The Hoek-Brown failure criterion is often applied to a wider range of rocks than the isotropic rock masses to which it is strictly applicable. This paper evaluates the patterns of defects in rock masses using spherical projection contour plots of samples of randomly-generated orientations and datasets from mapping of New Zealand closely-jointed greywacke rock masses. Two indices from the contour plots - the area within the 1% contour (A1% ) and maximum concentration (Cmax) are introduced for distinguishing uniform (isotropic), random or regular defect patterns. The indices provide a potential extension to current guidelines on the use of the Hoek-Brown failure criterion. The paper also outlines kinematic checks to assess the influence of defect clusters on rock mass strength.

Das Hoek-Brown Bruchkriterium, das strikte nur fuer isotrope Gefuege gilt, findet oft einen viel weiteren Anwendungsbereich. Diese Arbeit bewertet diverse Dispersionsmuster der Defekte einer Gesteinsmasse. Wir vergleichen in sphaerischer Projektion

  1. Beispiele zufaelliger Orientierung von Defekten und

  2. Felddaten ueber neuseelaendisache engstaendig geklueftete Grauwacken.

Die Projektionen werden durch die Indices A1% (Flaechenanteil des 1% -Bereichs) and Cmax (Hoechstkonzentration) gekennzeichnet, welche isotrope, zufaellige und regulaere Muster unterscheiden und die aktuellen Richtlinien zum Hoek-Brown Kriterium ergaenzen. Wir testen ferner Defektgruppen der Gesteine kinematisch auf ihren Einfluss auf Massenfestigkeit.

Le critère de rupture de Hoek-Brown est souvent utilise sur une variete de roches plus etendue que les masses rocheuses auxquelles il est strictement applicable. Ce travail evalue la distribution des defauts au sein d'une masse rocheuse en utilisant une projection spherique des contours d'echantillons d'orientations generees aleatoirement et de donnees de cartographie provenant de greywackes à joints rapproches de Nouvelle Zelande. Deux indices de cette projection - la zone des contours en dessous de 1 % (A1% ) et celle de concentration maximum (Cmax) sont introduites pour distinguer les repartitions uniformes (isotropes), aleatoires ou regulières. Ces indices fournissent une extension potentielle aux règles d'utilisation du critère de rupture de Hoek-Brown. Ce travail utilisent egalement les contrôles cinematiques pour evaluer l'influence des groupes de defauts sur la resistance de la masse rocheuse.


Rock defect patterns are an important attribute in rock mass strength. The Hoek-Brown failure criterion 1 is the commonly used method for assessing the strength of jointed rocks. Critical assumptions for the criterion are that the rock and the rock mass behaviour are isotropic and that the criterion should only be applied to those rock masses in which there are a sufficient number of closely spaced discontinuities that isotropic behaviour involving failure on discontinuities can be assumed1. Despite this clear warning about the limitation of the failure criterion, the procedure is often used fairly indiscriminately to assess the strength of rock masses with well ordered defect patterns.

Upper Paleozoic to Mesozoic-age greywacke rocks are widespread throughout New Zealand and these basement rocks form the foundations for many of the country's engineering projects. The rocks are commonly closely-jointed as a result of their complex tectonic and geological history.

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