- 12555 - The Influence of Matric Suction on the Shear Strength of Highly Plastic Compacted Swelling Clays Ahmed M. Elsharief Associate prof. Building and Road Research Institute, University of Khartoum Address: Khartoum, P.O.Box 321, Sudan Phone: 00249912144181 e-mail: aelsharief@yahoo.com Corresponding author Omaima A. Abdelaziz and Mohamed A. Dafallaa Formerly graduate students, Building and Road Research Institute, University of Khartoum, Sudan Address: Khartoum, P.O.Box 321, Sudan ABSTRACT This paper investigates the relationship between shear strength and matric suction of three highly plastic potentially expansive clays from Sudan, known for their very high matric suction. Due to difficulty in handling suction controlled shear tests for these soils, the shear strength and matric suction were independently measured. The direct shear test and the filter paper method (ASTM D 5298-03) were used for measurement of shear strength and matric suction, respectively. Each soil was prepared at the same dry density but different moisture content values. The soil water characteristic curve was defined for each soil and the peak shear resistance was obtained for each shear test. The data was analyzed. The results confirmed the bilinear relationship between shear strength and matric suction as reported earlier for low plastic soils. The break in the slope of the lines takes place at the air entry value of the tested soils. The shear parameters are close in value for suction values higher then or lower than the air entry value. The gain/loss in shear strength due to decrease/increase in moisture content could be attributed, mainly, to increase/decrease in total cohesion when the soil is quasi-saturated. KEYWORDS: Unsaturated soils, shear strength, matric suction, filter paper method. INTRODUCTION Earth fill dams and highway embankments are typical examples of earth structures constructed from compacted unsaturated soils. These embankments are often subjected to wetting and/or drying during their lifespan. The shear parameters used in the stability analysis of unsaturated soil embankments are affected by the changes in shear strength due to changes in the water and/or water vapor in the pores of these soils. Gravimetric water or moisture content is commonly used as a measure of the content of water or water vapor in the pores of soils. Previous studies have shown that Terzaghi (1925) effective stress equation is inadequate to account for the behavior of partially saturated soils [1]. Water pressure acts over part of the area of