Master of Engineering in Geotechnical Engineering

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Browsing Master of Engineering in Geotechnical Engineering by Author "Puswewala, UGA"

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    item: Thesis-Abstract
    Application of fuzzy sets and other statistical techniques in landslide hazard zonation mapping
    Weerasinghe, KM; Gunaratne, M; Ratnaweera, HGPA; Puswewala, UGA; Arambepola, NSMI
    The objectives of this research are, (I). establish a methodology that quantifies the severity levels of causative factor attributes that are rated subjectively, and (2). optimize weights and severity levels of the causative factor attributes in order to achieve maximum agreement between the predicted landslide hazard potential and the actual landslide intensities observed in the field. The landslide hazard potential is evaluated based on a decision tree consisting of two levels of attributes identified as primary and secondary. It is assumed that the primary and secondary attributes have independent contributions towards the final result. The relative importance of each component attribute (factor) among the other factors that are connected to a nodal point of this tree is indicated by a subjective (or linguistic) description. Fuzzy sets mathematics which is an effective tool for mathematical representation of subjective expressions such as "quite high", "medium" or ."not that high" etc. are used to describe the relative importance of component attributes. Moreover, linguistic ratings used to describe the severity of each secondary factor are also considered as fuzzy numbers in order to account for the vagueness or the subjectivity in the rating procedure. Computer routines are developed to express the fuzzy sets that describe the subjective ratings and weights and then manipulate them using a technique resembling the Monte-Carlo simulation in order to determine the overall hazard potential of a given site. Eventually, the resulting overall degree of hazard, which itself is a fuzzy set, is converted back to an appropriate linguistic scale containing 'very high', 'high', 'medium', 'low', or 'very low' designations, in order to interpret the overall landslide potential of the site. Once the hazard potential estimates are predicted for every zone demarcated on a selected study area, they are compared with actual landslide intensities estimated for the same zones based on available field data. Then, through an iterative procedure the initially assigned weights of the primary and secondary attributes are systematically adjusted until the field observed landslide intensity matches at a satisfactory level with the predicted hazard potential. When the developed methodology is applied to the remaining study areas in Sri Lanka, and weights are refined further, it would become an effective tool in accurate landslide hazard zonation.
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    item: Thesis-Abstract
    Application of non-destructive methods for testing of index properties of gneissic rocks in large-scale geotechnical investigations
    Samaradivakara, GVI; Puswewala, UGA
    There is an abundance of gneissic rock formations found in Sri Lanka. Gneiss is a banded rock with fairly continuos segregation of different minerals. Foundations of most of the large-scale civil engineering structures are extended up to or into the fresh gneissic rocks. Especially, the foundation of dams, bridges and high rise buildings are extended into fresh rock. In such large-scale geotechnical investigations, boreholes are advanced up to the interested depth of exploration and rock coring is compulsorily done using rotary core drilling machines. Borehole logging is followed by arranging the laboratory testing programme for testing of soil and rock materials. Hence the identification of engineering behaviour of gneissic rocks at the detailed investigation stage is a prime necessity in such projects. Laboratory testing of a large number of rock samples is time consuming and expensive. The general practice of selection of representative rock samples on visual inspection followed by laboratory destructive testing may not lead to a precise interpretation of engineering properties of the entire subsurface rock strata. Non-destructive testing of gneissic rock is identified as a fast and effective method of selection of representative rock samples for a laboratory-testing programme. A 100m deep exploratory rock core of diameter 54mm was selected for this research. More than two hundred samples were prepared and subjected to three different nondestructive tests, followed by destructive tests. Depending on the results of the study, samples having distinct characteristics could easily be identified. In particular, the strata having low compressive strength were clearly identified and hence those samples could be specified for destructive tests. The findings of this research will be immensely helpful to organise laboratory testing programmes on rock samples effectively and economically especially in large scale geotechnical investigations.
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    item: Thesis-Full-text
    Application of unsaturated shear strength properties in slope stability analysis
    Hapangama, MTCP; Puswewala, UGA
    It is very important to study applications of unsaturated soil properties in slope stability analysis due to the additional shear strength an unsaturated soil possesses. Shear strength of an unsaturated soil is strongly related to the amount of water in the voids of the soil, and therefore to the matric suction. It is postulated that the shear strength of an unsaturated soil should also bear a relationship to the soil-water characteristic curve. In this thesis, the effect of unsaturated shear strength properties on stability of slopes is investigated by analysing for the stability of hypothetical cut slopes. The effect of the position of water table below the failure surface of hypothetical cut slopes were analysed using Slope/w software for different water table positions for three different cut slope angles. For this work, analyses were done by replacing the cohesion by the apparent cohesion values. Apparent cohesion values were increased by increasing the depth of water table and increasing the effect of negative pore water pressure. Different apparent cohesion values corresponding to the different percentage of negative hydrostatic pressures were utilized. Spreadsheets prepared by a previous research were used to analyse the hypothetical cut slopes by the Modified Janbu’s Method of slices for unsaturated soils. The negative pore water pressure could be directly taken into account in this method, and the Factors of Safety (FOS) derived by this method are compared with the results from SLOPE/W software. Variation of FOS with slope angle and position of water table are investigated. The parametric study done here gives an insight into the problem of landslides. Lowering of the Water Table is been to increase the FOS against sliding failure, as expected.
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    item: Thesis-Abstract
    Matric suction characteristics of unsaturated residual soils of Sri Lanka
    Ratnajothy, T; Puswewala, UGA
    Residual soils from selected prominent landslide sites of the upcountry of Sri Lanka are tested under unsaturated conditions to evaluate pertinent unsaturated properties of the soils. The selected landslide sites are Naketiya, Kahagalla, Walhaputenna and Beragala. The tri-axial test apparatus was modified by using high air-entry disks so that poreair pressures and pore-water pressures of a soil sample could be maintained at separate, required magnitude. Soil samples obtained from Dambulla International Stadium site were used to verify the repeatability of the test results by the modified apparatus, and the reliability of the apparatus. Soil samples obtained from each of the selected landslides sites were prepared at the maximum dry density and optimum moisture content as determined by the proctor compaction test. These were tested in triaxial conditions under five different matric suction (which is the difference between pore-air and pore-water pressure) magnitudes. Time was allowed for each sample to reach an equilibrium state under the particular matric suction applied, before traxial testing commenced. The test results are used to develop the shear strength function for each soil tested, and the soil-water characteristic curve (SWCC) for each soil over the range of matric suction (30 kPa - 200 kPa) considered in the investigation. These results are of importance in interpreting the variation of shear strength with the moisture content for these residual soils, which will have a direct bearing on stability of slopes there.
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    item: Thesis-Abstract
    Numerical modeling of rock socketed piles
    Kodagoda, SSI; Puswewala, UGA
    The two basic types of pile foundations are skin friction piles and end- bearing piles. Friction piles resist the applied vertical load by utilising the shear stresses developed along the pile-soil interface, with the end-bearing at the tip of pile contributing a small proportion of the total pile resistance. End-bearing piles resist the applied vertical loads mainly by end bearing generated at the tip of pile. There are many instances where end-bearing piles piers are constructed with their tip located inside a cavity made in rock strata: such piles are referred to as rock-socketed piles. The resistance developed by rock-socketed piles and piers is partially due to the shearing stresses generated at the pile rock interface on the wall of the rock cavity and partially due to the end bearing at the tip of the pile. The load that can be carried by the rock socket is usually underestimated in the design practice due the lack of guidance in relevant codes. This thesis presents the findings of the research focused on computational modeling of rock socketed piles conducted to investigate the behavior of such piles. Ax symmetric finite element analyses were conducted to determine the percentages of load resisted by interface shear along the socket wall and end-bearing at pile tip, for various socket length/pile diameter ratios. Different interface conditions were modelled by varying cohesion and friction parameters input to the interface element, as well as by assuming perfect connection between pile and rock material The finite element results obtained are compared with some available field test data to verify the models. Finally a field test procedure is suggested for deriving load carrying capacity of the socket. Findings and further expansion of this research program would be of immense benefit to state and private organizations involved in deep foundation design and construction. It will highlight whether the use of rock-socketed piles can be economically advantageous over the simple end bearing piles with no sockets, and indicate optimum geometries of rock sockets.
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    item: Thesis-Abstract
    A Study on the effects of variation of elastic properties of asphalt concrete on the vertical stress distribution in layered road pavements
    (2014-08-07) Seneviratne, HMU; Puswewala, UGA
    107252-1.pdf Often the natural ground or soil is not mechanically strong enough to withstand the repeated applications of wheel loads without significant deformation. Therefore it is necessary to provide a structure between the wheel and natural ground or subgrade to spread the wheel loads within allowable and recoverable deformation limits. Road pavement is the structure, which serves this purpose, by supporting and distributing the applied load. The primary purposes of the pavement are to prevent excessive deformation of the subgrade and to prevent ultimate failure stage. In order to prevent excessive subgrade deformation the vertical stress oz at subgrade level has to be kept below some critical value. The two main types of pavements used in pavement construction are rigid pavements and flexible pavements. Rigid pavements are very rare in Sri Lanka and consists of very stiff reinforced or roller compacted concrete surfacing over comparatively much softer ground. Flexible pavements consist of three main layers from top to bottom, consisting of the bituminous surfacing, road base and sub base. Sometimes the subbase is not laid. The shear strength, stiffness and the quality of the material of the layers decrease in the downward direction of the pavement. The topmost layer of a flexible pavement is an asphalt concrete surfacing or any other bituminous surface dressing like Double Bituminous Surface Treatment (DBST), Single Bituminous Surface Treatment (SBST) or traditional metal and tar application. The most popular bituminous surface treatment in Sri Lanka is asphalt concrete due to its very good riding quality, durability, higher stiffness or stability, resistance to adverse climatic conditions and the ability to maintain consistent quality during manufacturing and laying. As asphalt concrete surfacing is the most modern and popular method used in flexible pavement construction, this work is limited to studies on flexible pavements with asphalt concrete surfacing. Primarily asphalt concrete is a mix of bitumen and a blend of densely graded aggregates. Mix proportions of the asphalt concrete are determined after a process of mix design to achieve the desired strength and durability against expected traffic and adverse climatic conditions by using available aggregate.