• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 4
  • 4
  • 1
  • Tagged with
  • 1485
  • 695
  • 550
  • 156
  • 129
  • 127
  • 69
  • 52
  • 48
  • 37
  • 35
  • 33
  • 32
  • 30
  • 29
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

A finite element analysis of soil structure interactions during consolidation

Tran, D. V. January 1988 (has links)
The importance of soil-structure interaction has been recognised for a long time, in fact since the early stage of modern soil mechanics. However, conventional designs of many superstructures still pay little attention to the true coupled nature of soil-structure interaction. Superstructures such as frames, buildings are normally analysed considering the bases to be either completely rigid or hinged. Flexible bulkheads, raft foundations and tunnels are often designed as if they are rigid; pore water pressures generated by various forms of . loading or unloading of the soils are computed using uncoupled Terzaghi theory and in many cases, the effects of rigidity of the superstructures are ignored.
2

A cost-effective, mobile platform-based, photogrammetric approach for continuous structural deformation monitoring

Wang, Chih-Heng January 2013 (has links)
With the evolution of construction techniques and materials technology, the design of modern civil engineering infrastructure has become increasingly advanced and complex. In parallel to this, the development and application of appropriate and efficient monitoring technologies has become essential. Improvement in the performance of structural monitoring systems, reduction of labour and total implementation costs have therefore become important issues that scientists and engineers are committed to solving. In this research, a non-intrusive structural monitoring system was developed based on close-range photogrammetric principles. This research aimed to combine the merits of photogrammetry and latest mobile phone technology to propose a cost-effective, compact (portable) and precise solution for structural monitoring applications. By combining the use of low-cost imaging devices (two or more mobile phone handsets) with in-house control software, a monitoring project can be undertaken within a relatively low budget when compared to conventional methods. The system uses programmable smart phones (Google Android v.2.2 OS) to replace conventional in-situ photogrammetric imaging stations. The developed software suite is able to control multiple handsets to continuously capture high-quality, synchronized image sequences for short or long-term structural monitoring purposes. The operations are fully automatic and the system can be remotely controlled, exempting the operator from having to attend the site, and thus saving considerable labour expense in long-term monitoring tasks. In order to prevent the system from crashing during a long-term monitoring scheme, an automatic system state monitoring program and a system recovery module were developed to enhance the stability. In considering that the image resolution for current mobile phone cameras is relatively low (in comparison to contemporary digital SLR cameras), a target detection algorithm was developed for the mobile platform that, when combined with dedicated target patterns, was found to improve the quality of photogrammetric target measurement. Comparing the photogrammetric results with physical measurements, which were measured using a Zeiss P3 analytical plotter, the returned accuracy achieved was 1/67,000. The feasibility of the system has been proven through the implementation of an indoor simulation test and an outdoor experiment. In terms of using this system for actual structural monitoring applications, the optimal relative accuracy of distance measurement was determined to be approximately 1/28,000 under laboratory conditions, and the outdoor experiment returned a relative accuracy of approximately 1/16,400.
3

Soil-pile interaction of bored and cast in-situ piles

Lee, Lin January 2001 (has links)
The research presented in this thesis was undertaken to investigate the changes of the engineering properties of clay surrounding bored piles induced by the ion migration and hence the variation of shaft carrying capacity with time. As lime forms one of the major chemical compositions in cement, it follows that the cement from the bored piles will have a similar effect of improving the engineering properties of the clay adjacent to the bored piles. A number of model piles were constructed in order to study the soil-pile chemical interaction. At a specific time, the piles were subjected to load tests and the clay surrounding the piles was tested for its engineering and chemical properties. The load-settlement curves show that failures take place at large displacements compared with the typical values of 0.5% to 2% of pile diameter normally used. From the tests and analysis of other researchers' works together with the results from this research, equations were drawn for determining the settlement to fully mobilize the shaft resistance of pile. The results obtained showed that the shaft resistance of the bored pile increased with time over the monitoring period investigated. Together with this, calcium and hydroxyl ions were detected in the clay surrounding the pile. It can be concluded that soil-pile chemical reaction does take place and it affects pile behaviour.
4

Identification of cracks in beams using vibration modal characteristics

Bahador, Amirabbas January 2014 (has links)
The research presented in this thesis investigates new vibration-based crack detection methods in simply-supported, cantilevered and clamped-clamped beams using signal processing techniques. Different cracked beams with cracks of various depths located at different positions along the beams were simulated using the ABAQUS Finite Element Analysis (FEA) software. The difference in the modelling of cracks in this thesis compared to previous works is that the cracks in beams are modelled as hairline cracks without any widths. In previous works, cracks are mostly modelled as slots of specified widths. Initially, a comprehensive mesh study based on natural frequencies and modal displacements is performed to obtain the optimum element type and mesh structures for intact and cracked beams and the accuracy of the FEA modal data is verified using computed analytical modal data. Subsequently, the ABAQUS FEA program is used to compute the modal displacements of intact and cracked beams. From these modal displacements, the first four transverse displacement mode shapes (DMS) of the beams are extracted. Also, the natural frequencies of intact and cracked beams with stationary roving masses are computed using the ABAQUS FEA program. The natural frequencies of a beam change as an auxiliary mass, that is the roving mass, is traversed from one end of the beam to the other. This variation in natural frequencies with locations of the mass gives rise to a natural frequency curve (NFC) for the beam. From the computed natural frequencies, NFCs for the first four transverse modes of beams are derived. The simulated DMS and NFCs are used in the development of the crack detection methods which are based on the stationary wavelet transform (SWT) of the DMS and NFCs and the derivatives of the NFCs. These resulted in four distinct crack detection procedures, namely: (1) The difference of the Stationary Wavelet Transform (SWT) of two sets of augmented modal data, (2) SWT of residuals from the difference of analytical baseline data and cracked simply-supported numerical data, (3) SWT of the natural frequency curves (NFCs), and (4) derivatives of the NFCs. It is shown that the first procedure, that is the difference of the SWT detail coefficients of two sets of augmented modal data, gives high accuracy and sensitivity in detecting double symmetric and multiple cracks as well as single cracks with various depths and different locations in simply-supported, clamped-clamped and cantilevered beams. The method takes the SWT detail difference of the extrapolated odd and even data points of the original signal which increases the sensitivity of the crack detection method and also eliminates the SWT end effects. Similarly, the second procedure, which is based on the SWT of the residuals, provides accurate crack identification of single and multiple cracks. The residuals are obtained from the difference of the modal displacements of the cracked beams and the analytical intact baseline data. The application of SWT to the residuals increases the accuracy and efficiency of crack detection method. Furthermore, it is shown that the third procedure, which is the SWT of NFCs, is capable of detecting single and multiple cracks in simply-supported, clamped-clamped and cantilevered beams with high accuracy. The SWT detail coefficients of intact beams are used as a baseline data to eliminate the end effects of the SWT detail coefficients of the cracked NFCs. Finally, it is shown that the fourth procedure, which is based on the first, second and third derivatives of the NFCs of the cracked beams, is capable of detecting, locating and determining the severity of single, double symmetric and multiple cracks in beams with various boundary conditions.
5

Adaptive and fixed mesh investigaiton of localisation in strain-softening geomaterials

Bakar, Abu January 1999 (has links)
The influence of localisation in strain-softening soils has been investigated using classical continuum theory, and both fixed mesh and adaptive mesh refinement (AMR) finite element approaches. For this purpose, a non-associated cohesion-softening Mohr-Coulomb soil model has been developed and, for the adaptive mesh study. an existing AMR algorithm has been upgraded by implementing the superconvergent patch recovery smoothing technique. Both fixed mesh and adaptive mesh approaches have been used to analyse the problem of passive earth pressure failure, with the computed results for limit load and failure plane orientation being compared with analytical solutions available in the literature. Parametric studies have been included to consider the influence of degree of softening and material dilation angle. The fixed mesh results highlight the findings of previous investigators who showed that, in the presence of strain-softening, the solution is non-unique: in particular. as the mesh density increases, so the post-peak load-displacement response becomes steeper, without there being any sign of convergence. However, this mesh dependency is shown to be less significant for a rough retaining wall (i.e. as compared to a smooth wall), due to the increased influence of progressive failure. The adaptive mesh results show that, in the absence of an internal length scale facility, shear band width reduces without limit as the minimum element size gets smaller. However- for both smooth and rough walls, load-displacement response converges to a unique solution, suggesting that adaptivity may be an alternative solution to non-standard continuum theories in countering mesh dependency in strain-softening computations. A major advantage of the adaptive mesh algorithm is its ability to update meshes during an analysis, to account for new and changing regions of strain concentration. Stress and strain contours show that this gives a better definition of developing failure mechanisms, compared to the fixed mesh approach. Using AMR, the solutions are relatively free from mesh alignment effects, with mesh configurations following strain concentrations, rather than vice versa. Furthermore, the efficient use of elements means that mesh adaptivity has an important role to play in the solution of much larger geotechnical problems.
6

Creep characteristics of soft rock and modelling of creep in tunnel : determination of creep characteristics of soft rock and development of non-linear creep analysis code for squeezing tunnel problem

Yu, Chi-Wen January 1998 (has links)
Squeezing or time-dependent creep deformation of rock has been encountered frequently in tunnels. It is particularly common in tunnels excavated in very soft rock or heavily fractured rock under significant in-situ stresses. In Taiwan, due to the inferior geological setting and the young sedimentary rock formations, squeezing in tunnels was found to be a very common problem from the recent case histories of some large span road tunnels. This research reviewed the existing rock creep models, and an adequate creep model for representing the creep behaviour of soft rock or a weak rock mass in general was selected based on testing data of some soft rock cores. A numerical code was developed and validated by using tunnel physical model test. Actual tunnel case histories were also used to develop the creep analysis method for practical engineering purpose.
7

Analysis of polyhedral domed structures composed of flat plates of sandwich material

Bettess, P. January 1971 (has links)
The finite element method was used to analyse a number of domed structures. Two new sandwich plate bending elements (rectangular and triangular) were devised. They were used to produce results for comparison with other solution methods. The agreement was excellent. They were also used to produce results for comparison with experimental work on sandwich plates. The agreement varied. To the triangular element was added a plane stress component, together with suitable apparatus for making transformations at plate boundaries. This element was used to solve three tetrahedral domes, a square pyramid, a hexagonal dome, and a 16-faced dome. All these were investigated experimentally. The agreements varied between very good and moderate. A comparison was also made with the only other published results on a sandwich folded plate structure, due to Benjamin. In conclusion some improvements were suggested.
8

A theoretical and experimental study of dynamic consolidation technique

Ghazireh, Nizar January 1991 (has links)
This thesis briefly summarises the various methods of ground improvement and then concentrates on the dynamic compaction technique. The history of the method is summarised and a review of the previous experimental and theoretical work is given. A simple theoretical study was carried out and an analytical equation was derived in order to predict the dynamic load on the soil surface. A computer programme was written in order to calculate the stresses and deformations in the soil when subjected to impact loads. The calculation of stresses and deformations was based on Boussinesq theory. Also the thesis investigates the behaviour of fully saturated sandy soils subjected to dynamic loading and examines the effects of falling mass, drop height and confining pressure on the loads, stresses, excess pore water pressure and both the dynamic and permanent displacements. Over 30 tests were carried out on specimens of 100 mm diameter and 200 mm height which were prepared in the laboratory. The falling masses ranged from 1 kg to 6 kg, the falling heights varied from 100 mm to 800 mm and the confining pressure ranged from 50 kPa to 300 kPa. The variation of porosity with the number of blows was studied. In addition, by carrying out a set of dynamic-static triaxial tests, the effect of compaction on the angle of shearing resistance was investigated. In these tests each specimen was first compacted to the desired level under certain conditions, and then it was tested in drained conditions to evaluate c'. A statistical analysis was carried out on the results obtained by the experimental work in order to find statistical models representing the variation of dynamic load, dynamic stress, dynamic p. w. p and both dynamic and permanent displacements. The variation of loads, stresses, p.w.p and displacements with time at the instant of impact was also studied. It was found that at impact a stress wave is created at the contact surface. This stress wave is reflected at the base and superposed with the original stress wave. This superposition dies with time.
9

Engineering soils to act as carbon sinks

Washbourne, Carla-Leanne January 2014 (has links)
Soils containing calcium (Ca) and magnesium (Mg) bearing waste silicate minerals may be intentionally engineered to capture and store atmospheric carbon (C). Within the soil environment these minerals can capture and store atmospheric C through the process of weathering that releases Ca and Mg which then precipitate as carbonate minerals. Like natural silicates, silicate ‘wastes’ and artificial silicates sequester C through carbonation of calcium (Ca2+) and magnesium (Mg2+). Terrestrial CO2 sequestration may be promoted by the inclusion of these reactive mineral substrates in soils, and many waste sites and urban and anthropogenic soils already contain quantities of these materials. The UK Government is currently committed to reducing carbon emissions by 80% in 2050 (against a 1990 baseline) and soils have a role to play, acting as sinks for carbon. It is proposed that soil engineering measures could harness the high C turnover of the global pedologic system, ~120Pg C a-1 , to develop an efficient method of enhanced weathering. Artificial silicates have the potential to capture 192-333 Mt C a-1 , representing 2.0-3.7% of contemporary global C emissions; natural silicates present a carbon capture potential many orders of magnitude greater. Mineral carbonation in an artificial soil setting has the potential to capture inorganic carbon comparable to organic carbon accumulation. Soils of this type can accumulate 20-30 kg C m2 as carbonates (≥ organic carbon content in natural soils, ~17.5 kg C m2 for rural soils in the UK). Laboratory investigations were carried out on a number of experimental scales, from meso-scale flow-through reactors to micro-scale batch experiments, to determine the rate at which Ca and Mg could be supplied from suitable materials in engineered soil systems to perform a carbon capture function. Environmental factors were controlled for each in order to constrain their contribution to the overall process. Batch experiments were carried out at standard temperature and pressure (STP) to investigate effects of changes in solute concentration, water chemistry, agitation and particle size. pH controlled experiments were run at STP from pH 3-8, to determine the effects of pH changes on the weathering of wollastonite. Flow-through weathering experiments at STP investigated the effects of time, water chemistry, hydrogeological conditions and addition of CO2 on the weathering of steel slag. Analytical results demonstrate that Ca leaches rapidly from a number of Ca-rich artificial minerals providing great potential for carbon capture to occur on human-relevant timescales. Steel slag was shown to weather at a log rate of -9.39 to -11.88 mol Ca m-2 sec-1 in laboratory settings and -7.11 to - 7.56 mol Ca m-2 sec-1 under ambient environmental conditions in the field over 975 days. Anthropogenic soils, known to contain substantial quantities of Ca and Mg-rich minerals derived from industrial and demolition activity (including iron and steel slag, cement and concrete), were systematically sampled across two field sites. Analysis illustrated mean soil carbonate values of 21.8 ± 4.7% wt to 41.16 ± 9.89 wt % demonstrating that a large quantity of soil carbonate forms and persists in these environments, formed at a rate of 18kg CO2 t-1 a-1 . Stable isotope data ( 13C, 18O) confirm that up to 81% of C in these pedogenic carbonates is atmospherically derived. 14 C data also suggest that a significant proportion of the C present in carbonates analysed is ‘modern’. Applying a current CO2 trading cost of £8-£12 t-1 CO2, the potential value of CO2 sequestration at a study site was calculated to be £51,843 £77,765 ha-1 after 58% of its carbonation potential had been exploited. The studies contained in this thesis add to a growing body of evidence for the formation of carbonate minerals in soil settings where Ca/Mg-bearing silicate minerals occur. They also support the idea that engineered soils could be effectively utilised for carbon sequestration. Soil engineering for carbon capture provides a comparatively cheap, easy and attractive way of beginning to offset the environmental impact of certain industrial processes. Carbonation of waste silicates is a useful exercise in ‘closing the loop’ on C emissions produced in their manufacture. Carbon capture taking place on sites containing industrial waste materials is of interest to a variety of stakeholders: site owners, third sector bodies and local and national legislative bodies. Effective, low- energy field-scale implementation of mineral carbonation through soil engineering could assuage current constraints on economic performance of enhanced weathering technologies and highlight the importance of soil carbon storage.
10

Multiphase modelling of desiccation cracking in compacted soil

Stirling, Ross Alexander January 2014 (has links)
The development of cracking as a result of desiccation is increasingly under investigation. This work is set within the context of climate change effects on surface processes influencing infrastructure slope stability. The inherent changes to the mechanical and hydrological behaviour of clayey soils subjected to desiccation are significant. The preferential transmission of water due to cracking is widely cited as a source of strength reduction that leads to infrastructure slope failure. In order to gain a better understanding of the cracking mechanism in typical compacted fill conditions, finite difference continuum modelling has been undertaken using FLAC 2D. The two-phase flow add-on has enabled the unsaturated behaviour of the desiccating soil to be included within the mesh. Physical behaviour observed in laboratory experiments has informed the development of the numerical model by allowing better constraint of boundary conditions. Model development has featured the inclusion of several non-linear processes that are fundamental to the changing soil response during drying. The influence of significant parameters has been identified and by means of a varied experimental program, the design, manufacture and testing of a laboratory test apparatus and procedure to define the tensile strength of compacted fills under varying saturation conditions was undertaken and subsequently validated. The factors affecting crack initiation and propagation have been investigated via parametric study. This demonstrated the significant influence of basal restraint on the generation of tensile stresses conducive to cracking and the fundamental importance of the tensile strength function within the proposed modelling methodology. Experimentation with the shape of the SWRC has shown the model to be very sensitive to the hydraulic properties of the material with not only the occurrence of primary cracking being affected but also the development of the desiccated crust. The findings of this work are relatable to the incorporation of desiccation effects in the development of coupled hydrological-mechanical continuum models where atmosphere-soil interactions are increasingly significant.

Page generated in 0.4669 seconds