow foundations located in seismically active zones typically takes a near zero tolerance approach to allowing relative movements between the foundation and underlying soil. This results in a rigid coupling of the foundation with the soil and hence the full transmission of the seismic energy into the foundation. Consequently the structure located on the foundation either has to be isolated from the shaking through manufactured damping systems or designed to withstand the full force of the dynamic shaking while the energy is dissipated through structural damping. There is, however, an increasing focus on reducing the coupling between the soil and foundation which consequently reduces the demands on the structure. Allowing the foundation to slide and/or rock isolates the foundation from the dynamic shaking and helps to dissipate energy through soil damping. The same level of seismic protection which is currently provided by manufactured solutions is still therefore possible but with the advantage of reduced costs and complexity in construction. This design concept has, however, not been widely adopted due to concerns regarding the possibility for excessive movement of the foundation, resulting in damage to the superstructure or overall toppling of the structure. In addition, the behaviour is currently difficult to model precisely and therefore it is challenging to quantify the exact level of seismic protection achieved. The work presented in this thesis strives to address some of these issues. A series of ten centrifuge tests were conducted on small-scale model structures founded on dry sand and subjected to a series of simulated earthquakes. The effect of a range of model parameters was investigated including relative density, bearing pressure, structural stiffness, aspect ratio, earthquake strength and earthquake frequency. For six of the tests, the movement of the soil beneath the foundation and the structure itself were monitored by analysing images collected from high speed photography (1000 frames per second) using particle image velocimetry software. In addition to the photogramrnetry, a series of miniature measurement transducers were used to record the acceleration in the soil and the structure. Displacement transducers were used to monitor the settlement of the structure and free-field. In total over one hundred earthquakes were carried out resulting in an extensive dataset, against which hypotheses and analytical models could be verified. It was found that the transition from the structure being stationary to it responding in a steady state fashion can be a critical period in which the response of the soil-foundation-structure system must be carefully analysed. The phase shift between the superstructure, foundation and soil can vary during this period. As a result, different modes of response are adopted by the system which, in certain circumstances, can result in a significant increase in the displacement magnitude experienced by the structure. The behaviour is not unexpected, as the same behaviour can be observed from the analytical analysis of a simple single degree of freedom system. Inaddition, a strong correlation between rocking, sliding and settlement was observed, with the degree of lift-off controlling the amount of settlement and sliding which takes place. A macro-element analytical model has been developed to predict the moment-rotation behaviour of shallow raft foundations. A hyperbolic model, used for predicting the stress-strain behaviour of soil, was adapted and used to create the backbone curve of the moment-rotation cycle. A modification was made to the hysteretic damping rules proposed by Masing which allows the energy dissipation to be included in the model. The model was found to mimic the experimental data accurately, with the correct prediction of the lift-off rotation magnitude, moment magnitude, small-rotation stiffness and energy dissipation. Finally, the soil deformation mechanism was observed and analysed. It was found that in some scenarios, when the strain level within the deformation mechanism was low, a trapped wedge was apparent under the foundation. The trapped wedge appeared as a triangular zone of low strain (referred to in some literature as a rigid block), with the foundation located along the top edge and two distinct shear bands on either side. However, as strain magnitudes increased, the shear bands appeared to widen and resulted in strain being apparent throughout the previously unstrained wedge. One of the main differences between the theoretical mechanisms proposed in the literature is the inclusion or exclusion of such a rigid block. Given the majority of the analytical mechanisms proposed in the literature are upper bound mechanisms, and therefore are a prediction of the mechanism at failure, it is inadvisable to include the rigid wedge within the analytical mechanism given that the strain magnitudes will inevitably be large at the point of bearing failure. Given complete failure of the supporting soil did not occur during any of the centrifuge tests performed, comparisons between the observed mechanism and one of these theoretical mechanisms is difficult. However, comparisons between the experimental deformation mechanisms and one analytical failure mechanism did show that the depth of the mechanism could be relatively well predicted as could the degree of separation between the foundation and the underlying soil. This information allows design engineers to know to what depth ground should be remediated below a shallow foundation and how strong the foundation needs to be to cope with the lift-off it will experience. The insight provided by this research into the true soil deformation mechanism, combined with the development of an analytical model of the moment-rotation behaviour, paves the way for engineers to implement designs which actively make use of the beneficial characteristics of soil-structure-interaction.

A nonlinear effective stress method of analysis for determining the static and dynamic response of 2-D embankments and soil-structure interaction systems is presented. The method of analysis is incorporated in the computer program TARA-3. The constitutive model in TARA-3 is expressed as a sum of a shear stress model and a normal stress model. The behavior in shear is assumed to be nonlinear and hysteretic, exhibiting Masing behavior under unloading and reloading. The response of the soil to uniform all round pressure is assumed to nonlinearly elastic and dependent on the mean normal effective stresses. The porewater pressures required in the dynamic effective stress method of analysis are obtained by the Martin-Finn-Seed porewater pressure generation model modified to include the effect of initial static shear. During dynamic analysis, the effective stress regime and consequently the soil properties are modified for the effect of seismically induced porewater pressures. A very attractive feature of TARA-3 is that all the parameters required for an analysis may be obtained from conventional geotechnical engineering tests either in-situ or in laboratory. A novel feature of the program is that the dynamic analysis can be conducted starting from the static stress-strain condition which leads to accumulating permanent deformations in the direction of the smallest residual resistance to deformation. The program can also start the dynamic analysis from a zero stress-zero strain condition as is done conventionally in engineering practice. The program includes an energy transmitting base and lateral energy transmitting boundaries to simulate the radiation of energy which occurs in the field. The program predicts accelerations, porewater pressures, instantaneous dynamic deformations, permanent deformations due to the hysteretic stress-strain response, deformations due to gravity acting on the softening soil and deformations due to consolidation as the seismic porewater pressures dissipate. The capability of TARA-3 to model the response of soil structures and soil-structure interaction systems during earthquakes has been validated using data from simulated earthquake tests on a variety of centrifuged models conducted on the large geotechnical centrifuge at Cambridge University in the United Kingdom. The data base includes acceleration time histories, porewater pressure time histories and deformations at many locations within the models. The program was able to successfully simulate acceleration and porewater pressure time histories and residual deformations in the models. The validation program suggests that TARA-3 is an efficient and reliable program for the nonlinear effective stress analysis of many important problems in geotechnical engineering for which 2-D plane strain representation is adequate. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

The problems associated with nonlinear analysis of the load-deformation response of soils and soil structures are investigated. Methods of incremental nonlinear analysis are reviewed and their relative advantages and disadvantages discussed. Stress-strain relations commonly used for soils are critically examined and their limitations discussed. These stress-strain relations are based on the assumption that soils are isotropic, incrementally elastic materials. Evidence reported by other authors and reviewed in this study shows that the stress-strain relations commonly used for soils have two major sources of error, the anisotropy of soils and the effects of stress-path are neglected. The representation of soil stress-strain behaviour after yield is discussed. Although soils act as plastic materials after yield, it is common practice to represent post-yield behaviour by models of elastic materials. Many researchers use a constant value of Poisson's ratio and merely reduce the value of Young's modulus at yield. It is shown, with numerical examples, that this practice results in yielded soil elements being unrealistically compressible after yield. It is shown, with further numerical examples, that the predicted behaviour of yielded soil elements is more realistic if the value of the shear modulus is reduced at yield and the bulk modulus is not reduced. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

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Soil is a complex heterogeneous system comprising of highly variable and dynamic micro-habitats that have significant impacts on the growth and activity of resident microbiota. A question addressed in this research is how soil structure affects the temporal dynamics and spatial distribution of bacteria. Using repacked microcosms, the effect of bulk-density, aggregate sizes and water content on growth and distribution of introduced Pseudomonas fluorescens and Bacillus subtilis bacteria was determined. Soil bulk-density and aggregate sizes were altered to manipulate the characteristics of the pore volume where bacteria reside and through which distribution of solutes and nutrients is controlled. X-ray CT was used to characterise the pore geometry of repacked soil microcosms. Soil porosity, connectivity and soil-pore interface area declined with increasing bulk-density. In samples that differ in pore geometry, its effect on growth and extent of spread of introduced bacteria was investigated. The growth rate of bacteria reduced with increasing bulk-density, consistent with a significant difference in pore geometry. To measure the ability of bacteria to spread thorough soil, placement experiments were developed. Bacteria were capable of spreading several cm’s through soil. The extent of spread of bacteria was faster and further in soil with larger and better connected pore volumes. To study the spatial distribution in detail, a methodology was developed where a combination of X-ray microtopography, to characterize the soil structure, and fluorescence microscopy, to visualize and quantify bacteria in soil sections was used. The influence of pore characteristics on distribution of bacteria was analysed at macro- and microscales. Soil porosity, connectivity and soil-pore interface influenced bacterial distribution only at the macroscale. The method developed was applied to investigate the effect of soil pore characteristics on the extent of spread of bacteria introduced locally towards a C source in soil. Soil-pore interface influenced spread of bacteria and colonization, therefore higher bacterial densities were found in soil with higher pore volumes. Therefore the results in this showed that pore geometry affects the growth and spread of bacteria in soil. The method developed showed showed how thin sectioning technique can be combined with 3D X-ray CT to visualize bacterial colonization of a 3D pore volume. This novel combination of methods is a significant step towards a full mechanistic understanding of microbial dynamics in structured soils.