Spelling suggestions: "subject:"[een] SAND LIQUEFACTION"" "subject:"[enn] SAND LIQUEFACTION""
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[en] STUDY OF CONSTITUTIVE MODELS TO PREDICT SOIL LIQUEFACTION UNDER MONOTONIC LOADING / [pt] ESTUDO DE MODELOS CONSTITUTIVOS PARA PREVISÃO DA LIQUEFAÇÃO EM SOLOS SOB CARREGAMENTO MONOTÔNICOJORGE LUIS CARDENAS GUILLEN 24 November 2004 (has links)
[pt] Historicamente é sabido que muitas das rupturas ocorridas
em barragens ou taludes naturais podem ser atribuídas ao
fenômeno da liquefação de solos arenosos, causada pela ação
de carregamentos dinâmicos gerados por explosão ou, mais
freqüentemente, por terremotos. Quando liquefação ocorre,
um súbito aumento da poropressão faz decrescer a
resistência ao cisalhamento do solo e sua capacidade de
suportar pontes e edifícios é significativamente reduzida.
Solo liquefeito também pode exercer altas pressões sobre
estruturas de contenção, causando inclinações da mesma e
movimentos do solo que, por sua vez, originam recalques e
destruição de estruturas localizadas sobre a superfície do
terreno. O termo liquefação tem sido empregado para
descrever fenômenos relacionados, que produzem efeitos
similares, mas cujos mecanismos de formação são bastante
diferentes. Estes fenômenos são modernamente descritos como
fluxo por liquefação e mobilidade cíclica. Fluxo por
liquefação é o fenômeno no qual o equilíbrio estático é
destruído por carregamentos estáticos ou dinâmicos em um
depósito de solo com baixa resistência residual. Colapsos
causados por fluxo por liquefação são freqüentemente
caracterizados por movimentos rápidos e de grande extensão.
Mobilidade cíclica, por outro lado, é causada por
carregamentos cíclicos em solos sob tensões cisalhantes
estáticas inferiores à resistência ao cisalhamento do
material, com as deformações desenvolvendo-se gradualmente.
A execução de barragens de rejeito usando a técnica de
construção à montante pode levar à ocorrência de liquefação
estática se a velocidade de construção for suficientemente
alta para causar o desenvolvimento de excessos de
poropressão. A resposta de liquefação pode ser observada em
amostras de solo fofo quando as tensões de cisalhamento
atingem um pico seguido por uma fase de amolecimento
aparente que, no caso de carregamento não drenado, é
associado com a tendência do material em contrair de
volume. Para alguns estados iniciais, a parte descendente
da resposta do material pode ser seguida por uma fase
crescente que se atenua à medida que o estado permanente ou
crítico for atingido. Nesta dissertação, a modelagem da
resposta de liquefação por carregamento estático, foi
feita considerando-se modelos propostos na literatura por
Juárez-Badillo (1999b) e Gutierrez e Verdugo (1995). Este
último, principalmente após modificação introduzida pela
dependência de alguns parâmetros em relação à tensão de
confinamento, levou a resultados bastante satisfatórios nas
retroanálises consideradas, apesar da relativa simplicidade
da formulação. / [en] Historically it is known that many failures in earth dams
and natural slopes can be attributed to the phenomenon of
sand liquefaction, caused by dynamic loads generated by
earthquake shaking or other rapid loading, such as blasts.
When liquefaction occurs, the strength of the soil
decreases and its ability to support foundations for
buildings and bridges is significantly reduced. Liquefied
soil can also exerts higher pressure on retaining walls,
which can cause them to tilt or slide, yielding settlement
of the retained soil with risks of destruction of
structures on the ground surface. The term liquefaction has
actually been used to describe a number of related
phenomena, which produce similar effects but whose
mechanisms are quite different in nature. These phenomena
can be divided into two main categories: flow liquefaction
and cyclic mobility. Flow liquefaction is a phenomenon in
which the static equilibrium is destroyed by static or
dynamic loads in a soil deposit with low residual strength.
Failures caused by flow liquefaction are often
characterized by large and rapid movements. Cyclic
mobility, on the other hand, is a liquefaction phenomenon
triggered by cyclic loading, occurring in soil deposits
with static shear stresses lower than the soil strength.
Deformations due to cyclic mobility develop incrementally
because of static and dynamic stresses that exist during an
earthquake. The rising of tailing dams using the upstream
construction technique can lead to static liquefaction
failure if the rate of construction is sufficiently
high to cause excess pore pressure to develop in the
tailings. The liquefaction response is observed for loose
specimens when the shear stress exhibits a peak followed by
a phase of apparent softening that, in undrained loading, is
associated with the tendency of the material to contract
(densify). For some initial loading states, the descending
part of the response is followed by an increasing part,
leveling-off eventually when the material reaches the
final, critical (steady) state. In this thesis, the
modeling of the phenomenon of static liquefaction is
carried out considering the constitutive models proposed in
the literature by Juárez-Badillo (1999b) and Gutierrez &
Verdugo (1995). The latter, mainly after introducing the
assumption that some material parameters are stress
dependent and not simple constants, as in the original
version, produced good matching between experimental and
predicted results, in spite the simplicity of the
mathematical formulation.
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Shaking Table Tests to Study the Influence of Ground Motion, Soil and Site Parameters on the Initiation of Liquefaction in SandsVarghese, Renjitha Mary January 2014 (has links) (PDF)
Liquefaction is a phenomenon in which soil loses a large percentage of its shear resistance due to increased pore water pressure and flows like a liquid. Undrained cyclic loading conditions during earthquakes cause liquefaction of soils, which can lead to catastrophic failures such as bearing capacity failures, slope failures and lateral spreads. The concepts and mechanisms of liquefaction were studied extensively by many researchers. Though the factors affecting the liquefaction response of soils during earthquakes are well documented in literature, there are still some gray areas in understanding the individual and combined effects of factors like frequency, gradation, fines content and surcharge pressure on the initiation of liquefaction. The objective of this thesis is to study the influence of ground motion, soil and site parameters on the initiation of liquefaction in saturated sand beds through laboratory shaking table model tests and numerical studies.
Shaking table tests are carried out using a uniaxial shaking table on sand beds of 600 mm thickness. The initiation of liquefaction was observed and identified by measuring the pore water pressure developed during the sinusoidal cyclic loading. Free field liquefaction studies are carried out on sand beds to study the influence of ground motion parameters, namely, input acceleration and frequency of shaking on liquefaction. These studies revealed that acceleration is one of the important parameters that can affect the initiation of liquefaction in sands. Increase in acceleration reduces the liquefaction resistance of sand and a small increase in acceleration can trigger liquefaction. Frequency of shaking did not affect the initiation of liquefaction at lower frequencies but a threshold frequency which triggered instant increase in the excess pore pressures is observed. Liquefaction caused slight initial amplification followed by de-amplification of accelerations due to the stiffness
reduction in soils during liquefaction, the effect being more pronounced in the top layers of the sand bed. Pore water pressure ratios during dynamic loading decreased with depth below the surface of the sand bed due to the low initial effective vertical stress and upward transmission of pore pressure during undrained loading.
Shaking table tests are carried out to study the influence of soil parameters such as relative density, thickness of dry overlying sand layer and gradation. Relative density of sand can influence the liquefaction potential of sand to a great extent, about 10% increase in relative density bringing down the probability of liquefaction by about 50%. With the increase in height of dry overlying sand layer, liquefaction potential has decreased nonlinearly. Change in grain size altered the pattern of liquefaction and pore pressure development and it is observed that the liquefaction in finer sands is influenced by the frequency of shaking to a larger extent. Surcharge pressure from building loads increased the liquefaction potential and heavier structures got liquefied at lower pore water pressure ratios. Significant post-liquefaction de-amplification was observed in sand beds with surcharge pressure.
Parametric numerical analyses are carried out using finite difference program FLAC (Fast Lagrangian Analysis of Continua) with FINN model to measure pore water pressures in the sand bed. Results from numerical analyses with change in the acceleration, surcharge pressure and thickness of dry overlying layer agreed well with the experimental results. However, effect of frequency in numerical studies did not match with the experimental observations, because of the inherent boundary effects in the experimental models. Results from this thesis provided important insights into the development of pore water pressures in sand beds during cyclic loading events, apart from enhancing the understanding towards the effect of various ground motion, site and soil parameters on the initiation of liquefaction in sand beds.
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