Results of triaxial tests conducted on a soft rock like material, under intact and disintegrated conditions, are presented. The test data are interpreted to propose an empirical failure criterion for rock masses over the brittle to ductile behaviour range. The parameters necessary for this criterion are evaluated from uniaxial compressive strength obtained from point load test and field data on rock mass quality index. Predictions by the proposed failure criterion are compared with those by the Hoek and Brown relationship.


On presente les resultats de tests triaxiaux effectues sur une roche tendre, d'abord intacte, puis desagregee. Les donnees des tests sont interpretees de manière à proposer un critère empirique de rupture pour les masses rocheuses dans la gamme de comportement fragile à malleable. Les paramètres necessaires à ce critère sont evalues à partir de la force compressive uniaxiale obtenue au moyen du test de charge ponctuelle et de donnees experimentales relatives à l'indice de qualite de la masse rocheuse. Les previsions basees sur le critère de rupture propose sont comparees à celles fournies par la relation de Hoek et Brown.


Die Ergebnisse von Dreiachsialversuchen, die mit unversehrtem und zerbrochenem Fels aehnlichem Material durchgefuehrt wurden, werden gezeigt. Kit der Deutung der Versuchsergebnisse wird ein empirisches Bruchkriterium fuer Fels abgeleitet, das fuer den sproeden und verformbaren Bereich gueltig ist. Die fuer das Kriterium notwendigen Parameter werden mit der einachsialen Druckfestigkeit, bestimmt mit Punkt-Last Versuchen, sowie mit Felsgueteziffern van Feldaufnahmen ermittelt. Aussagen mit Hilfe dieses Bruchkriteriums und aus der Beziehung nach Hoek und Brown werden verglichen.


For design of slopes and foundations on rock masses - it is well recognised now that engineers need a failure criterion which ia applicable over the stress range from brittle to ductile behaviour of rocks. The criterion should also be able to encompass the whole range of conditions varying from intact rock to highly jointed and disintegrated rock mass. The relationship should preferably be formulated in terms of parameters that are easy to evaluate and are correlated to rock mass quality indices such as Rock Mass Rating, RMR proposed by, Bieniawski (1974a, 1976) and/or rock mass Quality, Q proposed by Barton et al (1974). Hoek and Brown (1980) have proposed an empirical criterion which attempts to satisfy some of these requirements. Hoek and Brown criterion was developed from intuitive reasoning and by trial and error process based upon experience of both theoretical and experimental studies of rock failure". In this paper results of a laboratory experimental study on a rock-like material simulating soft rock are presented. The main objectives of this study were to examine: 1. the possibility of an engineering failure criterion for rock masses covering the whole range from brittle to ductile behaviour. 2. the stress range of applicability of the Hoek-Brown - criterion in the case of soft rock, and 3. the stress range where the transition from brittle to ductile rock behaviour takes place.


Stimpson (1970) has discussed in detail the large variety of model materials used in rock mechanics studies. Einstein et al (1970) have discussed the selection criteria for model materials to simulate rock behaviour. " In this study a mixture of gypsum and celite was used as a model material. From 20 trial samples, with varying mixture ratios and curing procedures, it was established that a soft rock-like material with combination of brittle behaviour, acceptable strength, lack of bleeding, smooth surface characteristics, and a minimum number of bubble holes was found for a gypsum-celite-water mixture with water/gypsum ratio = 0.55, & water/celite ratio = 32 (celite/gypsum ratio = 1.72 % by weight).

1. Characteristics of model material

The most important material properties and n-factors of the model material are given in table I. Also shown in table I are the corresponding values for 3 soft rocks.

2. Preparation of model material

Gypsum and celite were dry mixed for 3 minutes and then mixed with water for 1 minute - a duration of mixing that led to practically no bleeding. Then the mix was poured into a mould which was placed on a vibrating table and vibrated for l-minute to ensure expulsion of much of the air without causing segregation of the mix components. After 30 minutes when it was at room temperature, the sample was extruded, wrapped in a tight plastic wrap and cured in a humidity room for 3 days.

3. Model samples

A total of 122 model samples were tested during the investigation. Fig. 1 shows types of specimens used to simulate (a) intact rock, (b) smooth jointed rock, (c) disintegrated rock and (d) rough jointed rock. Disintegrated rock samples were prepared at two densities, viz 1.65 & 1.25 t/m3. These fractions were mixed uniformly and recompacted by static compaction to the required value of density (γ).

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