Thesis (M.S.)--University of Nevada, Reno, 2007. / "May 2007." Includes bibliographical references (leaves 97-99). Online version available on the World Wide Web.

Thesis (M.S.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed October 20, 2008) Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1940. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 68-73).

Loess has a metastable structure and undergoes structural hydrocollapse when loaded and wetted, leading to subsidence and damage of overlying structures, which presents enormous engineering problems in many countries in the world. This problem is a focus of much research on both the nature of the phenomenon and the means. of overcoming its negative consequences. However, there remains a need for fundamental experimental and theoretical studies aimed at understanding the many uncertainties involved in the hydrocollapse phenomenon. An experimental programme has been conducted to study the fundamental nature of the hydrocollapse phenomenon in loess, and the primary features of the metastable structure controlling it. High quality disc-shaped specimens were prepared from natural undisturbed, very soft and slightly cemented loess soils using two simple devices which were designed and manufactured in the laboratory. Artificial loess materials were prepared by mixing pure silt particles of different geometrical characteristics (crushed sand, ballotini glass balls) with various clay types (kaolinite, bentonite) to produce mixes with different clay/silt ratios. Soil specimens of metastable structure were created from natural and model loess materials using the air-fall technique, which allowed full control over the critical variables. The index properties of the materials were determined experimentally, together with their geometrical characteristics from scanning electron microscope (SEM) studies. The compressibility characteristics and hydrocollapse behaviour were measured via one-dimensional (oedometer) compression tests. Two testing methods were used: the single oedometer test and the double oedometer test. The results indicated that the experimental approach used in this research can be used successfully to investigate the hydrocollapse problem of loess deposits. The small-clay loess model was constructed and examined. The results proved that this model is valid. The collapse behaviour of reconstituted and undisturbed loess specimens prepared from the same material was qualitatively similar, although the reconstituted materials exhibited greater collapsibility, particularly under low normal effective stresses. Specimens prepared from pure silt revealed very little collapse in spite of relatively high initial voids ratios (e ≤ 0.9). The existence of bonding materials, such as clay minerals at the points of particle contact, is thus apparently essential for hydrocollapse to occur. Specimens prepared from pure silt-bentonite mixtures exhibited low hydrocollapse values in comparison with those for pure silt-kaolinite mixtures. It was observed that there is an optimum clay mineral content for maximum hydrocollapse and it depends on the type of clay mineral and the level of applied stress. Higher values of hydrocollapse were obtained with smooth, spherical glass balls as the silt fraction, thus confirming that the geometrical properties of the silt particles also have a significant effect on the hydrocollapse behaviour of loess deposits. The position and distribution (mixing method) of clay particles inside the soil structure were equally found to have a significant effect on the hydrocollapse behaviour. The index properties of Libyan loess were found to be very similar to those of other loess deposits world-wide. The Tripoli loess can be classified as silty loess with high susceptibility to structural hydrocollapse in the same manner as other better known loess deposits in the world.

The purpose of this study is to investigate the primary factors which control the engineering behavior of silts. The methods of classifying silts at present do not provide much basis to infer how silts will behave in the field for design purposes. In the course of this research study, series of UU and CU triaxial test results of different silt samples were gathered to investigate the factors that control the strength parameters of silts. Effective and total stress strength interpretation were done on the test results. The effective stress friction angle were evaluated using the CU triaxial test results. Undrained strength parameters were determined from the CU and UU triaxial test results, and comparison were performed using the undrained strength ratio S_u/p. Silt samples which exhibit dilative behavior during undrained shear make it difficult to define failure. Because of this behavior, the test results were interpreted using different failure criteria to assess the effective stress friction angle and the undrained strength parameters. Test results indicate that the effective stress friction angle can be evaluated using the CU triaxial test will little uncertainty. The undrained shear strength of silt samples are influenced by the type of failure criteria. The results indicate that a unique undrained shear strength can not be determined because the samples get stronger with increasing strain. A reliable method of evaluating the undrained shear strength of silt samples is to determine the relationship between the undrained strength ratio and the changes in pore pressure. / Master of Science

No description available.

Neste estudo, o tamanho médio do Sortable Silt da fração ‹ 63 micra dos sedimentos terrígenos foi medido em um testemunho para os últimos 800 ka na Bacia de Santos. Os estágios isotópicos marinhos (MIS) foram identificados até MIS o 19. Investigamos as variações oceanográficas passadas para obter uma melhor compreensão sobre a intensidade do fluxo de corrente de fundo durante os períodos glaciais / interglaciais. Os registros contínuos do Sortable Silt, isótopos de oxigênio e carbono, susceptibilidade magnética, taxa de sedimentação, fluxos de massa e conteúdo de carbonato foram obtidos e mostraram uma forte modulação com a intensidade do fluxo de corrente de fundo em resposta a ciclos glaciais e interglaciais. Em geral, para os períodos glaciais, foi possível observar uma dominância do deslocamento da água do fundo em direção ao norte. Uma comparação entre diferentes métodos de análise granulométrica (laser e técnica de pipetagem), revelou que ambos caracterizam os períodos interglacial e glacial. As análises espectrais mostraram um forte sinal no Sortable Silt para os ciclos da excentricidade (∼ 100 ka), o que foi confirmado pelos isótopos de oxigênio e susceptibilidade magnética, que apresentaram um sinal similar. Variações concomitantes no Sortable Silt e nos diferentes traçadores confirmam o potencial do primeiro como um proxy para a intensidade do fluxo de corrente de fundo. / In this study, the Sortable Silt mean size of the ‹ 63micra terrigenous sediment fraction were measured in a core for the last 800kyrs in Santos Basin. Marine Isotopic Stages (MIS) down to MIS 19 were identified. We investigate the past oceanographic variations to obtain a better understanding about bottom-current flow intensity during glacial/interglacial periods. Continuous records of Sortable Silt, oxygen and carbon isotopes, magnetic susceptibility, sedimentation rate, mass fluxes and carbonate content were obtained and show a strong modulation of bottom-current flux intensity in response to glacial and interglacial cycles. In general, for glacial periods it was possible to observe a dominance of northward bottom water. A comparison between different methods of grain-size analysis (laser and the pipetting technique), revealed that both characterize the interglacial and glacial periods. The spectral analyzes showed a strong signal in the Sortable Silt for the cycles of the eccentricity (~ 100 ka), which was confirmed by the oxygen isotopes and magnetic susceptibility, which presented a similar signal. Concomitant variations in the sortable silt and in the different tracers confirms the potential of the first as a proxy for bottom-current flow intensity.

Neste estudo, o tamanho médio do Sortable Silt da fração ‹ 63 micra dos sedimentos terrígenos foi medido em um testemunho para os últimos 800 ka na Bacia de Santos. Os estágios isotópicos marinhos (MIS) foram identificados até MIS o 19. Investigamos as variações oceanográficas passadas para obter uma melhor compreensão sobre a intensidade do fluxo de corrente de fundo durante os períodos glaciais / interglaciais. Os registros contínuos do Sortable Silt, isótopos de oxigênio e carbono, susceptibilidade magnética, taxa de sedimentação, fluxos de massa e conteúdo de carbonato foram obtidos e mostraram uma forte modulação com a intensidade do fluxo de corrente de fundo em resposta a ciclos glaciais e interglaciais. Em geral, para os períodos glaciais, foi possível observar uma dominância do deslocamento da água do fundo em direção ao norte. Uma comparação entre diferentes métodos de análise granulométrica (laser e técnica de pipetagem), revelou que ambos caracterizam os períodos interglacial e glacial. As análises espectrais mostraram um forte sinal no Sortable Silt para os ciclos da excentricidade (∼ 100 ka), o que foi confirmado pelos isótopos de oxigênio e susceptibilidade magnética, que apresentaram um sinal similar. Variações concomitantes no Sortable Silt e nos diferentes traçadores confirmam o potencial do primeiro como um proxy para a intensidade do fluxo de corrente de fundo. / In this study, the Sortable Silt mean size of the ‹ 63micra terrigenous sediment fraction were measured in a core for the last 800kyrs in Santos Basin. Marine Isotopic Stages (MIS) down to MIS 19 were identified. We investigate the past oceanographic variations to obtain a better understanding about bottom-current flow intensity during glacial/interglacial periods. Continuous records of Sortable Silt, oxygen and carbon isotopes, magnetic susceptibility, sedimentation rate, mass fluxes and carbonate content were obtained and show a strong modulation of bottom-current flux intensity in response to glacial and interglacial cycles. In general, for glacial periods it was possible to observe a dominance of northward bottom water. A comparison between different methods of grain-size analysis (laser and the pipetting technique), revealed that both characterize the interglacial and glacial periods. The spectral analyzes showed a strong signal in the Sortable Silt for the cycles of the eccentricity (~ 100 ka), which was confirmed by the oxygen isotopes and magnetic susceptibility, which presented a similar signal. Concomitant variations in the sortable silt and in the different tracers confirms the potential of the first as a proxy for bottom-current flow intensity.

Doctor of Philosophy / Suction caissons have been used in the offshore industry in the last two decades as both temporary mooring anchorages and permanent foundation systems. Although there have been more than 500 suction caissons installed in various locations around the world,understanding of this concept is still limited. This thesis investigates the installation aspect of suction caissons, focusing on the installation in dense sand and layered soils, where sand is inter-bedded by silt and weakly cemented layers. The research was mainly experimental, at both normal gravity and elevated acceleration levels in a geotechnical centrifuge, with some numerical simulations to complement the experimental observations. This study firstly explored the suction caisson installation response in the laboratory at 1g. The influence and effect of different design parameters, which include caisson size and wall thickness, and operational parameters including pumping rate and the use of surcharge were investigated in dense silica sand. The sand heave inside the caisson formed during these installations was also recorded and compared between tests. The 1g study also investigated the possibility of installing suction caissons in layered sand-silt soil, where caissons were installed by both slow and rapid pumping. The heave formation in this case is also discussed. The mechanism of heave formation in dense sand and deformation of the silt layer was further investigated using a half-caisson model and the particle image velocimetry (PIV) technique. The installation response at prototype soil stress conditions was then investigated in a geotechnical centrifuge. The effects of caisson size, wall thickness, as well as surcharge were investigated in various types of sand, including silica sand, calcareous sand dredged from the North Rankin site in the North West Shelf (Australia), and mixed soil where silica sand was mixed with different contents of silica flour. Comparison with the 1g results was also made. The general trend for the suction pressure during installation in homogenous sand was identified. The installation in layered soil was also investigated in the centrifuge. The installation tests were performed in various sand-silt profiles, where the silt layers were on the surface and embedded within the sand. Comparison with the results in homogenous sand was made to explore the influence of the silt layer. Installations in calcareous sand with cemented layers were also conducted. The penetration mechanism through the cemented layer is discussed, and also compared with the penetration mechanism through the silt layer. Finite element modelling was performed to simulate key installation behaviour. In particular, it was applied to simulate the sand deformation observed in the PIV tests. The likely loosening range of the internal sand plug during suction installation in silica sand was estimated. By investigating the development of hydraulic gradient along the inner wall, the principle underlying the suction response for different combinations of selfweight and wall thickness was identified. FE modelling was also performed to explore the influence of the hydraulic blockage by the silt layer. This study found that the caissons could penetrate into all soils by suction installation. Among the key findings are the observations that the suction pressure increases with depth following a distinct pressure slope, corresponding to a critical hydraulic condition along the inner wall; and the installation was possible in both layered sand-silt and uncemented-cemented soils if sufficient pumping was available. While the caisson could penetrate the weakly cemented layers well with no notable adverse effects, problems were observed in the installation in layered sand-silt soil. These include piping failure in slow pumping rate installation at 1g, and the formation of extremely unstable soil heave during installation.

The cyclic resistance of predominantly fine-grained soils has received considerable attention following ground and foundation failures at sites underlain by silt-rich soils during recent earthquakes. In several cases substantial ground deformation and reduced bearing capacity of silt soils has been attributed to excess pore pressure generation during cyclic loading. These field case studies are significant due to the occurrence of liquefaction related phenomena in soils that would be characterized as not susceptible to liquefaction using current geotechnical screening criteria. The most widely used of these criteria, the "Chinese Criteria" and its derivatives, are based solely on soil composition and they are essentially diagnostic tools that categorize the soil in a binary fashion as either liquefiable or non-liquefiable. The most significant limitations of these screening tools are that they fail to account for the characteristics of the cyclic loading. This investigation was undertaken to elucidate the potential for strain development in silts during cyclic loading, and to develop a practice-oriented procedure for evaluating the seismic performance of silts as a function of material properties, in situ stresses, and the characteristics of the cyclic loading. This dissertation presents the results of a multi-faceted investigation of the potential for seismically induced pore pressures and large strain development in silt soils. The primary focus of the research was on the synthesis of laboratory testing results on fine grained soils. Laboratory data from cyclic tests performed at Oregon State University and other universities formed the basis for enhanced screening criteria for potentially liquefiable silts. This data was supplemented with field data from sites at which excess pore pressure generation, liquefaction, and/or ground failures were observed during recent earthquakes. This investigation specifically addressed the behavior of silts during loading in cyclic triaxial tests due to the relative abundance of data obtained for this test. The data was used in conjunction with standard geotechnical index tests to enhance an existing energy based procedure for estimating excess pore pressure generation in silts. This pore pressure model can be used with the uncoupled, stress-based methods for estimating the post-cyclic loading volumetric strain developed in this investigation. The energy-based excess pore pressure model and empirical volumetric strain relationship were used to calibrate for applications involving silt soils a nonlinear, effective stress model for dynamic soil response (SUMDES). The SUMDES model was employed, along with the equivalent linear total stress model SHAKE, to estimate excess pore pressures generated at un-instrumented field sites that have exhibited evidence of liquefaction during recent earthquakes. A comparison of the SUMDES and SHAKE results highlighted the limitations of the latter model for simulating dynamic soil response at various levels of shaking and pore pressure response. The results of the SUMDES modeling at several well documented case study sites are presented in this dissertation. These comparisons are valuable for demonstrating the uncertainties associated with modeling of the effective stress behavior of silt during seismic loading. / Graduation date: 2005