Centrifugal experimental study of suction bucket foundations under dynamic loading

Centrifugal experiments were carried out to investigate the responses of suction bucket foundations under horizontal and vertical dynamic loading. It is shown that when the loading amplitude is over a critical value, the sand at the upper part around the bucket is softened or even liquefied. The excess pore pressure decreases from the upper part to the lower part of the sand layer in the vertical direction and decreases radially from the bucket's side wall in the horizontal direction. Large settlements of the bucket and the sand layer around the bucket are induced by dynamic loading. The dynamic responses of the bucket with smaller height (the same diameter) are heavier.

冲击载荷下饱和砂土中流动和破坏的X光观测

介绍脉冲X光机和医用X光机的特性，应用这两种设备进行一系列饱和砂土的冲击加载实验。利用医用X光机拍摄到了饱和砂土在冲击载荷作用下产生的横断裂缝、纵向排水通道以及密实沉降的照片，得到了横断裂缝和纵向排水通道的出现规律，从而为研究饱和砂土冲击液化后结构破坏与密实沉降的机理提供了一种实验观测手段。

受冲击的饱和砂土产生裂缝和通道的实验研究

本文首先对饱和砂土在动载荷下的液化、密实和结构破坏等方面做了文献综述，同时也叙述了化工冶金中的液固流态化的有关情况。本文探讨了饱和砂土中产生水平裂缝和纵向通道等现象的机理。为此，进行了扁平砂柱的落锤冲击实验和模拟圆柱落锤冲击实验中产生水平裂缝和纵向通道等现象的底部加压实验。扁平砂柱落锤冲击实验的目的是为了更清楚地观察饱和砂土结构破坏和孔隙水流动的情况，观察纵向通道和水平裂缝的产生和发展，以及水平裂缝和纵向通道之间的联系。为了探讨水平裂缝和纵向通道产生的条件和机理，设计和进行了底部加压实验，用来模拟落锤冲击实验的后期变形和渗流效应，测量出现水平裂缝和纵向通道时的超孔隙水压力和平均水流速度，统计分析采用什么样的参数可以表征水平裂缝和纵向通道的出现及其特性，发现液化指数等于1是出现水平裂缝的必要条件。此外，针对含有细砂层的饱和砂土在该层出现水平裂缝的现象，提出了一个简化模型，推导出砂柱的液化指数一维分布，根据液化指数等于1才能出现水平裂缝，能够说明水平裂缝出现的条件和位置。

Pipeline uplift response in thawed sandy backfills: Preliminary findings

Offshore and onshore buried pipelines under high operating temperature and pressures may lead to upheaval buckling (UHB) if sufficient soil cover is not present to prevent the upward movement of the pipeline. In regions where seasonal changes involve ground soil undergoing freezing-thawing cycles, the uplift resistance from soil cover may be minimum when the soil is undergoing thawing. This paper presents the results from 2 directly-comparable minidrum centrifuge tests conducted at the Schofield Centre, University of Cambridge, to investigate the difference in uplift resistance responses between fully-saturated and thawed sandy backfill conditions. Both tests were conducted drained at 30g using an 8.6 mm diameter aluminium model pipe, corresponding to a prototype pipe diameter of 258 mm. The soil cover/pipe diameter ratio, H/D, was kept at 1. Fraction E fine silica sand was used as the backfill. Preliminary experimental results indicated that the ultimate uplift resistance of a thawing sand backfill to be lower than that of a fully saturated sand backfill. This suggests that in regions where backfill soil undergoes freeze-thaw cycles, the thawing backfill may be more critical than fully saturated backfill for uplift resistance. The 2-dimensional displacement field during the experiment was accurately measured and analysed using the Particle Image Velocimetry technique. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE).

Deformation and fracture of impulsively loaded sandwich panels

Light metal sandwich panel structures with cellular cores have attracted interest for multifunctional applications which exploit their high bend strength and impact energy absorption. This concept has been explored here using a model 6061-T6 aluminum alloy system fabricated by friction stir weld joining extruded sandwich panels with a triangular corrugated core. Micro-hardness and miniature tensile coupon testing revealed that friction stir welding reduced the strength and ductility in the welds and a narrow heat affected zone on either side of the weld by approximately 30%. Square, edge clamped sandwich panels and solid plates of equal mass per unit area were subjected to localized impulsive loading by the impact of explosively accelerated, water saturated, sand shells. The hydrodynamic load and impulse applied by the sand were gradually increased by reducing the stand-off distance between the test charge and panel surfaces. The sandwich panels suffered global bending and stretching, and localized core crushing. As the pressure applied by the sand increased, face sheet fracture by a combination of tensile stretching and shear-off occurred first at the two clamped edges of the panels that were parallel with the corrugation and weld direction. The plane of these fractures always lay within the heat affected zone of the longitudinal welds. For the most intensively loaded panels additional cracks occurred at the other clamped boundaries and in the center of the panel. To investigate the dynamic deformation and fracture processes, a particle-based method has been used to simulate the impulsive loading of the panels. This has been combined with a finite element analysis utilizing a modified Johnson-Cook constitutive relation and a Cockcroft-Latham fracture criterion that accounted for local variation in material properties. The fully coupled simulation approach enabled the relationships between the soil-explosive test charge design, panel geometry, spatially varying material properties and the panel's deformation and dynamic failure responses to be explored. This comprehensive study reveals the existence of a strong instability in the loading that results from changes in sand particle reflection during dynamic evolution of the panel's surface topology. Significant fluid-structure interaction effects are also discovered at the sample sides and corners due to changes of the sand reflection angle by the edge clamping system. © 2012 Elsevier Ltd. All rights reserved.

A zona de contato entre as formações Motuca e Sambaíba, Permo-Triássico da bacia do Parnaíba, regiões de Filadélfia (TO), Riachão (MA) e Loreto (MA)

O intervalo que compreende o final do Paleozóico e início do Mesozóico foi marcado por mudanças globais paleogeográficas e paleoclimáticas, em parte atribuídas a eventos catastróficos. A intensa continentalização do supercontinente Pangéia, com a implantação de extensos desertos, sucedeu os ambientes costeiros-plataformais do início do Permiano. Os registros desses eventos no norte do Brasil são encontrados nas bacias intracratônicas, particularmente na Bacia do Parnaíba, representados pela zona de contato entre as formações Motuca e Sambaíba. A Formação Motuca é constituída predominantemente por pelitos vermelhos laminados com lentes de gipsita, calcita e marga. Na porção leste da Bacia do Parnaíba, as fácies da Formação Motuca tornam-se mais arenosas com a ocorrência expressiva de arenitos com estratificação cruzada sigmoidal. A Formação Sambaíba consiste em arenitos de coloração creme alaranjada com estratificação plano-paralela e estratificação cruzada de médio a grande porte. Em geral, o contato entre as unidades é brusco, representado pela passagem de arenitos finos com laminação cruzada cavalgante e acamamento flaser/wavy da Formação Motuca para arenitos médios com falhas/microfalhas sinsedimentares e laminações convolutas da Formação Sambaíba. Foram individualizadas 14 fácies sedimentares, agrupadas em quatro associações: AF1 – Lacustre raso / Planície de lama (mudflat), AF2 – “Panela” salina (saline pan), AF3 – Lençol de areia e AF4 – Campo de dunas. A AF1 foi depositada dominantemente por processos de decantação em um extenso ambiente lacustre raso de baixa energia, influenciado por influxos esporádicos de areias oriundos de rios efêmeros. Este sistema lacustre foi, provavelmente, influenciado por períodos de contração e expansão, devido às variações das condições climáticas predominantemente áridas. Os mais expressivos períodos de contração ocorreram na porção oeste da Bacia do Parnaíba, representados pelo desenvolvimento de planícies de lama (mudflats) associadas a lagoas efêmeras saturadas em carbonatos e a “panelas” salinas (saline pans- AF2). Os lençóis de areia (AF3) são planícies arenosas extensas, localmente com área úmidas, intensamente retrabalhadas por processos eólicos. A AF4 é interpretada como parte de um erg composto por dunas/draas em zona saturada em areia, com interdunas secas subordinadas. Intervalos deformados lateralmente contínuos por centenas de quilômetros ocorrem na zona de contato entre as formações Motuca e Sambaíba. Pelitos com camadas contorcidas e brechadas (Formação Motuca) e arenitos com falhas/microfalhas sinsedimentares, laminação convoluta e diques de injeção preenchidos por argilitos (Formação Sambaíba) são interpretados como sismitos induzidos por terremotos de alta magnitude (>8 na escala Ritcher). Anomalias geoquímicas de elementos traços como Mn, Cr, Co, Cu e Ni na zona de contato entre as formações, juntamente com a presença de micropartículas de composição metálica na matriz argilosa dos sismitos, corroboram com impactos de meteoritos no limite c, possivelmente do astroblema Riachão.

Swash-aquifer interaction in the vicinity of the water table exit point on a sandy beach

The coupling of sandy beach aquifers with the swash zone in the vicinity of the water table exit point is investigated through simultaneous measurements of the instantaneous shoreline (swash front) location, pore pressures and the water table exit point. The field observations reveal new insights into swash-aquifer coupling not previously gleaned from measurements of pore pressure only. In particular, for the case where the exit point is seaward of the observation point, the pore pressure response is correlated with the distance between the exit point and the shoreline in that when the distance is large the rate of pressure drop is fast and when the distance is small the rate decreases. The observations expose limitations in a simple model describing exit point dynamics which is based only on the force balance on a particle of water at the sand surface and neglects subsurface pressures. A new modified form of the model is shown to significantly improve the model-data comparison through a parameterization of the effects of capillarity into the aquifer storage coefficient. The model enables sufficiently accurate predictions of the exit point to determine when the swash uprush propagates over a saturated or a partially saturated sand surface, potentially an important factor in the morphological evolution of the beach face. Observations of the shoreward propagation of the swash-induced pore pressure waves ahead of the runup limit shows that the magnitude of the pressure fluctuation decays exponentially and that there is a linear increase in time lags, behavior similar to that of tidally induced water table waves. The location of the exit point and the intermittency of wave runup events is also shown to be significant in terms of the shore-normal energy distribution. Seaward of the mean exit point location, peak energies are small because of the saturated sand surface within the seepage face acting as a "rigid lid'' and limiting pressure fluctuations. Landward of the mean exit point the peak energies grow before decreasing landward of the maximum shoreline position.

Dynamic properties of sand from dry to fully saturated states

By using bender and extender elements test, the velocities of the primary and shear waves, V(P) and V(s) respectively, were measured for a sandy material by gradually varying the degree of saturation, S(r), between the dry and fully saturated states. The effect on the results of varying the relative density and effective confining pressure was also studied. The measurements clearly reveal that for a certain optimum S(r), which is around 0.7-0.9% for the chosen sand, the value of the shear modulus G reaches a maximum value, whereas the corresponding Poisson's ratio nu attains a minimum value. The values of the shear modulus corresponding to S(r) approximate to 0% and S(r) = 100% tend towards the same value. For values of Skempton's B parameter greater than 0.99, the values of V(P) and nu rise very sharply to those of water. The predictions from Biot's theory with respect to the variation of V(P) with S(r) match well with the measured experimental data.

Comparison of the Dynamic Response of a Saturated Soil Between Two Models

By comparing the dynamic responses of saturated soil to Biot's and Yamamoto's models, the properties of the two models have be pointed out. First of all, an analysis has been made for energy loss of each model from the basic equations. Then the damping of elastic waves in coarse sand and fine sand with loading frequency and soil's parameters have been calculated and the representation of viscous friction and Coulomb friction in the two models has been concluded. Finally, the variations of loading wave damping and stress phase angles with water depth and soil's parameters have been obtained as loading waves range in ocean waves.

Measurement of coefficient of consolidation during reconsolidation of liquefied sand

Saturated sands particularly at low relative density commonly exhibit rises in excess pore pressure when subjected to earthquake loading. The excess pore pressure can approach a maximum value, limited by the initial vertical effective stress. After the completion of earthquake shaking, these excess pore pressures dissipate according to the consolidation equation, which can be solved to produce a Fourier series solution. It will be shown by manipulation of this Fourier series that excess pore pressure traces provide a method for back-calculation of coefficient of consolidation Cv. This method is validated against dissipation curves generated using known values of C v and seen to be more accurate in the middle of the layer. The method is then applied to data recorded in centrifuge tests to evaluate Cv throughout the reconsolidation process following liquefaction conditions. C v is seen to fit better as a function of excess pore pressure ratio than effective stress for the stress levels considered. For the soil investigated, Cv is about three times smaller at excess pore pressure ratio of 0.9 compared to excess pore pressure ratio of 0. Copyright © 1996-2011 ASTM.

Quantifying the influence of humic acid adsorption on colloidal microsphere deposition onto iron-oxide-coated sand.

This article describes an approach for quantifying microsphere deposition onto iron-oxide-coated sand under the influence of adsorbed Suwannee River Humic Acid (SRHA). The experimental technique involved a triple pulse injection of model latex microspheres (microspheres) in pulses of (1) microspheres, followed by (2) SRHA, and then (3) microspheres, into a column filled with iron-coated quartz sand as a water-saturated porous medium. A random sequential adsorption model (RSA) simulated the gradual rise in the first (microsphere) breakthrough curve (BTC). Using the same model calibration parameters a dramatic increase in concentration at the start of the second particle BTC, generated after SRHA injection, could be simulated by matching microsphere concentrations to extrapolated RSA output. RSA results and microsphere/SRHA recoveries showed that 1 mg of SRHA could block 5.90 plus or minus 0.14 x 10^9 microsphere deposition sites. This figure was consistent between experiments injecting different SRHA masses, despite contrasting microsphere deposition/release regimes generating the second microsphere BTC.

Quantifying mircoorganism/organic matter interactions in saturated porous media.

Proteins and humic acids are common constituents of waste water. Latex colloids (colloids) acted as surrogates for microorganisms in multiple pulse dynamic column experiments (MPEs) that permitted colloid mobility to be quantified before and after the injection of either BSA (a protein), or Suwannee River humic acid (SRHA).
At low OM coverage colloid breakthrough curves demonstrated both BSA and SRHA reduced colloid deposition rates, but did not affect colloid irreversible deposition mechanisms. By contrast, high levels of SRHA surface coverage not only further reduced the matrix’s ability to attenuate colloids, but also resulted in reversible adsorption of a significant fraction of colloids deposited. Modelling of colloid responses using random sequential adsorption modelling suggested that 1 microgram of SRHA had the same effect as the deposition of 5.90±0.14 x109 colloids; the model suggested that adsorption of the same mass of BSA was equivalent to the deposition of between 7.1x108 and 2.3x109 colloids.
Colloid responses in MPEs where BSA coverage of colloid deposition sites approached saturation demonstrated the sand matrix remained capable of adsorbing colloids. However, in contrast to responses observed in MPEs at low surface coverage, continued colloid injection showed that the sand’s attenuation capacity increased with time, i.e. colloid concentrations declined as more were deposited (filter ripening).
Importance: Study results highlight the contrasting responses that may arise due to the interactions between colloids and OM in porous media. Results not only underscore that colloids can interact differently with various forms of deposited OM, but also that a single type of OM may generate dramatically different responses depending on the degree of surface coverage. The MPE method provides a means of quantifying the influence of OM on microorganism mobility in porous media such as filter beds, which may be used for either drinking water treatment or waste water treatment. In the wider environment study findings have potential to allow more confident predictions of the mobility of sewage derived pathogens discharging to groundwater.

Comparing the influence of two different natural organic matter types on colloid deposition in saturated porous medium

Humic acid and protein are two major organic matter types encountered in natural and polluted environment, respectively. This study employed Triple Pulse Experiments (TPEs) to investigate and compare the influence of Suwannee River Humic Acid (SRHA) (model humic acid) and Bovine Serum Albumin (BSA) (model protein) on colloid deposition in a column packed with saturated iron oxide-coated quartz sand. Study results suggest that adsorbed SRHA may inhibit colloid deposition by occupying colloid sites on the porous medium. Conversely, BSA may promote colloid deposition by a 'filter ripening' mechanism. This study provides insight to understand the complex behavior of colloids in organic matter-presented aquifers and sand filters. © (2012) Trans Tech Publications, Switzerland.