Master of Engineering in Geotechnical Engineering

Permanent URI for this collectionhttp://192.248.9.226/handle/123/16121

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Now showing 1 - 12 of 12

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.
item: Thesis-Abstract
Improvement of Sri Lankan peaty clays by deep mixing and electro-osmosis
(2014-07-07) Sarojini, WAS; Kulathilaka, SAS
As a result of the rapid increase in population and the development taken place in the city of Colombo and suburbs during the last few decades, lands with good subsoil condition have been already utilized. The lands required for development of industrial, commercial and residential facilities and development of infrastructure facilities such as highways in Colombo and suburbs are presently found mainly in low-lying areas where the sub soil conditions are not very favorable for civil engineering structures. The geotechnical engineer is faced with the problem of finding an economical solution to this problem. This may involve improving the ground condition or designing suitable types of foundations, or both. A prominent feature of these low laying areas which are in the flood pains of main rivers and lakes is the presence of sub soils that are predominantly organic and possess unique physical and engineering properties quite different from those of inorganic soils. It has been revealed by the geotechnical investigations carried out by various institutions in the past, that these low laying areas have highly compressible peat and soft inorganic clays of thickness varying from lm to 15m. Peat encountered in these sites possesses organic contents in the range of 20% to 30%. As such, they should be referred to as peaty clays and not as peat. The most significant features of peaty clays are very high water contents and void ratios and low specific gravity. As a result they are of very low shear strength and high compressibility. They are characterised by the very high secondary consolidation settlements. The ground water table is almost at the ground level in most of the situations. These sub-soils clearly are susceptible to undergo very high settlements even under a small imposed load and to experience catastrophic shear failure.
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.
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.
item: Thesis-Abstract
Simulation of deep excavation using finite element method
Jayakody, NJ; Peiris, TA
Finite Element calculations are frequently used in the design of deep excavations because prediction of ground movements and wall deformation is not possible with classical limit equilibrium fixed earth support and free earth support methods. To solve such geotechnical boundary value problems successfully, appropriate constitutive laws should be used for the description of mechanical behaviour of the soils. Two possible behaviour of soils can be categorized as those with a constitutive law based on plasticity and those based on elasticity. A key distinction between the plasticity and the elasticity group is that in the latter, strains are recoverable upon decrease in stresses, where as in elastoplastic models strain are only partly recoverable. A crucial point in making the choice of a suitable soil constitutive model is the ease with which values can be assigned to the model constants. The level of investigations carried out and the type of data available is another factor. Considering the above points mentioned, the study summarizes the application of a simple linear elastic model and a non linear elastic [Hyperbolic] model for modeling the 15m deep base excavation supported by an anchored secant bored pile wall. Finite element software SIGMAW is applied for a 2-D plain strain type analysis. The mechanical behaviour of the soil is modeled with linear and non linear elastic constitutive model using reasonable soil parameters, derived through limited tests. The actual excavation was carried out in 4 steps. Pre-stressed anchors were used as additional supports, just before each excavation step. The wall deformations were obtained through inclinometers installed at several locations. The FEM simulation was carried out in this research and the computed and observed deformation patterns were compared.
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.
item: Thesis-Abstract
Probabilistic analysis of shallow translational slides
Rathnayake, PLW; Kulathllaka, SAS
Slope instability is a major environmental hazard, which is widely researched by Geotechnical engineers in the world. In general, there is a vast range of different mechanisms of slope failures. Shallow translational mode of slope failure is one such important mode. This thesis concentrates on shallow translational form of slope failures. Analysis of shallow translational form of failure is carried out deterministically as well as probabilistically. The probabilistic analysis gives due consideration to the uncertainty of soil strength parameters and pore water pressures. Analysis are carried out under both saturated soil mechanics theories and unsaturated soil mechanics theories. Two hypothetical examples representative of natural lateritic slopes in Sri Lanka are analysed deterministically and probabilistically. Finally, the influence of rainfall and wetting front propagation are analysed. The corresponding changes of the probability of failure of the previously unsaturated slope is highlighted. The transition of slope from unsaturated situation to saturated situation with the propagation of wetting band thickness, and the resulting changes in the factor of safety and probability of failure are discussed. A method of evaluating the overall probability of failure is suggested for further research.
item: Thesis-Abstract
Improvement of engineering properties of peat by preconsolidation : a comparison of field and laboratory test results
Karunawardena, WA; Kulathilake, SAS
Sri Lanka lies in the monsoon region of South- East Asia. Colombo, the Capital city situated on the Western cost of the island experiences a humid tropical climate with an annual rainfall of 2500 mm. In the Colombo region there exists flood plains of Kelani Gaga to the North and in the in the coastal belt there are some water bodies such as the Kotte Lake, Bolgoda Lake etc. The fluctuation of water table due to excessive rainfall provides suitable condition to accumilate peat deposits in the above areas, as these are poorly drained grounds. The above process has been occuring for a long time and as a result of that now there exists around 2500 acres of marshy land underlain with peat in and around Colombo city. These marshy lands consist of peat layers having thickness around 5m-10m. As most of the lands with good subsoil condition around Colombo are already used up with the development of the country, it is now necessary to use these unused marshy lands for housing and infrastructure development.
item: Thesis-Full-text
Methods for improvement of engineering properties of peat - a comparative study
Munasinghe, WGS; Kulathilake, SAS
Development of cost effective methods for improvement of engineering properties of peat is a need of the hour in Sri Lanka in view of the number of major infrastructure development projects that are proposed over the lands underlain by peat. In this research several different methods of improvements were tried out in Sri Lankan peat, which have a rather low organic content around 20%-40%. The improvement methods tried out were namely; pre-consolidation through preloading, mixing with cement at percentages of 5%, 10% and 15% and mixing 15% of lime. Peats at different levels of humification were used in the study. It was shown that the preconsolidation causes a significant improvement in both the primary and secondary consolidation characteristics irrespective of the degree of humification. Improvements were achieved in both the fibrous and amorphous peat. Even after the mixing of 15% of cement or 15% of lime significant improvements of consolidation characteristics could not be achieved in fibrous peat. But, even the mixing of 5% cement caused significant improvements in both the primary and secondary consolidation characteristics in amorphous peats. The organic contents of the two types of peat considered were similar. Improvements of shear strength were achieved in all types of peat due to preconsolidation. Mixing with cement also caused some improvements in undrained shear strength of Peat. However, these improvements were not as high as those reported for inorganic soils. Consolidation tests were conducted with simultaneous measurement of settlement and pore water pressure, in a new experimental setup developed. The data obtained were used to check the validity of the Terzaghi theory to model the consolidation behaviour of peat. Some experiments were conducted to derive Bjerrum curves for Peat.
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.
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.
item: Thesis-Abstract
Use of barden model for consolidation of Sri Lankan peaty soils
Hearth, HMSD; Tennekoon, BL
Peat is an accumulation of partially decomposed and disintegrated plant remains, which have been fossilized under conditions of incomplete aeration and high water content. Development of peat is a progressive process with the continuous decomposition of plant matter. With time plant fiber may disappear leaving organic soils (organic clay/ organic slit), or organic matter may decompose in to an amorphous form. As the decomposition of plant fiber/vegetation is a time dependent process, different stages of decomposition could exist. Investigation done in low-laying areas of Colombo reveal the existence of 3 different types of peaty soils. 1. Fiber peats 2. Amorphous peat 3. Organic clays mixed with peat.

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