This paper discusses the basis of a fractal model of scale effects on rock joints. The fractal model is applied to the case of a theoretical joint, and the predictions of this approach are compared with the application of empirical JRC correction factors. It further presents the justification for a postulated stress boundary condition scaling law, and presents experimental results which demonstrate these scaling laws on both simplified joints comprising regular triangular asperities, and on complex joints derived from fractal geometry principles.


Cet article discute la base du modele fractal des effets d'echelles sur I'interface rocher-rocher. Le modele fractal est applique dans Ie cas d'une interface theorique. Les predictions de cette approche sont comparees avec I'application des facteurs de correction empiriques JRC. De plus, cet article presente la justifcation d'úne loi d'echelle regie par des contraintes et des conditions aux limites. Des resultats experimentaux sont presentes pour demontrer ces lois d'echelles sur des interfaces sirnplifiees comprenant des asperites triangulaires regulieres et sur des interfaces complexes derivees des principes de la geometric fractale.


Diese Abhandlung erörtert die Grundlagen eines fraktalen Models fuer Gesteinsfugen und den Einfluß der Größenordnung. Das fraktalen Model wird auf den Fall einer theoretischen Fuge angewendet und die voraus gesagten Resultate werden mit den Ergebnissen von empirischen JRC-korrektur Factoren verglichen. Weiterhin behandelt sie die Rechtfertigung von angenommen Spannungsgrenzwert Bedingungen einfacher Fugen mit norrnaler dreieckiger Rauheit sowie komplizierter Fugen ZlI den Prinzipien del' fraktalen Geometrie demonstrieren.


It is well known that scale plays an important role in the shear behaviour of rock joints. The classic tests of Barton and Choubey (1977) demonstrated that the parent rock joint, had a lower peak shear stress than measured for the rock joint subsamples. Further, peak shear stress occurred at larger shear displacements for the parent rock joint. With regard to the appropriate rock joint length for testing, Bandis, Lumsden and Barton (1981) recommended the use of natural block sizes. As a result of this, and other work, Barton and Bandis (1982) recommended the following expression for scale correction of both the Joint Roughness Coefficient (JRC) and Joint Wall Compressive Strength (JCS). These empirically-based recommendations have been widely and successfully implemented in rock mechanics practice. The authors have approached the problem of scale effect as a result of a need to extend their laboratory based concrete-rock joint investigations to joints of natural scale (Johnston et aI., 1993; Seidel, 1993). The particular joints being investigated are the joints that form the contact between a drilled socket and the cast-in-situ concrete of piles and anchors. These investigations have been premised on developing a theoretical, rather than empirical approach to predicting the shear behaviour of rock joints. In the most part, the investigations have been limited to soft rocks typified by Melbourne mudstone, however, the approach is considered to be of more general applicability and other rock types are currently being incorporated in the research program. Work is also progressing on laboratory testing of rock-rock joints. The fundamental laboratory investigations have largely involved the so-called constant normal stiffness (CNS) condition, as the dilation of the concrete-rock pile interface is confined. Dilation of the surrounding elastic medium (by cavity expansion) is approximately governed by a constant stiffness, K. Such confined dilation against an external stiffness applies to many other joint dilation problems in rock mechanics. It is noted that the constant normal load (CNL) condition which has dominated laboratory direct shear testing of rock joints, is only a special case of the CNS condition, for which K=O.

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