To simulate brittle rocks, a mixture of glastone, sand and water was used as a model material. Thin galvanized sheets were used to create joints in blocks made out of the model material. To investigate the failure modes and strength, both the intact material blocks as well as jointed model material blocks of size 35.6 × 17.8 × 2.5 cm having different joint geometry configurations were subjected to uniaxial and biaxial compressive loadings. Results obtained from the intact model material blocks are used to validate a new intact rock failure criterion for biaxial loading. Results obtained from both the intact and jointed model material blocks are used to develop a strongly non-linear new rock mass failure criterion for biaxial loading. In this failure criterion, the fracture tensor component is used to incorporate the directional effect of fracture geometry system on jointed block strength. The failure criterion shows the important role the intermediate principal stress plays on rock mass strength.

Zur Simulation spröder Gesteinsarten wurde ein Modellmaterial aus Dentalgips, Sand und Wasser hergestellt. Daraus wurden Blöcke gegossen, in denen mittels duennen, galvanisierten Blechen Trennflachen erzeugt wurden. Die Blöcke mit Abmessungen von 35.6 × 17.8 × 2.5 cm wurden in einaxialen und biaxialen Druckversuchen belastet. Es wurde das Versagensverhalten und die Festigkeit von intakten Blöcken sowie durchtrennten Blöcken mit unterschiedlicher Trennflachengeometrie untersucht. Aus den Ergebnissen der Versuche an intaktem Material wurde ein neues Versagenskriterium fuer biaxiale Belastungen validiert. Die Ergebnisse der Versuche an intakten und durchtrennten Blöcken wurden verwendet, um ein neues stark nicht-lineares Versagenskriterium fuer geklueftetes Gebirge unter biaxialer Belastung abzuleiten. Darin wird der Bruchtensor verwendet, um den Einfluss der Trennflachengeometrie auf die Festigkeit zu erfassen. Das Versagenskriterium stellt heraus, welche Bedeutung die mittlere Hauptspannung fuer die Gebirgsfestigkeit hat.

Un melange de platre dentaire, de sable et d'eau a ete utilise pour developper un materiau analogue des roches fragiles. De fines feuilles galvanisees permettent de creer des discontinuites representatives des discontinuites naturelles dans des bloques fabriques avec ce materiau analogue. Les modes de rupture et la resistance, tant pour le materiau intact que pour le materiau avec ses discontinuites, ont ete etudies sur des blocs de 35.6 × 17.8 × 2.5 cm soumis à des efforts de compression uniaxiaux et biaxiaux. Differentes configurations de geometrie de discontinuites ont ete considerees. Les resultats obtenus à partir des blocs intacts permettent de verifier un nouveau critère de rupture pour les roches intactes soumises à des efforts biaxiaux. Les resultats obtenus, tant avec les blocs intacts qu'avec les blocs avec discontinuites, sont utilises pour developper un nouveau critère de rupture, fortement non lineaire, pour les massifs rocheux soumis à des chargements biaxiaux. Dans ce critère de rupture, un tenseur de fractures est introduit pour caracteriser les effets directionnels de la geometrie des systèmes de discontinuite sur la resistance des blocs avec discontinuites. Ce critère de rupture demontre le rôle important de la contrainte principale intermediaire sur la resistance des massifs rocheux.


A good understanding of the mechanical properties of jointed rock masses is vital to arrive at safe and economical designs for structures built in and on rock masses. The presence of complicated discontinuity patterns, the inherent statistical nature of their geometrical parameters, and the uncertainties involved in the estimation of their geomechanical and geometrical properties make accurate prediction of mechanical properties of jointed rock masses difficult.

Some in situ tests have been performed to study the effect of size on rock mass compressive strength (Bieniawski 1975). Heuze (1980) had reviewed the work done prior to 1980 on scale effects on mass strength. The reported results of these investigations show the reduction of mass strength and modulus with size up to a certain size at which change becomes insignificant. Note that the obtained relations from these different sites primarily depend on the fracture network of the tested rock mass. However, unfortunately, in these early investigations, no attempt had been made to map the fracture network before subjecting the rock mass to mechanical behavior testing. Therefore, the reported relations are highly site dependent and have qualitative value only.

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