Recent studies by the authors into the behaviour of jointed rock masses using direct shear testing have confirmed that joint spacing has a significant effect on rock mass strength. The rock mass failure envelopes assessed from these tests have been compared to failure envelopes obtained using the Hoek-Brown GSI criterion. Good agreement between measured and estimated GSI strength envelopes where obtained when joint spacing was considered in the assessment of GSI.

Les etudes recentes conduites par les auteurs sur le comportement des joints rocheux employant l'essai direct de cisaillement ont confirme que l'espacement a un effet significatif sur la force de la masse rocheuse. Les enveloppes de ruptures obtenues pour ces essais ont ete comparees à l'enveloppe de rupture obtenue en utilisant le critère GSI Hoek-Brown. Une bonne correlation a ete constatee entre les enveloppes obtenues experimentalement et les enveloppes calculees par la methode GSI quand l'espacement des joints a ete pris en consideration dans l'evaluation de GSI.

Kuerzliche Untersuchungen der Autoren betreffs Reaktionen von gebrochenen Felsmassen zu direkter Scherkraft, bestaetigen, dass die Abstaende von Rissen einen deutlichen Einfluss auf den Widerstand der Felsmassen hat. Die Bruchbereiche der Felsmassen die von diesen Untersuchungen entwickelt wurden, wurden verglichen mit den Bruchbereichen, die man mit der Anwendund der Hoek-Brown GSI Formulierung erhaelt. Eine gute Uebereinstimmung zwischen gemessenen und errechneten GSI Festigkeitsbereichen wurde erzielt, wenn der Abstand der Risse in der Berechnung der GSI in Betracht gezogen wurde.


The interactions that occur within rock masses as they are subjected to loading are complex and difficult to model analytically. This has prompted the use of case studies and laboratory test results to develop empirical criteria that can be used to assess rock mass strength.

Hoek and Brown [1] introduced a criterion to assess the strength of good to reasonable quality rock masses. However, this method was applied to rock masses over the spectrum of rock quality. To include rock masses of all qualities, Hoek [2] introduced the concept of geological strength index (GSI). This has been refined by others [3], [4], [5], over time. While the strength of jointed rock masses relative to geological conditions can be assessed using the Hoek-Brown GSI criterion, the accuracy of the assessments remains largely unknown unless it can be correlated with measured behaviour.

A recent study at Monash University has used laboratory testing to investigate the strength of jointed soft rock masses. The peak strengths of synthetic, relatively large-scale samples were measured in a direct shear apparatus. The results of these tests have been normalised by dividing the peak shear stress by the unconfined compressive strength (UCS) of the intact rock. These normalised results are compared to strength envelopes assessed by the Hoek-Brown GSI criterion.

Hoek-Brown GSI criterion

The concept of GSI requires the selection of a suitable range of GSI values from the chart shown in Figure 1. The values selected are based on the structure of the rock mass and the condition of the joints.

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