Geotextiles and Geomembranes 26 (2008) 82–90 Technical note The influence of bentonite extrusion on shear strength of GCL/geomembrane interface Ana Vukelic´ a,Ã , Antun Szavits-Nossan b , Predrag Kvasnicˇka c a Civil Engineering Institute of Croatia, Janka Rakusˇe 1, 10000 Zagreb, Croatia b Faculty of Civil Engineering, Fra Andrije Kacˇic´a-Miosˇic´a 26, 10000 Zagreb, Croatia c Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia Received 23 January 2006; received in revised form 5 April 2007; accepted 6 April 2007 Available online 29 May 2007 Abstract A series of shear strength tests of needle-punched geosynthetic clay liner (GCL)/textured geomembrane interface were conducted at normal stresses ranging from 10 to 400 kPa. The geomembrane was in contact with the woven geotextile of GCL. One half of the tests were carried out on prehydrated GCL samples at low normal stress (about 1 kPa), whereas the other half on non-prehydrated samples. The prehydrated samples exposed during shearing to normal stresses of 100 kPa and above demonstrated bentonite extrusion to the area in contact with geomembrane, which was visible to the naked eye. The bentonite extruded was quantified by introducing an extrusion coefficient. The quantity of the bentonite extruded increased with an increase in normal stress, and lubrication of the contact area with bentonite resulted in reduced shear strength. Finally, the testing showed that for tests carried out on prehydrated samples at lower shear rates, lower contact shear strengths were obtained and more extensive bentonite extrusion to the contact area was observed. r 2007 Elsevier Ltd. All rights reserved. Keywords: GCL; Geomembrane; Shear strength; Bentonite extrusion 1. Introduction Geosynthetic clay liners (GCLs) are installed as liners. In landfills, they are installed (1) in a landfill bottom lining to reduce the permeation of contaminated leachate into the ground, and (2) in cover systems, to minimize infiltration of precipitation water into the deposited waste and thus the resulting leachate as well. Geosynthetic clay liners represent a composite material consisting primarily of bentonite (either powdered or granular) and geosynthetics (either geotextiles or geomem- branes) (Bouazza and Vangpaisal, 2003). They are being investigated intensively, especially in regard to their hydraulic and diffusion characteristics, chemical compat- ibility, and mechanical behavior (Bouazza and Vangpaisal, 2003). In the last few years, special attention was given to their hydraulic performance (Barroso et al., 2006; Lor- enzetti et al., 2005; Rowe et al., 2007; Shan and Chen, 2003; Southen and Rowe, 2007; Touze-Foltz et al., 2006), cation exchange (Bouazza et al., 2006; Touze-Foltz et al., 2006), thermally induced desiccation (Southen and Rowe, 2005), internal erosion (Rowe and Orsini, 2003), volatile organic compound sorption (Lake and Rowe, 2005), deformation (Dickinson and Brachman, 2006), internal strength (Hurst and Rowe, 2006; Koerner et al., 2001), interface shear strength (Bergado et al., 2006), and gas migration through them (Bouazza and Vangpaisal, 2003, 2006). This paper investigates the interface shear strength of GCL. For the bottom lining of a municipal waste landfill and of a hazardous waste landfill, very often a composite liner is used consisting of a geomembrane placed on the GCL. Such liner systems are installed to enhance safety with respect to leachate percolation or water inflow through waste as specified by The council of the European Union (1999) and by the United States Environmental Protection Agency (EPA). The Agency also recommends that composite liner should be installed in municipal waste covering. In such a composite liner, one layer is always a geomembrane, whereas the other is either a natural ARTICLE IN PRESS www.elsevier.com/locate/geotexmem 0266-1144/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2007.04.001 Ã Corresponding author. Tel.: +385 1 6125 491; fax: +385 1 6125 481. E-mail address: ana.vukelic@igh.hr (A. Vukelic´).