A. Hamidi, 1 G. Habibagahi, 1 and M. Ajdari 2 A Modified Osmotic Direct Shear Apparatus for Testing Unsaturated Soils REFERENCE: Hamidi, A., Habibagahi, G., and Ajdari, M., “A Modified Osmotic Direct Shear Apparatus for Testing Unsaturated Soils,” Geotechnical Testing Journal, Vol. 36, No. 1, 2013, pp. 1–10, doi:10.1520/GTJ20120092. ISSN 0149-6115. ABSTRACT: Most hydro-mechanical models for unsaturated soil models require shear strength characteristics of the soil in order to determine the pertinent model parameters. Moreover, one needs to know the shear strength of unsaturated soils when dealing with stability problems in engi- neering practices. Because matric suction affects the shear strength of unsaturated soils, a suitable apparatus must be capable of imposing and/or measuring suction during tests. Shear tests on unsaturated soils are usually performed using the axis translation technique. This method covers a low range of matric suctions that corresponds to relatively high degrees of saturation for fine grained soils. Thus, it is essential to develop new tech- niques to extend the range of suctions for these soils. In this study, a modified direct shear device is introduced that uses the osmotic method to impose the desired matric suction. A polyethylene glycol (PEG) solution is circulated all around the bottom of the soil sample to impose the suc- tion, and a semi-permeable membrane is introduced between the soil and the PEG solution that permits water swap but prevents the PEG macro- molecules from crossing the membrane. Using this technique, the suction is controlled and kept constant during the test. The special shear box design described in this paper minimizes the potential for membrane rupture during the test. Shear stress, as well as the vertical and horizontal dis- placement, is recorded using an auto-logging device. The suitability of the apparatus to perform direct shear tests at relatively high suction values is investigated via tests carried out on Shiraz silty clay. In these tests, soil specimens are subjected to suctions of up to 800 kPa during shearing. The results indicate that the shear strength and shear induced dilation of the soil samples increase with suction. KEYWORDS: osmotic method, direct shear, unsaturated soil, matric suction Introduction The assessment of soil shear strength is an essential step in the sta- bility analysis of earth structures, foundations, and natural slopes. In these problems, matric suction plays a significant role. The sta- bility of residual soil slopes in Singapore, where shallow slides are frequent, was illustrated by Rahardjo et al. (1995). Oloo and Fredlund (1996) centered their study on understanding the impor- tance of matric suction in the bearing capacity of pavement struc- tures. More recently, Zhan and Ng (2006) carried out some direct shear tests on slightly expansive natural and remolded soils to determine the shear response and soil water retention curve (SWRC) of these materials. An axis translation technique was used in these research works, and the tests were performed for a range of low suction values (up to 500 kPa). Besides the importance of shear strength in engineering prac- tice, shear strength is a fundamental ingredient of any constitutive elastic-plastic framework presented for unsaturated soils (Alonso et al. 1999; Russell and Khalili 2006; Sheng et al. 2008). Further- more, considerable experimental research has been done on the shear strength features of unsaturated soils using the axis transla- tion method, covering relatively low matric suctions that corre- spond to rather high degrees of saturation for fine grained soils (Bishop and Blight 1963; Ho and Fredlund 1982; Rassam and Williams 1999; Rampino et al. 2000; Khalili et al. 2004; Rahardjo et al. 2004; Lee et al. 2005; Fazeli et al. 2009). In addition to the experimental works at low matric suctions, some other shear tests at different ranges of suction have been reported. Ajdari et al. (2010a) performed some direct shear tests on an expansive silt–bentonite mixture utilizing the vapor equilibrium method. This technique is valid for suction values greater than 10 MPa, but their study did not cover the shear characteristics of the soil in the lower range of suctions. Boso et al. (2005) used the os- motic technique to perform direct shear tests on unsaturated soils at suctions of up to 800 kPa. However, the shear rate (0.0056 mm/min) was too high for consolidated drained tests to be performed, and the matric suction was not controlled during the shearing process. Investigations of the shear response of unsaturated plastic clays often need experimental programs capable of dealing with a rela- tively high range of suction values in order to develop the continu- ous air phase in the soil sample. This paper describes a modified direct shear device designed and constructed to enable the study of the shear strength of unsaturated soils with a low to medium range of suctions. In this study, matric suction of up to 800kPa is imposed on the soil samples utilizing the osmotic method. The details of the osmotic method and the osmotic direct shear Manuscript received January 20, 2011; accepted for publication July 30, 2012; published online December 10, 2012. 1 Dept. of Civil Engineering, School of Engineering, Shiraz Univ., Shiraz, 7134851156, Fars, Iran. 2 Dept. of Civil Engineering, School of Engineering, Fasa Univ., Fasa, 7461781189, Fars, Iran. Copyright V C 2012 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 1 Geotechnical Testing Journal, Vol. 36, No. 1, 2013 Available online at www.astm.org doi:10.1520/GTJ20120092 Copyright by ASTM Int'l (all rights reserved); Wed Feb 6 15:16:15 EST 2013 Downloaded/printed by Syracuse Univ pursuant to License Agreement. No further reproductions authorized.