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The effect of matric suction on shear strength of soil, because of its dominant role in geotechnical modelling, has received the attention of researchers in this filed. This research is focused in investigating the effect of matric suction on shear strength of a fine soil. The physical properties of the soil are determined. All samples are passed through No. 200 sieve. The soil is a high plasticity soil which is classified CH according to the unified classification system. The filter paper method is performed to extract the soil water characteristic curve. In addition to its simplicity, one of the major advantages of this method is the vast range of suctions can be measured by it. Specimens were compacted at identical void ratio with different water contents. Suctions are inferred from empirical relations that relate the suction to the measured moisture content of filter paper which has been in contact with soil sample. Air entry suction is determined 140 kPa from SWCC curve. In order to examine moisture content effects on shear strength parameters (cohesion and friction angle) of the clay, soil samples are prepared with different moisture content and consequently different matric suctions with respect to soil water characteristic curve. The specimens with different initial moisture contents are statically compacted at dry density of 1.3 gr/cm3into direct shear test mould. The moisture contents of samples are in the range of 10 to 28 precent. Hence, according to soil water characteristic curve the initial matric suctions are between 42 to 25238 kPa. Direct shear tests are conducted in unsaturated state. After applying vertical stress the samples are left at least for 48 hours to allow the equalization of internal stresses. Based on experimental data the shear stress is plotted against horizontal displacement in different vertical stresses and moisture contents and the results and analysed and discussed. The effective stress equation proposed by Bishop (1959) is used to interpret the results. The effective stress parameter χ in each moisture content is calculated bythe equation suggested by Khalili & Khabbaz (1998) using the air entry suction extracted from SWCC curve. The results reveal that strength parameters do not substantially change at different moisture contents. The negligible changes in strength parameters may be attributed to slight changes in pore water pressure during shearing experiment. Such changes in pore pressure during shearing would be eliminated in constant suction consolidated drained experiments. The results of this paper support the validity of effective stress concept in unsaturated soils. In other words, with a proper estimate of effective stress, the saturated parameters are valid in unsaturated state and it is not necessary to measure strength parameters in unsaturated state. The unsaturated geotechnical experiments are time consuming and expensive and need special equipment and skills. The method proposed in this paper has a good degree of accuracy. Therefore it can decrease the costs of geotechnical investigation projects in arid lands.

Keywords: Initial water content, Cohesion, internal friction angle, plastic soil, filter paper method

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