1,720,993 research outputs found
Large Strain Analysis of Electro-Osmosis Consolidation for Clays
No full textConsolidation of soft clay creates a lot of problems in foundation engineering, because of the very low clay permeability and high compressibility. Primary consolidation takes a long time to complete if the material is left consolidating under atmospheric evaporation, and traditional dewatering techniques, such as surcharge preloading, vacuum preloading or vertical drains, have been used for decades to shorten the consolidation time. Among new soft ground improvement technologies, electro-osmosis consolidation is receiving much attention as a possible time efficient solution. Electro-osmosis is a novel technique to consolidate soft clays, and involves the flow of pore fluid in a soil mass in response to an applied electrical field. The electrodes (positive and negative) are installed in pairs in the soil mass; the direct current then forces ions in the mobile part of the electric double layer (EDL) to move from the anode towards the cathode, causing water flow. Electro-osmosis is found to be more effective in clayey soils because the electro-osmosis permeability is independent of the grain size. This means that electroosmosis can generate flow rates that are 100 to 1000 times greater than hydraulic flows in fine grained soft clays. The purpose of this thesis is to develop a numerical model for simulating multidimensional and fully coupled multi-physics electro-osmosis consolidation, including the elasto-plastic behaviour of soil and time dependent transport parameters at large strain. Special attention is paid to the simulation of complicated geometries and boundary conditions, and to the inclusion of more advanced elasto-plastic constitutive models. The overall goal is to develop a more realistic numerical tool which addresses the main features of electro-osmosis consolidation, and that has potential use in the design and optimization of field applications. In this thesis, numerical models for the electro-osmosis consolidation of soft clays in multi-dimensional domains at large strain are presented, which consider the full coupling of the soil mechanical behaviour, pore water transport, pore gas transport and electric flow. In particular, elasto-plastic constitutive models (i.e. the Modified Cam Clay model and Barcelona Basic Model) are employed to describe the mechanical behaviour of the clay, and some empirical expressions are employed to describe the nonlinear transport parameters. The proposed models have been verified against analytical/numerical solutions and also evaluated with results obtained formlaboratory experiments. Overall, excellent agreement has been found, which demonstrates the accuracy and efficiency of the proposed models. Updated Lagrangian formulations are employed to account for the geometric nonlinearity. The importance of considering large strains in a consistent and proper way is demonstrated, and differences with models based on small strain theory are highlighted. As deformation is the key concern during consolidation behaviour, various numerical examples are investigated to study the deformation characteristics. The ratio of electro-osmosis permeability to hydraulic permeability keo/kw is a key factor in electro-osmosis consolidation. Generally, electro-osmosis permeability and hydraulic permeability decrease with a decrease in the void ratio and degree of water saturation, but the decrease in hydraulic conductivity is much faster than the decrease of electro-osmosis permeability, so the ratio keo/kw increases during the consolidation process. A field test of electro-osmosis consolidation has been analysed, showing excellent agreement between the computed and measured settlements. Various electrode configurations, as well as current intermittence and current reversal approaches for electro-osmosis consolidation have also been investigated using the proposed model. The particular contribution of this thesis is that it introduces a realistic numerical tool for the simulation of electro-osmosis consolidation. It is able to simulate field applications with complicated boundary and geometry conditions, as well as practical applications such as current intermittence and polarity reversal, which are often employed in the field to achieve efficient and economical consolidation. Feasibility studies and a proper design are important for the field application of electro-osmosis consolidation. Hence this numerical tool has potential use in the design and analysis of electro-osmosis consolidation, including the assessment of factors for achieving optimal dewatering effects and estimating the cost.Geoscience & EngineeringCivil Engineering and Geoscience
Dynamic Finite Element Investigation of Wave Attack on Sea Dikes: A Coupled Approach using Plate and Volume Elements
No full textApproximately 400 kilometres of Dutch sea dikes are protected by bituminous concrete revetments to prevent damage from erosion and repeated wave attacks during storms. The numerical analysis of sea dikes subjected to cyclic wave loading needs to consider the behaviour of the bituminous concrete revetment, and the behaviour of the subsoil including the generation and dissipation of the excess pore pressures, as well as the interaction between the revetment and subsoil. This thesis develops and evaluates numerical methods for investigating this problem, using the dynamic Finite Element Method with the coupling of plate and volume elements. In dynamic finite element analysis, the arbitrary selected boundary generates wave reflections which normally causes oscillation problems. Absorbing boundary conditions are therefore adopted to minimise the wave reflection at the artificial boundaries. The effects of the absorbing boundary conditions are investigated in detail for both solid and water phases, and appropriate sets of parameters are recommended. In order to model the bituminous concrete revetment, which is a thin layer of material with a high stiffness, a very fine mesh is often needed in order to obtain accurate results. However, an explicit time integration algorithm is normally needed for dynamic analysis, which reduces the critical time step size required for numerical stability if the same element type is used for both the revetment and the soil, severely affecting simulation performance. In order to avoid this problem, in this thesis a structural plate element is developed based on the classical Kirchhoff thin plate bending theory for modelling thin layer materials instead of the volume element. For consideration of both stretching and bending problems, the Kirchhoff thin plate bending theory has been extended. Specifically, a more conventional 3-noded triangular plate element, with 1 translational and 2 rotational degrees of freedom per node, has been extended to include 2 extra translational degrees of freedom per node, i.e. giving a total of 3 translational and 2 rotational degrees of freedom per node. Moreover, an enhanced lumped mass matrix for the plate element has been constructed by considering both translational and rotational degrees of freedom. Numerical validations indicate that the use of plate elements for simulating the thin layer material provides a great improvement on the accuracy of the results, as well as a significant decrease in the computational cost. For the limit state design of some engineering problems, non-linear material models provide more accurate results than elastic material models. Here an elastic viscoplastic model is investigated and implemented for the new plate element, using the Drucker-Prager yield condition and a non-associated flow rule. Typical creep behaviour of the bituminous concrete is presented and a parametric study of the viscoplastic model has been carried out for modelling the behaviour of Delft bituminous concrete. It is concluded that the viscosity value for a particular material changes from different conditions, that it depends not only on the material model and stress level, but also on the temperature. The developed numerical methods have been used to investigate the behaviour of sea dikes subjected to wave attacks. The results of the bending moments and stresses in the bituminous concrete, as well as the excess pore pressures and displacements in the soil, are presented. The tendency of a so-called "shake down" behaviour is observed after 10 cycles of wave attack during a severe storm, indicating that the displacement of the dike converges asymptotically to a particular final deformation.Geoscience & EngineeringCivil Engineering and Geoscience
Backward erosion piping: Initiation and progression
No full textBackward erosion piping is an internal erosion mechanism during which shallow pipes are formed in the direction opposite to the flow underneath water-retaining structures as a result of the gradual removal of sandy material by the action of water. It is an important failure mechanism in both dikes and dams where sandy layers are covered by a cohesive layer. Sand boils can indicate that backward erosion is present and they are observed regularly during high water and floods. Although failure resulting from backward erosion piping is not common, several dike failures in the US, China and the Netherlands have been attributed to this mechanism. Given the impact that climate change is expected to have, prediction models for backward erosion piping are becoming increasingly important in flood-risk assessment. The prediction models available until now, such as Bligh’s rule and the Sellmeijer model, were validated in the research programme ‘Strength and loads on flood defence structures’ (SBW: Sterkte en Belastingen Waterkeringen) in the period 2008-2010 using small-, medium- and large-scale experiments. These experiments showed that an empirical adjustment of the Sellmeijer model was required to take the effect of the sand type into account and that validation was not possible for loose sand types because the erosion mode is different in those conditions. However, the absence of a theoretical basis makes this proposed empirical adjustment unsatisfactory because it lacks robustness. The main question addressed by this dissertation is how to explain and predict the pipe-forming erosion processes in uniform sands. A review of the literature, in conjunction with additional experiments, showed that the critical head in pipe formation leading to dike failure depends on either pipe initiation or pipe progression. In some experiments, the critical head for pipe initiation exceeds that of pipe progression and equilibrium is therefore prevented. The experiments in which no equilibrium was observed allowed for the development of a model for pipe initiation. It was possible to relate the observed differences in critical gradient caused by scale, sand type and configuration to the fluidisation of sand very close to the exit, where the local gradients are high. In the field, pipe progression is likely to determine the critical gradient. The Sellmeijer model predicts the progression of the pipe on the basis of the equilibrium of particles on the bottom of the pipe. The literature, and an analysis of the pipe width, depth, gradient and erosion process in experiments, indicate that pipe progression relies on two processes: primary erosion, which causes the removal of particles at the pipe tip, and secondary erosion, which causes the erosion of the pipe walls and bottom. Although the Sellmeijer model does not include primary erosion, it does function well for sand layers with a 2D exit configuration in which there is no variation in the grain size along the pipe path. The adaptation of the Sellmeijer model that was found necessary to account for the effect of sand type can be replaced by using the original model in combination with a variable bedding angle based on incipient motion experiments from the literature. The Sellmeijer model does not predict the critical gradient well for 3D configurations such as flow towards a single point, or for heterogeneous soils. Variations in the grain size in the pipe path were found to result in significantly higher critical gradients than expected, whereas a strong concentration of the flow towards the exit led to a fall in the critical gradient. 3D numerical calculations and the inclusion of primary erosion in the Sellmeijer model are needed to predict piping under these conditions.Geoscience & EngineeringCivil Engineering and Geoscience
First Order Reliability Method: Concepts and Application
No full textFirst/second-order reliability method (FORM/SORM) is considered to be one of the most reliable computational methods for structural reliability. A relative advantage of such analytical methods is that they provide physical interpretations and do not require much computation time. Designs based on FORM/SORM are usually performed using commercial software packages in which the underlying concept of the Reliability method is hidden. Also, the available literature is not easy to read and the basic concept is buried in complex mathematical equations. This document aims to give a comprehensive understanding of First Order Reliability Methods. In this document, practical application of FORM is demonstrated with a retaining wall and slope stability problem, both analysed using a spreadsheet model developed by Low (2003). Both applications presented are existing examples by Low (2003, 2005). These are briefly explained, and later modified to understand the efficiency of the model, and to investigate the effect of geometrical uncertainties in a slope’s stability. Additional Graduation Thesis - The efficiency of spreadsheet model is investigated by considering uncertainty of geometrical parameters. Taking advantage of FORM’s ability to reflect sensitivity of the parameters, a sensitivity interpretation of the parameters involved in the slope stability problem is made. The influence of uncertainty of soil layering on the stability of the slope is analysed. Additional investigation on the effect of one dimensional spatial variation on the outcome of slope reliability is made. The spreadsheet model uses intuitive First Order Reliability approach and MS Excels’s inbuilt solver with constrained optimisation to compute Reliability index and probability of failure. It was found to be relatively less user friendly when compared to the existing commercial software packages but it serves as a very efficient tool to understand the concepts of FORM better. The major disadvantage of Monte Carlo regarding its high computational cost has triggered the need to find better alternatives. In most applications, FORM only needs a small number of iterations for convergence, making it more computationally efficient than MCS. This is particularly so when the failure probabilities are low. With the limited research here, it is safe to say that FORM could serve as a first step in Reliability based design to study the relative importance of parameters.Civil Engineering and GeosciencesGeoscience and Engineerin
Implementation and inspection of a high-cycle accumulation model
No full textThe prediction of permanent deformation accretion in granular materials as a result of long-term cyclic loading is a complex challenge that receives great attention in engineering fields, including infrastructural design and design of offshore wind turbines. Many models have been proposed in the past which aim to predict this cyclic deformation accumulation for specific or general purpose. One of the most advanced models available is the "High-Cycle Accumulation (HCA) model" byNiemunis et al (2005) [1]. Although very promising, this model is as of yet rarely used in Dutch geotechnical engineering practice. One of the possible reasons for this is the lack of understanding and availability of the model. This thesis has the aim to overcome these problems by gathering relevant information on the HCA model, implementing it in Plaxis 2D finite element software and inspecting the capabilities of the resulting FE routine. After gathering the information on the model from literature, it was concluded that the model has great potential and is themost extensively validated model available. However, there are also some boundary conditions for which the model fails to predict the deformations correctly. The model also requires a vast number of advanced laboratory tests to be carried out in order to compute the model parameters. The implementation of the constitutive equations in Plaxis 2D was performed successfully and was verified for a single-element type case. When applying the FE routine to a boundary value problem, numerical issues arising from the finite element calculation kernel conflict with the sensitivity of the HCA model to the state parameters, resulting in a severe over-estimation of accumulated settlements. A sensitivity analysis was carried out, which shows that the model is highly sensitive to the parameters of the hypoplastic model (which is used in the initial calculation phase of the FE routine). It is concluded that for the HCA routine to be used in Plaxis, the numerical issues should be investigated and possibly a numerical interface should be designed. The high sensitivity to the model parameters, combined with the vast amount of required effort (and cost) to compute these parameters for a sand sample make the model less favourable for practical design applications.Civil Engineering and GeosciencesGeoscience and EngineeringGeo-engineerin
Efficiency of a Column Supported Embankment in Sabkha Soil
No full textThis thesis report presents an investigation of a soil improvement technique that is being executed for a Van Oord project in Kuwait. The soil is improved by the use of a column supported embankment, consisting of sand columns installed in a soft soil layer and a sand platform. The efficiency of this method is defined in terms of stress transfer and settlement reduction. When the soil improvement is finished and the land will be used, there are conditions concerning bearing capacity and settlement behavior. To this extent two important parameters were defined. i.e. the incremental efficiency (the load increase in a sand column over the total surface load increase) and incremental settlement reduction ratio (the settlement of the improved soil over the settlement of the unimproved soil (i.e. soil that has not been improved by sand columns), under loading). To determine the efficiency of the soil improvement, a number of tests were performed on site. Tests included plate load tests (in this thesis referred to as zone load tests). The load tests were simulated in Plaxis, with the known load/settlement results the model could be benchmarked. Furthermore soil samples were taken and tested to determine the local soil characteristics. The parameters derived from the soil tests are also used in the Plaxis calculations. Plaxis allows for a step-by-step consolidation of the soft soil in which the columns were installed. It can be seen that the stress distribution changes for different stages of consolidation. The columns are first constrained by the very stiff soft soil layer (due to high excess pore pressures under loading). When the pore pressures dissipate the constraining stress is lowered and the column head expands. Under vertical loading the stress in the column head has a funnel shape, due to the displacements in the outer ring of the column head. Based on the Plaxis calculations it can be concluded that when a load is activated on top of a surface of soil that has been improved by the use of sand columns (with a center-to-center distance of three meters), given that the platform is thick enough, 60% of that load is transferred to the column. With a greater center-to-center distance between the columns that percentage decreases, e.g. 28% for a column spacing of five meters. Compared to existing theories by Hewlett and Randolph (1988) and Zaeske (2001) (it should be noted that most existing theories assume presence of geosynthetic reinforcement, which is not the case for this project) the calculated column force is relatively low. A minimal thickness of the sand platform is needed to facilitate maximum efficiency. The thickness as determined by the Plaxis calculations are lower compared to existing literature. With platform heights of up to seven meters no full arching was observed, however partial arching did occur as evidenced by the efficiency values.Section Geo-EngineeringCivil Engineering and Geoscience
Shear strength of saturated sand-steel interfaces: Geotechnical issues found at landfall operations
No full textLandfall operations are conducted for connecting an offshore pipeline with process facilities on the shore. During a landfall operation an offshore pipeline is pulled by means of a steel wire rope on the shore with a velocity of 7 cm/s. During this process the steel wire rope interacts with the soil located on the seabed and on the shore. Allseas Engineering BV has a long experience on landfall operations and it was occasionally noticed that the steel wire ropes were buried beneath sand dunes which resulted from sedimentation and wave actions. In these instances excessive pulling forces were required to mobilise the steel wire ropes. A prototype experimental setup was developed for simulating the pulling process in medium scale. Physical modelling involved the pulling of a steel element through saturated sand with a relative density of 80%. The aim of this apparatus was to examine the shear failure mechanisms that develop in saturated sand while a steel pipe, with a significantly rough surface, is pulled through it. The main focus points of this study were the peak pulling force of the steel pipe and the change of pore pressures around the steel pipe's circumference during the pulling process. The latter two were examined with respect to six burial depths (0 - 0.31 m) and three pulling velocities (1, 4 and 7 cm/s). It was observed that at every test a momentary decrease of pore pressures was taking place around the steel pipe during the pulling process. A peak was always recorded at the same time as the peak pulling force and this was attributed to the tendency of sand particles to dilate around the steel pipe. Undrained loading conditions were caused by the high pulling velocities and low permeability of the soil. Therefore, dilation was restrained by the pore water and consequently tensile pore pressures developed which increased the shear strength of the soil, momentarily. Burying the steel pipe at different depths influenced, as it was expected, the peak pulling force due to the increase of the vertical effective stresses. In addition, the peak decrease of pore pressures was found to increase in magnitude while the burial depth ranged from 0 to 0.09 m and this was an unexpected event as the tendency of the soil to dilate was expected to be restrained. The magnitude of peak pore pressure decrease was also found to reduce at burial depths ranging from 0.09 m to 0.31 m due to the increase of vertical effective stresses that restrained the soil's tendency to dilation. The effect of pulling velocity to the peak pulling force and the peak pore pressure reduction values was also examined. The latter were found to increase linearly with the increase of pulling velocity, at each burial depth that was tested. Also, the peak pulling forces were found to increase linearly while comparing tests at the same burial depth, conducted with different pulling velocities. In addition, the increase of pulling velocity caused a linear increase on the stiffness index of the test specimens. The scientific significance of the results of the current study can assist to performing landfall operations in a more efficient way. It is recommended that during a landfall operation the initiation of the pulling of the steel wire rope should take place at the lowest possible rate. As a result, the maximum pulling force will be minimised and the pulling velocity can be increased gradually once the steel wire rope is mobilised. Moreover, the findings of this study can be useful for the (un-)installation of (offshore) piles, sheet pile walls, soil nails and dredging operations on saturated sands.Civil Engineering and GeosciencesGeoscience & EngineeringGeo-Engineerin
Reliability of long heterogeneous slopes in 3d: Model performance and conditional simulation
No full textHighway embankments, river dykes and sea dykes usually have a uniform cross-section and extend for a long distance in the third dimension. These long soil structures are generally characterised by spatially varying soil properties, i.e. soil heterogeneity. Slope stability failures of these structures may have significant economic and societal consequences. Thus, it is of particular interest for engineers to investigate the influence of soil spatial variability on the stability and failure mechanisms of long ‘linear’ structures. For example, earthen levee flood protection systems can be viewed as series systems, where failure at one location, or failure of one component, can result in catastrophic failure of the entire flood protection system and result in tragic loss of life, damage to fundamental infrastructure, and substantial economic impact to the immediate and surrounding regions. In order to ensure the desired level of flood protection system performance, standards in the Netherlands explicitly require probabilistic designs. For example, this may include the use of semi-analytical tools, such as Calle’s 2.5D method and the 3D method of Vanmarcke. However, these (semi-) analytical models make certain simplifying assumptions; in particular, that of a finite length cylindrical failure mechanism. The random finite element method (RFEM) has found increasing use for long engineered slopes in recent years, due to its conceptual simplicity to implement and its capability to comprehensively analyse the effects of soil spatial variability. As a simulation method, RFEM can be applied to large and complex systems, without the need to include some of the rigid idealisations and/or simplifications necessary for analytical solutions, resulting in more realistic models. Therefore, RFEMcan be used as a comparative tool in investigating the performance of simpler methods. Its biggest disadvantage is that it tends to be computationally expensive. The main body of this thesis is devoted to comparative studies (in terms of statistics of the realised factor of safety, the reliability and the failure consequences with respect to potential failure length and volume) of the three above models, for a range of spatial statistics of the soil shear strength. In particular, the relative performance of RFEM and Vanmarcke’s model is investigated for a relatively short slope (of length 10 times the height) for which the length effect may be ignored. For horizontal scales of fluctuation that are large compared to the slope length, the two approaches give similar results, because most of the failure surfaces computed in the RFEM analyses are then approximately cylindrical and propagate along the entire length of the slope, thereby matching Vanmarcke’s assumption and resulting failure length for this limiting condition. In contrast, for smaller values (i.e. less than the slope length), the two approaches can give significantly different results, with the RFEM response of the slope generally being much weaker than the Vanmarcke solution, apparently due to different predicted failure lengths and the influence of the cylinder ends in the simpler model. A second comparative study using all three models involves the so-called length effect (i.e. the increase of the probability of failure as the total slope length increases) for very long slopes (of length up to 100 times the height), using HPC strategies developed in this thesis. In contrast to the level crossing approach adopted in the two (semi-) analytical models, a simple power law equation was utilised with RFEM, which was validated based on the principles of probability of multiple independent (failure) events within the length of the slope. It is shown that RFEM predicts the smallest reliability indices for the range of cases considered. However, the solutions predicted by Vanmarcke’s and Calle’s models move closer to the RFEMresults at larger horizontal scales of fluctuation. Discrete failure lengths have been quantified in RFEMand comparedwith predicted failure lengths using the simpler models, in order to provide a rational explanation for the differences observed. Moreover, the ® factor used with Calle’s model in Dutch practice was investigated thoroughly via random fields for various degrees of spatial variability, enabling a comprehensive evaluation of its influence. While the unconditional RFEM is used as a baseline stochastic method to make the comparative studies, the conditional RFEM was implemented and applied in the last part of the thesis to two example geotechnical applications. The first example focuses on the efficient design of site investigation plans (i.e. optimum locations and sampling intensity) in a 3D soil deposit. A sampling efficiency index was defined and used as an indicator of the efficiency of a site’s plan. A ‘posterior’ distribution of the structure performance, after taking account of the spatial distribution of all the measured CPT data, was derived and showed a significant reduction in the uncertainty compared to the ‘prior’ distribution of the structure response by using the unconditional simulation based on random field theory. An optimal sampling position for the excavation of a slope was identified, both for a single stage of site investigation and a two stage site investigation. Moreover, an optimal sampling distance of half the horizontal scale of fluctuation was identified when an exponential correlation function is used. The second example is devoted to cost-effective designs of an excavated 3D slope. For the problem analysed, a steeper slope was found to be sufficiently reliable (i.e. in line with Eurocode 7) when conditional random fields were used. This was in contrast to the finding of unconditional simulations, due to the greater uncertainty due to only making partial use of available measurement data. The potential benefit of a 3D conditional simulation in geotechnical cost-effective designs has therefore been highlighted
Небольшие заметки о значимой проблеме. Рецензия на работу “Руководство по политической социологии: государства, гражданские общества и глобализация”
Full text linkThe material presents a review of the book of the western researchers in its original language: “The handbook of political sociology: states, civil societies and globalisation” (ed. by T. Janoski, R. Alford, A. Hicks, M.A. Schwartz).В материале представлена рецензия на англоязычную работу западных исследователей “Руководство по политической социологии: государства, гражданские общества и глобализация” (под редакцией Т. Яноски, Р. Алфорда, А. Хикса, М.А. Шварца)
Slope stability analysis based on random finite element method and probabilistic approach
No full textIn this work, an investigation into generic slope stability is carried out. The investigation takes into account slope parameters of height (H) and slope gradient (?) as well as material parameters of shear strength (vc) and spatial correlation length (?). A Monte Carlo analysis of many slopes with randomly generated shear strength parameters is carried out with the intention of determining ‘predicting functions’: the relations between failure probability(Pf) and the aforementioned parameters.Applied Earth ScienceGeoscience & EngineeringCivil Engineering and Geoscience
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