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[pt] O Peru se localiza na região denominada cinturão de fogo do Pacífico, onde se registra aproximadamente 85 porcento da atividade sísmica mundial. Como grande produtor de minérios, importantes estruturas são construídas no país para desenvolvimento de projetos de mineração, incluindo barragens de rejeitos. Nesta dissertação é investigado o potencial de liquefação dinâmica de uma barragem de rejeitos localizada na região central do Peru, que atualmente passa por uma etapa de alteamento pelo método construtivo da linha de centro. A liquefação dos solos é um fenômeno caracterizado pela perda de rigidez e resistência durante curto intervalo de tempo, mas suficiente para causar instabilidades e, em casos extremos, o colapso da estrutura. Análises numéricas foram realizadas para verificar o potencial de liquefação dinâmica da barragem, representando o comportamento cíclico dos materiais suscetíveis à liquefação pelo modelo constitutivo UBCSand e de Byrne, considerando terremotos de magnitude 8,2, 7,8 e 8,1. Resultados numéricos obtidos foram comparados com formulações simplificadas, utilizadas na prática de engenharia para determinação do fator de segurança contra liquefação dinâmica. As análises indicaram que rejeitos finos, abaixo do nível de lençol freático, apresentam potencial de liquefação, conforme valores do parâmetro de razão de poropressão (ru) computados, porém sem provocar risco à estabilidade geral da estrutura. São também apresentados resultados de deslocamentos permanentes em alguns pontos da barragem e dos rejeitos. / [en] Peru is located in the region called the Pacific fire belt, where approximately 85 percent of the world seismic activity is recorded. As a major ore producer, important structures are built in the country to develop mining projects, including tailings dam. The dissertation investigates the potential for dynamic liquefaction of a tailings dam located in the central region of Peru, whose height is currently being raised by the center line construction method. Liquefaction of soils is a phenomenon characterized by loss of stiffness and resistance over a short period of time, but sufficient to cause instability and, in extreme cases, the collapse of the structure. Numerical analyses were carried out to verify the dynamic liquefaction potential of the dam, representing the cyclical behavior of the materials susceptible to liquefaction by the constitutive model UBCSand and Byrne, considering an earthquakes of magnitude 8,2, 7,8 and 8,1. Numerical results obtained were compared with simplified formulations used in the practice of engineering in order to determine the safety factor against dynamic liquefaction. The analyses indicated that fine tailings, below the water table level, have potential for liquefaction, according to the values of the poropression parameter (ru) computed, but without risk to the general stability of the structure. Results are also presented in terms of permanent displacements at some points of the dam and tailings.

The static cone penetrometer is an in-situ testing tool which was originally developed to derive information on soil type and soil strength. More recently, it has found application in liquefaction assessment. Typical cone penetrometers are heavy duty devices which are operated with the assistance of a drill rig. However, this capacity is not necessary in the case of field studies of liquefaction, since liquefaction usually occurs at relatively shallow depths. This thesis is directed to the goal of the development of a miniature, lightweight cone penetrometer which can be used in earthquake reconnaissance studies related to liquefaction problems. The research for this thesis involved four principal objectives: 1. Development of procedures to automatically acquire and process measurements from a miniature electrical cone; 2. Develop and perform tests in a model soil-filled bin to calibrate the cone; 3. Evaluate the utility and accuracy of the cone results as a means to assess conventional soil properties; and, 4. Conduct a preliminary evaluation of the cone results in the context of recently developed methods to predict liquefaction potential. The work in regard to the first objective involved assembling and writing software for a microcomputer based data acquisition system. Successful implementation of this system allowed data from the tests to be rapidly processed and displayed. Calibration tests with the cone were carried out in a four foot high model bin which was filled ten times with sand formed to variety of densities. The sand used is Monterey No. 0/30, a standard material with well known behavioral characteristics under static and dynamic loading. The test results showed the cone to produce consistent data, and to be able to readily distinguish the varying density configurations of the sand. Using the results in conventional methods for converting cone data into soil parameters yielded values which were consistent with those expected. Liquefaction potential predictions were less satisfying, although not unreasonable. Further research is needed in this area both to check the reliability of the prediction procedures and the ability to achieve the desired objectives. / M.S.

[pt] O Peru está localizado em uma área de alta atividade sísmica conhecida como cinturão de fogo. Os terremotos registrados com diferentes mecanismos de falha resultam da interação entre as placas tectônicas de Nazca e Sul-americana. É nesta região onde acontecem atividades de mineração no país, juntamente com a construção de grandes estruturas civis, como barragens de rejeitos, projetadas para armazenar grande quantidade de material na menor área possível. Neste trabalho, foi avaliado o comportamento sísmico de uma barragem de rejeitos localizada em Arequipa, Peru, construída com o método a montante. Em evento sísmico ocorrido em 2013, a barragem sofreu trincas no seu corpo, recalques na crista e pequenos vulcões de areia, indicando a redução dos parâmetros de resistência no rejeito devido a possível fenômeno de liquefação. Nesta pesquisa, a estimativa da ameaça sísmica foi feita por métodos probabilísticos, com o terremoto artificial de projeto obtido por método de ajuste espectral. Foram feitas análises pseudo-estáticas de estabilidade dos taludes, bem como estimados deslocamentos permanentes e o potencial de liquefação dinâmica por métodos simplificados. A análise global da barragem de rejeitos foi executada pelo método dos elementos finitos, considerando o modelo constituivo UBC3D-PLM para simular o comportamento mecânico dos rejeitos sob carregamento cíclico. / [en] Peru is located in an area of high seismic activity known as the fire belt. Earthquakes recorded with different failure mechanisms result from the interaction between the Nazca and South-American tectonic plates. It is in this region where mining activities take place in the country, along with the construction of large civil structures, such as tailings dams, designed to store large amounts of material in the smallest possible area. In this work, the seismic behavior of a tailings dam located in Arequipa, Peru, built with the upstream method, was evaluated. In a seismic event that occurred in 2013, the dam suffered cracks in its body, settlement in the crest and small sand boils, indicating the reduction of the tailings resistance parameters due to possible dynamic liquefaction. In this research, the seismic hazard assessment was made by probabilistic methods, with the artificial design earthquake obtained by the spectral adjustment method. Pseudo-static analyzes of slope stability were performed, as well as the estimated permanent displacements and the potential of dynamic liquefaction were obtainded by simplified methods. The global seismic analysis of the tailings dam was carried out through the finite element method, considering the UBC3D-PLM constitutive model to simulate the mechanical behavior of tailings under cyclic loading.

[pt] A ocorrência de dois acidentes de proporções inaceitáveis no Brasil, num período de 4 anos, resultou em problemas de confiança nos projetos e na operação de barragens de rejeitos, em especial as construídas pelo método de montante. Esse trabalho tem o objetivo de auxiliar no entendimento dos riscos de barragens de rejeitos, propondo uma metodologia para estimativa desses riscos e indicação de valores para limites de aceitabilidade e tolerabilidade. A metodologia proposta foi aplicada em duas barragens que romperam recentemente, Barragem de Fundão (Mariana/MG) e Barragem de Feijão (Brumadinho/MG), e uma barragem existente, a Barragem do Germano. A aplicação do método probabilístico FOSM em conjunto com a análise de equilíbrio limite por Spencer foi sugerida para estimar a probabilidade de ruptura por liquefação. Adicionalmente, a probabilidade de ocorrência dos gatilhos foi estimada de acordo com as características e operação da barragem. Nos casos estudados nesse trabalho, as probabilidades de ruptura determinadas pela metodologia desenvolvida são iguais a 36 por cento no caso de Fundão, 47 por cento no caso de Feijão e 3 por cento no caso de Germano. Os resultados mostraram que o risco associado às barragens que romperam era incompatível com qualquer obra de engenharia e muito acima dos limites normativos considerando as consequências da ruptura. / [en] Two accidents in tailings dams occurred in the last few years in Brazil and they resulted in the lack of trust in their design and performance. The aim of this work is to provide a better understanding of the risks on existing tailings dams, presenting a methodology for risk estimate and recommendation of values for acceptability and tolerability limits. The combination of the Spencer’s limit equilibrium method and the first-order second-moment (FOSM) probabilistic method (Christian et al., 1992) were chosen to be applied in the Fundão Dam (collapsed in 2015 in Mariana/MG), Feijão Dam (collapsed in 2019 in Brumadinho/MG), and Germano Dam. There is also a methodology suggested for trigger probability evaluation according to the dam s characteristics and operation. The probabilities of rupture through liquefaction during the time of the accidents are 36 per cent in case of Fundão Dam, 47 per cent in case of Feijão Dam and 3 per cent in case of Germano Dam. The results shows that the risk associated with the collapsed structures was much higher than the tolerable level of the standards.

Deep soil mixing to construct stiff columns is one of the methods used today to improve performance of loose ground and remediate liquefaction problems. This research adopts a numerical approach to study seismic performance of soil-cement columnar reinforcements in loose sandy profiles. Different constitutive models were investigated in order to find a model that can properly predict soil behavior during seismic excitations. These models included NorSand, Dafalias-Manzari, Plasticity Model for Sands (PM4Sand) and Pressure-Dependent-Multi-Yield-02 (PDMY02) model. They were employed to predict behavior of soils with different relative densities and under different confining pressures during monotonic and cyclic loading. PDMY02 was identified as the most suitable model to represent soil seismic behavior for the system studied herein. The numerical aspects of the finite element approach were investigated to minimize the unintended numerical miscalculations. The focus was put on convergence tolerance, solver time-step, constraint definition, and, integration, material and Rayleigh damping. This resulted in forming a robust numerical configuration for 3-D nonlinear models that were later used for studying behavior of the reinforced grounds. Nonlinear finite element models were developed to capture the seismic response of columnar reinforced ground during dynamic centrifuge testing. The models were calibrated with results from tests with unreinforced profiles. Thereafter, they were implemented to predict the response of two reinforced profiles during seismic excitations with different intensities and liquefaction triggering. Model predictions were compared with recordings and the possible effects from the reinforcements were discussed. Finally, parametric studies were performed to further evaluate the efficiency of the reinforcements with different extension depths and area replacement ratios. The results collectively showed that the stiff elements, if constructed appropriately, can withstand seismic excitations with different intensities, and provide a firm base for overlying structures. However, the presence of the stiff elements within the loose ground resulted in stronger seismic intensities on the soil surface. The columns were not able to considerably reduce pore water pressure generation, nor prevent liquefaction triggering. The reinforced profiles, comparing to the free-field profiles, had larger settlements on the soil surface but smaller settlements on the columns. The results concluded that utilization of the columnar reinforcements requires great attention as these reinforcements may result in larger seismic intensities at the ground surface, while not considerably reducing the ground deformations. / Ph. D. / The mitigation of seismic damage potential of soft soil sites remains one of the leading challenges in geotechnical earthquake engineering. It is well-established that structures located on these sites generally experience more damage due to excessive ground deformation during earthquakes. Ground reinforcements are often required to improve these sites for support of overlying structures. A remediating approach is to construct stiff columns within these sites by mechanically mixing soil with cementitious materials. Cemented soil has higher strength, and thereby, undergoes less deformations. Moreover, stiff columns can provide resistance against movement of their surrounding soil providing a firm base for possible above foundations. The primary focus of this research is to evaluate the effect of stiff column reinforcements on seismic behavior of loose ground. For this purpose, a numerical model was developed for the reinforced ground, and it was validated with results from experiments. The model was then used to study the performance of the reinforced ground during earthquake excitations with different intensities. The observed behavior was discussed and compared with findings from previous studies in literature. Finally, the numerical model was employed to evaluate efficiency of the reinforcements with different extension depths and occupied area. The results collectively showed that stiff columns, if constructed appropriately, can withstand different shaking levels, and provide a firm support for overlying structures. However, they were not efficient in reducing deformation of the surrounding soils. The presence of the stiff elements within the loose ground resulted in stronger seismic intensities on the soil surface. The study concluded that utilization of stiff columns requires great attention and understanding of the reinforcing mechanism. These columns might increase seismic intensity below foundations, while not considerably reducing the ground deformations.

[pt] Muitos autores concordam que areias em estado fofo, sob carregamento não drenado crescente e granulometria favorável à liquefação, necessariamente liquefazem em algum momento. Este trabalho tem como objetivo principal avaliar se duas amostras de areia, com características muito distintas uma da outra, submetidas às mesmas condições descritas acima, entram em liquefação. A primeira amostra é uma areia limpa que foi extraída de um aterro hidráulico localizado em Camboinhas, Niterói, RJ, Brasil. A segunda é um rejeito arenoso de ferro extraído de uma barragem de rejeitos localizada em Mariana, MG, Brasil. As avalições foram feitas através de ensaios de compressão triaxial com deformação controlada. Avaliou-se o comportamento desses materiais em condições de baixa compacidade e sob diferentes tensões de confinamento, com a finalidade de observar se ocorreria a liquefação. Em caso de não ocorrência da liquefação, o objetivo era compreender os motivos de tal resposta e no caso de ocorrência, avaliar a influência da tensão de confinamento, assim como delimitar as zonas de instabilidade não drenada. Verificou-se que a areia de Camboinhas entrou em liquefação total apenas para tensões de confinamento baixas, e apresenta comportamento oposto ao comportamento normal de solos arenosos fofos, tendo baixa resistência à liquefação em baixa tensão de confinamento e alta resistência à liquefação em grandes tensões de confinamento. O rejeito arenoso de Mariana entrou em liquefação total em tensões de confinamento baixas e altas. O rejeito apresentou comportamento diferente da areia de Camboinhas e semelhante ao comportamento normalmente obtido em solos arenosos fofos sob condições não drenadas. / [en] Many authors agree that loose sands, under increasing undrained loading and liquefaction favorable granulometry, necessarily liquefy at some moment. This work has as main objective to evaluate if two sand specimens, with characteristics very different from each other, under the same conditions as described above, enter into liquefaction. The first specimen is a clean sand that was extracted from a hydraulic embankment located in Camboinhas, Niterói, RJ, Brazil. The second is a sandy tailing of iron extracted from a tailing dam located in Mariana, MG, Brazil. The evaluations were made through triaxial compression tests with controlled deformation. The behavior of these materials was evaluated under conditions of low compactness and under different confininig pressure, in order to observe if liquefaction would occur. In case of no liquefaction, the objective was understand the reasons for such response and in case of occurrence, evaluate the influence of the confining pressure, as well as mark the zones of instability undrained. It was verified that the Camboinhas sand entered in total liquefaction only for low confining pressure, and it has the opposite behavior to the normal behavior of loose granular soils, having low strength to liquefaction in low confining pressure and high strength to liquefaction in large confining pressure. The Mariana granular tailing also entered in total liquefaction for low and higth confinig pressure. The tailing showed different behavior of the Camboinhas sand and similar to the behavior normally obtained in loose granular soils under undrained conditions.

Liquefaction-induced deformations associated with lateral spreading represent a significant hazard that can cause substantial damage during earthquakes. The ability to accurately predict lateral-spreading displacement is hampered by a lack of field data from previous earthquakes. Remote sensing via optical image correlation can fill this gap and provide data regarding liquefaction-induced lateral spreading displacements. In this thesis, deformations from three earthquakes (2010 Darfield, February 2011 Christchurch, and 2011 Tohoku Earthquakes) are measured using optical image correlation applied to 0.5-m resolution satellite imagery. The resulting deformations from optical image correlation are compared to the geologic conditions, as well as field observations and measurements of liquefaction. Measurements from optical image correlation are found to have a precision within 0.40 m in all three cases, and results agree well with field measurements. / text

The recent 2011 Tōhoku earthquake and tsunami served as a stark reminder of the destructive capabilities of such combined events. Civil Engineers are increasingly tasked with protecting coastal populations and infrastructure against more severe multi-hazard events. Whilst the protective measures must be robust, their deployment over long stretches of coastline necessitates an economical and environmentally friendly design. The dual row retaining wall concept, which features two parallel sheet pile walls with a sand infill between them and tie rods connecting the wall heads, is potentially an efficient and resilient system in the face of both earthquake and tsunami loading. Optimal use of the soil's strength and stiffness as part of the structural system is an elegant geotechnical solution which could also be applied to harbours or elevated roads. However, both the static equilibrium and dynamic response of these types of constructions are not well understood and raise many academic and practical challenges. A combination of centrifuge and numerical modelling was utilised to investigate the problem. Studying the mechanics of the walls in dry sand from the soil stresses to the system displacements revealed the complex nature of the soil structure interaction. Increased wall flexibility can allow more utilisation of the soil's plastic capacity without necessarily increasing the total displacements. Recognising the dynamically varying vertical effective stresses promotes a purer understanding of the earth pressures mobilised around the walls and may encourage a move away from historically used dynamic earth pressure coefficients. In a similar vein, the proposed modified Winkler method can form the basis of an efficient preliminary design tool for practice with a reduced disconnect between the wall movements and mobilised soil stresses. When founded in liquefiable soil and subjected to harmonic base motion, the dual row walls were resilient to catastrophic collapse and only accrued deformation in a ratcheting fashion. The experiments and numerical simulations highlighted the importance of relative suction between the walls, shear-induced dilation and regained strength outside the walls and partial drainage in the co-seismic period. The use of surrogate modelling to automatically optimise parameter selection for the advanced constitutive model was successfully explored. Ultimately, focussing on the mechanics of the dual row walls has helped further the academic and practical understanding of these complex but life-saving systems.