Doctor of Philosophy (Ph.D.)

Permanent URI for this collectionhttp://192.248.9.226/handle/123/8871

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item: Thesis-Full-text
Consolidation analysis of Sri Lankan peaty clay using elasto-viscoplastic theory
(7/16/2011) Karunawardena, WA; Kulathilaka, SAS
is necessary to take these properties into account in order to obtain better predictions from peat consolidation analyses. In the present study, the consolidation behavior of peaty clay found in Sri Lanka is extensively studied using a model based on the elasto-viscoplastic theory. The model can describe the prominent creep behavior of peaty soil as a continuous process. In addition, the model can accommodate the effect of structural degradation on the consolidation process. The analysis takes into account all the main features involved in the peat consolidation process, namely, finite strain, variable permeability, and the effect of secondary compression. Also, it considers the variable compressibility for stage constructed embankments which exert high levels of pressure on the peaty subsoil. The constitutive equations used in the model and the procedure adapted to account for the above-mentioned features of the analysis are described. The constitutive model is based on Perzyna's type viscoplastic theory and the Cambridge elasto-plastic theory combined with empirical evidence. In the finite element formulations, which are based on the finite deformation theory, an updated Lagrangian method is adopted. A description of the material parameters used in the model and the procedures applied to evaluate them, with standard laboratory and field tests, are explained. In addition,. a performance of the model incorporating the original and the modified Cam-clay theory is evaluated by simulating triaxial test results. A comparison shows that with the present definition of the parameters, the original model yields more representative results than the model based on the modified Cam-clay theory. Initially, the capability of the constitutive model to capture the consolidation behavior is verified using the consolidation model test data on peaty clay found in Sri Lanka. It is confirmed that the constitutive model is able to predict the observed creep characteristics and the effect of sample thickness on settlement predictions for the material under consideration. The performance of the model in predicting the consolidation behavior under field conditions is studied using field data on instrumented earth fill constructed on peaty clay. One-dimensional compression is assumed for the peaty clay due to the large plane area of the fill. Separate analyses are carried out by the model considering the infinitesimal strain theory, the finite strain theory, and the finite strain theory together with the effect of structural degradation in order to explore how these features describe the observed field behavior. Analyses reveal that it is necessary to consider finite deformation together with the effect of structural degradation in order to successfully simulate the resultant large strain and the stagnated pore water pressure observed in the field. The construction of road embankments over peat deposits is quite problematic, and thus, it is often done after first improving the properties of the peaty soil through the utilization of appropriate ground-improvement techniques. Understanding the field response of peaty clay during this improvement process is naturally of great importance. A constitutive model is applied to predict the field performance of embankments constructed on peaty clay using different ground-improvement techniques. The back analysis of embankments constructed with the preloading method indicates that the model can be successfully applied to predict both the deformation and the stability of structures constructed on peaty clays. The stability of the embankment during and after construction is verified by investigating the stress-strain characteristics of the subsoil. The model applications used to predict the consolidation behavior of embankments constructed by the preloading method, combined with other ground-improvement techniques, are then discussed. Embankments constructed with prefabricated vertical drains (PVDs) and sand compaction piles (SCPs) are considered, and finite element analyses are carried out in all cases by converting the actual three-dimensional conditions that exist around the drains into simplified two-dimensional plane strain conditions. The field behavior when PVDs are installed in the peaty clay is simulated using the equivalent vertical permeability for the PVD-improved subsoil. In the case of SCPs, a conversion scheme is used to transform the axisymmetric nature of sand columns into equivalent plane strain conditions. A comparison of the predicted results with the field observations shows a reasonable agreement. An analysis of the PVD-improved foundation indicates that the installation of PVDs not only accelerates the rate of consolidation, but influences the deformation pattern of the subsoil due to embankment loading. The analysis also shows that the use of PVDs can significantly increase embankment stability. The model prediction for the SCP-improved foundation reveals that the stiffness and the area replacement ratio used in the conversion scheme play vital roles in predicting the behavior of SCP-improved soft grounds. The observed improvements in the bearing capacity of the subsoil and in the stability of the embankment, brought about by the installation of SCPs, can be simulated by the model.
item: Thesis-Abstract
Degenerating Orr-Sommerfeld Eigenmodes and Development of Three-dimensional Perturbations
(5/24/2011) Shanthini, R
Degeneracies of temporally damped Orr-Sommerfeld eigenmodes are studied primarily for plane Poiseuille flow. The discrete spectrum of the eigenmodes is shown to possess infinitely much degeneracy, each appearing at a certain combination of k (the modulus of resultant wavenumber) and αR (the streamwise wavenumber times the Reynolds number). The streamwise phase velocities of the degeneracies are close to about 2/3of the centreline velocity, and their damping rates decrease with increasing Reynolds number. Interestingly, several degeneracies are operative even at sub-transitional Reynolds number. Degeneracies are found also in water table flow and some of these degeneracies are shown to have the same characteristics and modal structures as those in plane Poiseuille flow. The responses of the degeneracies in plane Poiseuille flow are investigated through the initial-value problem. The initial perturbation field is chosen to be free of normal vorticity, and its velocity component normal to the walls is described by the generalized Orr-Sommerfeld eigenfunction in the normal direction. The subsequent temporal development of this normal velocity is shown to be that of monotonical decay, in the linear regime. Notable initial growth is exhibited by the streamwise velocity of the two-dimensional perturbation flow in case of the least damped degeneracy. Nevertheless, in all cases of the degeneracies investigated, the two-dimensional perturbation flow looses its kinetic energy with increasing time. In the case of a three-dimensional perturbation flow, the span wise variations in the normal velocity induce normal vorticity. The amplitude of the induced vorticity and, hence, that of the streamwise perturbation velocity are shown to grow to significant peak values, before the exponential decay predicted by the linear theory sets in. The amplitude of the induced vorticity is shown to increase also with increasing Reynolds number. The presence of normal vorticity causes the perturbation flow to gain kinetic energy from the basic Poiseuille flow. This gain is so great in cases of the least damped symmetric and anti symmetric degeneracies, that the perturbation flow overcomes its energy loss due to viscous dissipation and exhibits significant initial growth of its kinetic energy, at crucial Reynolds numbers such as the transitional one. These growths, according to the linear theory, do not influence the normal velocity that is monotonically decaying with time. It is shown, however, that these growths are to be followed by nonlinear equations at about the transitional Reynolds number. It is also shown that describing the solution of the nonlinear system as waves travelling in one single oblique direction does not change the monotonous behaviour of the normal velocity. Key words: degeneracy, double eigenvalue, Orr-Sornmerfeld equation, direct resonance, stability, plane Poiseuille flow, water table flow, initial-value problem, three-dimensionality, vortex stretching, induced vorticity, nonlinearity, Reynolds stress, perturbation energy.
item: Thesis-Abstract
The growth of buddhist monastic institutions in Sri Lanka from Brahmi inscriptions :
(2001) Dias M; Reynolds CHB
The propagation of Buddhism in Sri Lanka in the third century B.C. as witnessed by the lithic records and the chronicles was due to the efforts of the Buddhist monks who spread Theravada or the orthodox tradition of the religion in the first few centuries. Its spread went parallel wtith the growth of monastic institutions in the island. At first the patronage extended by the kings and the people towards the Buddhist monks was to safe-guard the religion. They were endowed with dwellings and requisites for their maintenance. To make them self-sufficient the monks were endowed with permanent and valuable gifts consisting of land, tanks, revenue from taxes and so forth. When these endowments were made, the monks became landholders and gained a role of leadership in society. On the one hand, the relationship with the masses made thc Saitgha an institution which dominated society in its own way. On the other, it became a major factor to check the royal power. The monks generally regarded themselves as having a two-fold task; striving for their own salvation and preaching the Dhamma to lead others to find their salvation. It was the scholar and the preacher in them that were useful to society. Buddhism was flourishing in.the entire island by the second century A.D. Even before that dissensions among members of the Sattgha were visible in the premiere establishments such as Mahavihara and Abhayagirivihara. Some individuals struggled for power and tried to gain favours from the kings. By about the third century A.D. new trends of thought in Buddhism, coming from the subcontinent of India, were making headway in the island. By contact with these new movements, the monks of the Abhayagirivihara were becoming liberal in their views. The new schools of thought such as Vaitulyavada, described as heresies in the chronicles, divided them into sects. These schools of new thought had the backing of some kings though the Mahavihara vehemently opposed them. By about the fourth century, there were three fraternities, namely, Mahavihara, Abhayagiri and Jetavana. The inscriptions and the chronicles bear clear evidence to the manifestation of Mahayana ideals such as the Bodhisattva worship. The sixth and seventh century inscriptions attach great importance to the merit-making rituals that accompanied them. Here we find the Safigha in the role of the priest invoking blessings on the masses and becoming an indispensable figure in society.
item: Thesis-Abstract
Optimising earthmoving by linear programming and computer simulation
(5/24/2011) Jayawardane, AKW
The accuracy of planning and estimating of earthmoving operations in any highway construction is important for both successful tendering and high profit margins. Mass-haul diagrams and experienced engineering judgment together with deterministic methods have been the key factors in planning and estimating earthmoving operations. Despite this, the limited use of Mass-haul diagrams and inaccuracy of deterministic estimates are well known. Although Stochastic and Linear Programming methods were developed to overcome some of these limitations, those available hitherto are relatively fundamental and are not bold enough to incorporate most real life problems. After identifying the need for a relatively quick and accurate planning and estimating procedure, a new approach was developed by combining Computer Simulation and Linear/Integer Programming. The developed model was named RESOM; an acronym for Roadwork Earthmoving System Optimisation Model and was developed in three basic stages: simulation model; LPIIP model; and network model. RESOM can be applied to any road project to obtain an optimum earthmoving plan including material distribution, plant utilisation incorporating real life problems and constraints. The main aims of the simulation model were to obtain realistic unit costs and production rates using balanced plant teams. Various cycle element times of earthmoving equipment were obtained from standard distributions fitted onto field data collected from four sites in Sri Lanka. Comprehensive LPIIP formulations were developed incorporating constraints like project duration, plant availability, sequence of operations etc. to obtain an optimum earthmoving plan using the simulated results. The third stage of RESOM involved the presentation of the selected earthmoving plan in the form of network diagrams and barcharts. RESOM was successfully validated using two actual case studies (Anamaduwa Gam Udawa, Sri Lanka, and the A42 - Measham and Ashby By-pass, UK). Application and experimentation with RESOM were explained using two other case studies (the M40 - Banbury By-pass and a hypothetical example) and proved that about 20% cost savings can be obtained. The experimentation process revealed that RESOM could be of considerable help in planning, estimating and obtaining optimum earthmoving plans.
item: Thesis-Abstract
Porting a distributed operating system to a shared memory parallel computer
Atukorala, GS
This thesis describes the porting of a distributed operating system, Helios, to a bus based, shared memory, parallel processing computer. Helios design was originally based on a distributed memory, transputer network, connected via serial links. After examining the design of distributed operating systems in general, the design issues and the implementation of porting Helios to a Motorola MC68020jMC68881 based, shared memory, parallel computer, are described. Performance measurements that were taken on the new version of the operating system are then described. The results are compared with that of the original version and also with that of the Tripos operating system. Running a parallel programming application to test the efficiency in utilizing the available parallel processing power is also described. Finally, future work is suggested, particularly in the areas of inter-processor communication, the use of shared memory and porting Helios to other processors
item: Thesis-Abstract
Representation of transformers in switching surge studies
Lucas, JR
A Transformer representation suitable for switching surge analysis is derived in terms of 'primitive' inductance and capacitance matrices. Connection matrices are used to account for all types of winding connections. Frequency dependance of elements is considered. The transmission line is formulated on a frequency basis, and the earth return path is represented by Carson's formula. Series and shunt compensation are also included with the line. The cable is similarly formulated. The problem is solved in the frequency plane using the Fourier Transform, and then transformed back into the time domain. Modal analysis is used for both transmission line and cable two port admittance matrix formulation. Energisation on both low voltage as well as high voltage sides of the source side transformer are considered for transformer feeders. A few comparisons with field test oscillograms are given to validate the formulation of the problem. Voltage non-linearities when surge divertors are present are considered using a step response of the linear system and Duhamel's integral, together with the Newton-Raphson iterative technique for both single phase and three operation. Finally, the Newton-Raphson algorithm is further extended to take into account the magnetic non-linearity of the transformer 'core. Flux coupling between phases during saturation is calculated based on the instantaneous reluctance values of the individual limbs and the leakage air path.
item: Thesis-Full-text
Some factors influencing the engineering properties of Hawkesbury Sandstone
De Silva, MDK
The Hawkesbury Sandstone is a major rock unit within the Permian-Triassic sediments which form the Sydney Basin, Australia. It covers an area of about 12,5000 square kilometers and has a thickness of about 300 meters. As a result of its widespread occurrence in the region, particularly in the metropolitan area, it is of considerable importance to both the civil engineer and the geologist. This thesis examines the factors causing variations in the engineering properties of Hawkesbury Sandstone. An investigation has also been included to determine the suitability of crushed sandstone as a concrete and road aggregate. The determination of the mechanical properties of fresh Hawkesbury Sandstone was carried out in the laboratory on small specimens of intact rock, free from obvious macroscopic discontinuities, sampled at different locations in accordance with the standard test procedures. A number of sandstone exposures, in the form of quarries and roadside cuttings, were examined to understand the natural processes of weathering occurring in the field. Samples of weathered rock, representing all stages of weathering, were also tested in the laboratory to determine the effects of weathering on the properties of this sandstone. Thin section studies were made on all fresh and weathered samples to study the mineralogy and the texture of the rock. The clay fraction of the rock was analysed using X-ray diffraction and differential thermal analysis techniques. Samples of fresh Hawkesbury Sandstone were also collected in the field to produce crushed aggregates having a nominal particle size of 19 mm. Two trial mixes of concrete were made in the laboratory using crushed sandstone as the coarse aggregate. After curing, the strength and the deformation measurements of concrete were made. The stress-strain relationship of Hawkesbury Sandstone subject to uniaxial compression exhibits strong non-linearity at low stress levels. The results obtained in the measurement of mechanical properties of sandstone showed wide variations. The analysis of these results based on statistical techniques, revealed that the material characteristics of the rock, such as density, porosity, moisture content, mineralogy and texture, significantly influence the strength properties and the modulus of elasticity of the rock. The physical processes of weathering appears to be more widespread than the chemical process and cover a number of changes. These changes include opening of discontinuities and formation of new discontinuities in the rock mass; opening of grain boundaries and grain fracturing in the rock material. Alternate wetting and drying was found to be the most significant single process of weathering which contributes largely towards the degradation of the rock. The strength and modulus of elasticity of the weathered rock were appreciably lower than those of fresh rock. The failure of certain properties of the sandstone aggregate to satisfy the limits set by the currently available specifications has proved it to be an unsatisfactory material in the upper courses of a road-pavement. The success of its use in the lower courses, mainly in the sub-base, depends on its grading, durability and stability to environmental and traffic variations. The performances of crushed sandstone as a concrete aggregate where much poorer than those of conventional types of aggregates.
item: Thesis-Abstract
Studies in pressure .filtration on upward facing surfaces from a structural point of view
(5/24/2011) Herath Banda, SM
A direct technique to obtain information about the structure of a filter cake is described. Cakes formed from two different materials are consolidated and the porosity and pore size distribution in polished sections of these cakes are measured directly by using an image analyser. Two methods of impregnating filter cakes have been developed and are discussed in the paper. They are also suitable for consolidation of cakes formed from sub-sieve size material (<38μm).
item: Thesis-Abstract
Use of rice hull ash in water treatment
(2014-04-08) Mampitiyaarachchi, TR
Rice hulls, the largest milling byproduct of rice, constitute one fifth of the paddy by weight. The hulls which can be obtained at relatively low cost, are in abundant supply in most developing countries, particularly in the Asian region. Hull contains approximately 20% silica by weight and, on combustion, yields a porous ash having a silica content of approximately 90 percent. This thesis investigates the following two possible applications of rice hull ash in water treatment processes: (i) the use of rice hull ash as a filter medium; (ii) the use of rice hull ash as a coagulant aid. Rice hull ash comprising amorphous silica was produced from locally available rice hulls, using a specially constructed incinerator. Scanning electron microscopic studies were conducted to evaluate the microstructure of this ash as well as diatomaceous earth and filter sand. The laboratory filtration experiments were conducted at slow to semi-rapid filtration rates in order to investigate the effectiveness of rice hull ash medium in removing turbidity, bacteria and colour from water, and to compare its performance with a conventional sand filter. Synthetic water was made by adding suitable amounts of kaolin clay, Escherichia coli suspension, coffee/leaf extract, to laboratory tap water. Filtrate quality and head loss were considered as the major parameters in assessing the performance of these filters. A number of thin layer filter experiments were conducted to obtain the variation of turbidity with depth in rice hull ash filters, under selected operating conditions. Solubility studies of rice hull ash at various strengths of sodium hydroxide and for various steeping periods were made to evaluate the optimum conditions for silica solubilisation. A dilution procedure including partial neutralization and Subsequent aging prior to further dilution was adopted to convert alkali soluble rice hull ash into activated silica. Coagulation of dilute clay suspensions (£ 40 m g / L ) , using alum, activated silica and selected polyelectrolytes were conducted with the aid of a jar test apparatus. The filtrate turbidity for approximately 750 mm depth of rice hull ash medium was equal to or less than that of a sand medium, at rates of filtration 0.25 to 2.0 m 3 / m 2h and a turbidity range of 20 to 60 FTU. The rate of head loss in the sand medium was 1.5 to 5 times more than in an ash filter. The optimum rate of filtration for the ash filter occurred at 1.0 m 3 / m 2 h , with a rate of head loss of 52 mm/d. At filtration rates of 0.5 and 1.0 m 3 / m 2 h , for an influent Escherichia coli concentration of 100 - 2000 no/mL, approximately 70% to 90% reductions in bacterial numbers were achieved by 750 mm depths of rice hull ash media. Colour removal of at least 30% was achieved by shallow depths (< 320 mm) of ash media, at slow rates of filtration (< 0.25 m 3 / m 2 h ). The results obtained from thin layer filtration experiments were analysed using a statistical filtration model known as the chi-square distribution analogy. This technique was successful in predicting the performance of rice hull ash filters at specific operating conditions. The optimum removal of silica from rice hull ash occurred when ash was steeped in 5% NaOH solution for a period of 24 h. A procedure for the preparation of activated silica from rice hull ash was developed. The addition of 5 mg/L of activated silica as a coagulant aid during the coagulation of turbid water (4 0 mg/L of kaolin clay) with 50 mg/L of alum at pH value of 6, was sufficient to achieve a residual turbidity of 1.2 FTU. The coagulation of the same water with alum or alum-polyelectrolyte at similar conditions led to higher residual turbidity.

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