Master of Science in Geotechnical Engineering

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Browsing Master of Science in Geotechnical Engineering by Author "Kulathilaka, SAS"

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    item: Thesis-Abstract
    Modelling the field application of electro-osmotic consolidation to improve engineering properties of soft peaty clays
    Sagarika, DKNS; Kulathilaka, SAS
    In Sri Lanka, there are large areas underlain by soft peaty clays in and around Colombo and its suburbs. Soft peaty clay deposits are highly compressible and have very low shear strength. High primary and secondary consolidation settlement problems are associated with the low shear strength. Due to scarcity of land with good subsoil condition, Geotechnical engineers are compelled to use these grounds for new development works. Therefore, it is needed to improve the properties of peaty clay deposits before doing any construction works on them. As such, there is a need to find cost effective and efficient ground improvement techniques. The potential of electro-osmotic consolidation as a technique for improvement of Sri Lankan peaty clay was studied in this project. There are records in literature to indicate that this technique was successful with soft inorganic clays. But there are no records of the use of the method in organic soils. Sri Lankan peaty clays have very low organic contents in the range of 20% to 30%. The effectiveness of electro-osmotic consolidation technique with Sri Lankan peaty clay was studied at the University of Moratuwa first by performing a series of one dimensional electro-osmotic consolidation tests and the method was found to be quite successful. In the field, electro-osmotic consolidation is done by driving parallel lines of electrodes, and by applying direct current potential difference and pumping from the cathode. This configuration cannot be considered as one-dimensional and this would be essentially three-dimensional. In this research, the field arrangement of electro-osmotic consolidation was closely simulated in a model tank filled with remoulded peaty clay and series of tests were performed with the objective of understanding the aspects of electro-osmotic consolidation technique under three dimensional conditions. Specimens were taken from the remoulded peat mass after it was subjected to consolidation tests and shear strength tests. For comparison purposes, tests were done on untreated peaty clay remoulded in the same manner. The level of improvement achieved in compressibility characteristics in three dimensional condition is less than that achieved with one-dimensional condition. But it shows a preconsolidation effect especially near anodes. A significant reduction in water content and significant increase in shear strength were observed near anode compared to near the cathode. pH tests confirm that electro-chemical changes take place in the soil. pH values increased at cathode and decreased at anode. Electro-osmosis treatment has caused an increase in the liquid limit. The coefficient of electro-osmotic permeability of Sri Lankan peaty clay found to be in the range of 1* 10-9 to 1* 10-8 m2 /sec.- V, which was within the range suggested by Mitchell (1991) for fine-grained soils.
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    item: Thesis-Abstract
    Use of lightweight fill materials in construction of road embankments on soft peaty clay
    Muhunthan, A; Kulathilaka, SAS
    Number of proposed highways in Sri Lanka are to be constructed over sites underlain by soft peaty clay, due to scarcity of land. In order to ensure that the in-service settlements of these roads are small enough and the road could function satisfactorily, number of special ground improvements techniques are to be adopted. An alternate approach that could be considered is the use of a lightweight fill material in the construction. Extremely lightweight fill material such as expanded and extruded polystyrene blocks were used in a number of developed countries in the construction of road embankments over soft ground and in landslide repair. However, these materials are to be imported to the country and would be very expensive. As such, from a local point of view, a process involving the use of such materials would not be economically competitive. In order to find an economically feasible solution, the lightweight fill materials should be developed with the locally available inexpensive raw materials. As such, lightweight fill materials were developed locally by mixing with different proportions of tyre chips with lateritic soil, sawdust with lateritic soil and paddy husk with lateritic soil. Tyre chips were obtained by shredding discarded motorcar tyres. Sawdust was obtained from wood mill waste and paddy husk was obtained from rice mill waste. The developed fill material should of sufficiently low density and workable. Different mix proportions were tried out to get several suitable mixes. The developed material should be sufficiently incompressible and should possess adequate shear strength. Further detailed tests were conducted on selected mixes to establish their engineering characteristics in relation to strength and stiffness. The effectiveness of the use of lightweight fill material in the embankment construction was studied in detail by the finite element package CRISP. The set criterion was that the in-service settlement of the road should be less than 50mm. This was achieved through the preloading process. In this study a comparison was done for two different approaches; one constructed with lateritic soil and the other incorporating the developed lightweight fills in the preloading process. The placement of the fill layers, the settlement of the peaty clay, the effect of the removal of the preload and the application of the pavement and traffic load was studied with a fully coupled Modified Cam clay constitutive model. Parametric analyses were also done varying the thickness of the embankment and the peaty clay. The process was found to be helpful in reducing the construction period and consumed fill volume. The advantages were more prominent with the increase of embankment height and soft layer thickness.