Joints existing within a rock mass are normally filled with fine materials such as clay and silt, which decrease its ultimate strength and make it easier to deform. The shear behaviour of infilled rock joints is controlled by several parameters, such as infill thickness, joint roughness and drainage conditions (Ladanyi & Archambault, 1977; Lama, 1978; de Toledo & de Freitas, 1993; Indraratna et al., 2005). If the infill material is saturated and drainage is impeded, pore water pressure within the infilled joint will build up and change during shear displacement. The importance of pore water pressure in the hydromechanical stability of jointed rock mass has been recognised for several decades (Lane, 1970). Several catastrophic failures of natural rock slopes have occurred as a result of pore water pressure build-up in jointed rock (e.g. Kangaroo Valley rockslide, NSW, Australia; Indraratna & Ranjith, 2001). Limited experimental studies on the effect of pore water pressure in rock specimens (porous or fractured) have been reported by Lane (1970), Goodman & Ohnishi (1973) and Poirier et al. (1994). Even fewer studies have been dedicated to jointed rock behaviour under undrained conditions including pore water pressure behaviour, but without any infill (e.g. Archambault et al., 1998, 1999). In this paper, the authors describe some limited but interesting test results on the behaviour of clay-infilled joints sheared under triaxial, undrained conditions.