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In recent years use of geotextile as underground drainage pipes has come in to vogue. Because of the importance of the subject under consideration, a research program was conducted to investigate the hydraulic and mechanical properties of a special woven geotextile as an underground drainage pipe. The geotextile material used was of woven type made of polyester wefts and polypropylene warps of hollow reeds with the following specifications: average thic;kness 3.5 mm; weight per unit area 816 g/m2; internal dimension of op(mings or average diameter of pores 0.105 mm (subject to change as per density of pores) and its tensile strength 800 kg/meter width (based on the manufacturers'view). This type of geotextile is being presently manufactured in one of the local factories in forms of sheets with a dimension of 90 cm width and limitless length (between 100 to 500 meters). The test comprised of three sections, namely evaluation of permeability and drainage potential, filtration and sedimentation of silt in geotextile pipes in comparison with the prevalent drainage pipes, evaluation of resistance to deformation under soil pressure and finally the mineral clogging and obstraction potential of pores. In the implementation of the experiment two types of soiL_, namely, fine textured (silty clay, CL - ML)
and coarse textured (silty sand, 8M - 8C) based on the unified soil
classification system, were utilized. The results obtained from the tests revealed that the flow rate of water through the common drain pipes in the soil CL ¬ML was between 4.9 to 5.5 times greater than the geotextile drain pipes while in soil 8M - 8C it was 2.1 to 2.7 times greater. In addition, the rise of water table from the axes of geotextile pipe was 1.7 to 1.9 times than of common



pipes in use. This substantiates the greater resistance of geotextile pipes to the entry of water. The deformation susceptiblity of geotextile pipes in CL - ML soil was between 7.8 to 31.6 percent, while in SM-SC soil it reached 100 percent of the diameter of geotextile pipes (complete joining of upper and lower walls). The relative hydraulic gradient change resulting from the third test in CL - ML soil ranged from minimum 8 in the initial stage of the test to a maximum 80 at the end while in SM-SC the change ranged from 6.3 to 86 which being greater than one, denotes the greater potential capacity of clogging of geotextile sheet. The permeability of geotextile-soil system in the CL - ML soil sample was on an average 1.5 x 10-5 meter per second in the beginning and 4.35 xl 0-5 meter per second during the end of the test; while for SM - SC soil it was 4.74 x 10-5 at the beginning and l.06x 10-5 meter per second at the end of the test.