A double filter system, composed of a layer of granular soil overlying a nonwoven geotextile layer, is often applied in many geotechnical applications. However, until now, the filter and drainage efficiency of the double filter system have not been fully investigated. In addition, the material selection and design procedure for the double filter system have not well established. According, this thesis presents an experimental study to evaluate the drainage and filtration efficiency of double filter systems. An infiltration system including a large permeameter (H = 350 mm, D =158 mm), constant seepage apparatus, loading frame and manometer tubes was developed. A series of vertical downward seepage tests was conducted to quantify the system drainage and filtration efficiency by measuring the system hydraulic conductivity and the gradient ratio of the hydraulic gradient at the geotextile layer to that at the vegetative soil. The test variables include 2 geotextiles which have different thicknesses (Geotextile A with t =1.7 mm and Geotextile B with t =4.5 mm), 2 different granular filters (river and quartz sands) with 3 different granular filter thicknesses (0, 6 and 11cm) and test duration (0-7 days). The test specimen was prepared by filling the double filter system underlying a vegetative (local fine) soil. A surcharge of 10 kPa was applied on the top of specimens to model the soil overburden pressure above. The test results indicate the system hydraulic conductivity (vegetative soil + granular filter + geotextile) increases and the gradient ratio decreases as the granular filter thickness increases, suggesting that the granular filter can effectively reduce the long-term clogging of geotextile by preventing the fine particles of the vegetative soil from migrating through seepage. The tests with thinner geotextile (Geotextile A) and the granular filter with larger mean soil particle size (quartz sand) show better drainage efficiency and lower long-term clogging potential. In addition to the gradient ratio, the clogging of geotextile was also quantified by measuring the weight difference between geotextile before and after tests and observed using Scanning Electron Microscope (SEM) in this study. Based on the results in this study, an evaluation procedure was proposed for the material selection and design of the double filter system to encourage the practical application of double filter system in the geotechnical field.

A double filter system, composed of a layer of granular soil overlying a nonwoven geotextile layer, is often applied in many geotechnical applications. However, until now, the filter and drainage efficiency of the double filter system have not been fully investigated. In addition, the material selection and design procedure for the double filter system have not well established. According, this thesis presents an experimental study to evaluate the drainage and filtration efficiency of double filter systems. An infiltration system including a large permeameter (H = 350 mm, D =158 mm), constant seepage apparatus, loading frame and manometer tubes was developed. A series of vertical downward seepage tests was conducted to quantify the system drainage and filtration efficiency by measuring the system hydraulic conductivity and the gradient ratio of the hydraulic gradient at the geotextile layer to that at the vegetative soil. The test variables include 2 geotextiles which have different thicknesses (Geotextile A with t =1.7 mm and Geotextile B with t =4.5 mm), 2 different granular filters (river and quartz sands) with 3 different granular filter thicknesses (0, 6 and 11cm) and test duration (0-7 days). The test specimen was prepared by filling the double filter system underlying a vegetative (local fine) soil. A surcharge of 10 kPa was applied on the top of specimens to model the soil overburden pressure above. The test results indicate the system hydraulic conductivity (vegetative soil + granular filter + geotextile) increases and the gradient ratio decreases as the granular filter thickness increases, suggesting that the granular filter can effectively reduce the long-term clogging of geotextile by preventing the fine particles of the vegetative soil from migrating through seepage. The tests with thinner geotextile (Geotextile A) and the granular filter with larger mean soil particle size (quartz sand) show better drainage efficiency and lower long-term clogging potential. In addition to the gradient ratio, the clogging of geotextile was also quantified by measuring the weight difference between geotextile before and after tests and observed using Scanning Electron Microscope (SEM) in this study. Based on the results in this study, an evaluation procedure was proposed for the material selection and design of the double filter system to encourage the practical application of double filter system in the geotechnical field.

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