Indraratna, B. et al. (2010). Ge ´otechnique 60, No. 8, 623–633 [doi: 10.1680/geot.8.P.094] 623 A shear-displacement criterion for soil-infilled rock discontinuities B. INDRARATNA  , D. A. F. OLIVEIRA  and E. T. BROWN† An infilled rock joint is likely to be the weakest plane in a rock mass. The most pronounced effect of the presence of infill material is the reduction in friction of the discontinuity boundaries (i.e. rock to rock contact of the joint walls). The thicker the infill, the smaller the shear strength of the rock joint. Once the infill reaches a critical thickness, the joint walls (rock) play no significant role in the overall shear strength. Several models have been proposed to predict the peak shear strength of infilled joints under both constant normal load and con- stant normal stiffness boundary conditions, taking into account the ratio of infill thickness (t ) to the height of the joint wall asperity (a), that is the t/a ratio. Models based on the constant normal stiffness condition provide a much more accurate representation of the infilled joint behaviour in the field, but only a limited number of studies have focused on the more realistic constant nor- mal stiffness stress–strain behaviour. This paper presents a critical review of some of the earlier studies and the most recent advancement of a shear-strength model developed at University of Wollongong, Australia, supple- mented with laboratory data for model validation. The effect of different factors on the shear behaviour such as the t/a ratio, infill friction angle, joint wall roughness, joint stiffness and type of infill are presented. KEYWORDS: clays; laboratory tests; rocks/rock mechanics; shear strength Dans une masse rocheuse, un joint rocheux rempli est probablement le plan le plus faible. L’effet le plus saillant de la pre ´sence de matie `res de remplissage est la re ´duction du frottement dans les limites de discontinuite ´ (autrement dit le contact entre une roche et l’autre de parois con- jointes). Plus la matie `re de remplissage est e ´paisse, moins la re ´sistance au cisaillement du joint rocheux est e ´leve ´e. Lorsque la matie `re de remplissage atteint une e ´paisseur critique, les parois conjointes (roche) ne jouent plus un ro ˆle significatif dans la re ´sistance au cisaillement globale. Plusieurs mode `les ont e ´te ´ propose ´s pour pre ´dire la re ´sis- tance au cisaillement de pointe des joints remplis sous une charge normale constante (CNC) et une rigidite ´ normale constante (RNC) limites, en tenant compte du ratio d’e ´paisseur de remplissage (t ) jusqu’a ` la hauteur de l’aspe ´rite ´ des parois conjointes (a), autrement dit un ratio t/a. Les mode `les base ´s sur la RNC pre ´sentent une pre ´ci- sion nettement supe ´rieure du comportement du joint rempli sur le terrain, mais seul un nombre limite ´ d’e ´tudes se sont penche ´es sur le comportement contrainte – de ´for- mation RNC plus re ´aliste. Cette communication pre ´sente un examen critique de certaines e ´tudes pre ´ce ´dentes, ainsi que l’e ´volution la plus re ´cente d’un mode `le contrainte – de ´formation de ´veloppe ´a ` l’universite ´ de Wollongong, en Australie, que viennent comple ´ter des donne ´es de labor- atoire pour la validation du mode `le. L’effet de diffe ´rents facteurs sur le comportement au cisaillement, comme le ratio t/a, l’angle de frottement de remplissage, la rugosite ´ des parois conjointes, la rigidite ´ du joint, et le type de matie `re de remplissage, est pre ´sente ´. INTRODUCTION The presence of a filling material often produces the weakest planes in a rock mass mainly owing to its low frictional properties. Several catastrophic failures of rock slopes have been attributed to infilled joints. Examples of such failures are the Camara ´ Dam collapse in Brazil (Kanji, 2004) and the Kangaroo Valley rock slide in New South Wales, Aus- tralia (Indraratna & Ranjith, 2001). Despite the availability of several earlier models simulating the peak shear strength of infilled rock joints (Ladanyi & Archambault, 1977; Papaliangas et al., 1990; Phien-Wej et al. 1990; de Toledo & de Freitas, 1993), only limited research has been con- ducted to describe the shear-displacement behaviour under constant normal stiffness (CNS) conditions. It is the objec- tive of this paper to develop a mathematical model for representing the shear-displacement behaviour of soil-infilled rock joints that is capable of capturing all of the key parameters, including infill thickness, joint roughness, fric- tion angle of the infill material (ö fill ), basic friction angle of the rock interface (ö b ) and degradation of asperities (Indraratna et al. 1999, 2005, 2008). According to Indraratna & Haque (2000), the majority of the current rock joint models are capable of predicting the shear behaviour of relatively simplified joint surfaces, but many of these models do not allow for complex joint surface characteristics, the effect of infill properties and the degrada- tion behaviour of asperities. In addition, conventional labora- tory direct shear tests were the primary mode of assessing the shear behaviour of rock joints in the past. As a result, the majority of these models were developed under constant normal load (CNL) conditions, but some recent studies have shown that this approach may not be representative of some practical cases. Shear strength parameters obtained under CNS conditions are more likely to be representative of non- planar joints in which dilation takes place as result of shearing, and the surrounding rock mass inhibits some of this dilation. Owing to the lack of a generalised model that correctly simulates the behaviour of soil-infilled joints under CNS, especially under mining and underground tunnelling conditions, limited attempts to capture the numerous factors affecting infilled joints have been made in recent years (Indraratna et al., 1999, 2005, 2008). THEORETICAL BACKGROUND Besides the properties of the constituent materials (rock and infill), the infill thickness is the most important para- Manuscript received 14 July 2008; revised manuscript accepted 24 September 2009. Published online ahead of print 23 March 2010. Discussion on this paper closes on 1 January 2011, for further details see p. ii.  Faculty of Engineering, University of Wollongong, New South Wales, Australia. † Golder Associates Pty Ltd, Brisbane, Australia.