Master of Engineering in Foundation Engineering & Earth Retaining Systems

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Browsing Master of Engineering in Foundation Engineering & Earth Retaining Systems by Subject "Civil Engineering"

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    Establishment of fundamental characteristics of some unsaturated Sri Lankan residual soils
    Vasanthan, N; Kulathilaka, SAS
    Slope failure in tropical climates frequently occurs due to excessive rainfall. Heavy infiltration causes destruction of matric suctions, development of perched water table conditions and rise of ground water table. Severe erosion and surface destruction will also be caused by the heavy prolonged rainfall. In order to understand the threshold values of rainfall leading to instability it is necessary to model this process with a reasonable accuracy. Sri Lankan residual soil formations are formed by weathering of the metamorphic parent rock and have inherited significant abrupt variations in engineering characteristics. Basic characteristics of these soil formations such as soil water characteristic curves (SWCCs), variation of permeability with water content and unsaturated shear strength parameters are essential parameters in these analyses. These characteristics have not been established for typical residual soils forming slopes in Sri Lanka. This thesis highlights the need for detailed experimental studies and presents comprehensive studies that have been conducted at the University of Moratuwa and National Building Research Organization (NBRO) laboratories to establish the fundamental characteristics of unsaturated Sri Lankan residual soils. Undisturbed samples of soil obtained from the failed slope at Welipenna in the Southern Expressway were used in this study. Direct shear tests were done by modifying the conventional apparatus by incorporating a miniature tensiometer which allows for the simple and direct measurement of soil matric suction during shearing. Soil water characteristic curves (SWCCs) were also established using these apparatus. Alternatively, pressure plate apparatus was also used for this purpose. In addition to that, soil water characteristic curves (SWCCs) were developed from gradation curve also.Permeability of an unsaturated soil varies considerably with the level of saturation and will make a very significance influence on the infiltration process. Permeability function which defines the variation of permeability with matric suction was investigated on undisturbed samples. The method is based on continuously drying and wetting the soil sample while continuously monitoring the suction gradient and the change in soil mass. The thesis highlights the importance of these studies and presents the procedures that are being used.
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    A Study on most suitable slurry cutoff wall material to mitigate seepage in vendrasan dam, Trincomalee
    Wijayawardhana, HMJT; De Silva, LIN
    Earthen dams are constructed to store water for the purposes of human consumption, food production, electricity production, industrial use and flood protection. Failure of Earthen dams may be due to hydraulic failure, structural failure or seepage failure. Hydraulic failure may be due to overtopping, erosion of the upstream/downstream surface/toe and piping. Structural failures can occur in either the embankment or the appurtenances. Failure of a spillway, lake drain, or other appurtenance may lead to failure of the embankment. Cracking, settlement, and slides are the more common signs of structural failure of embankments. Seepage Failure occurs due to the uncontrolled seepage in both velocity and quantity. Water permeating slowly through the dam and progressively erode the soil in the embankment or the foundation toward the reservoir. Eventually with increased seepage flow rate the direct connection is made to the reservoir causing the piping. Piping may occur through the dam or the foundation causing dam failure. Seepage problems in large dams should be addressed in proper way on time to prevent before it becomes a massive disaster. Field investigation and data evaluation reveal the type of seepage and its extent. Then the most appropriate remedial measures can be adopted. Construction of toe filter, toe drain, downstream seepage berm, conducting cement/clay grouting and Cutoff walls construction are more frequently practiced to prevent seepage through and beneath the dam. Cutoff walls make the seepage paths longer, decrease the exit gradient at the toe and reduce the seepage quantities. Compacted impervious trench cutoffs, concrete cutoff walls, sheet piles, slurry trenches/cutoff walls are some different types of cutoff walls currently being utilized.