Summary

As part of an extensive study of the mechanics of block-jointed systems the authors have come to recognize that failure modes other than those commonly discussed in the literature are likely to have a profound effect on the collapse of block-jointed rock masses. Such modes of failure are described and discussed in terms of a previously developed effective stress equation. Particular attention is given to the importance of «splitting» failures at low confining pressures and this phenomenon is described in terms of the effective stress equation.

Resume

En faisant des etudes approfondies de la mecanique des systèmes de roches à joints les auteurs sont venus au point de reconnaître qu'il est bien probable que des modes de rupture autres que celles generalement examinees dans les ecrits ont un effet profond sur l'effondrement des masses de roches à joints. De telles modes de rupture sont examinees et evaluees par moyen d'une equation de resistance effective anterieurement developpee. L'on se preoccupe en particulier de l'importance des ruptures «en refente» à des basses pressions confinantes et l'on evalue ce phenomène au moyen de l'equation de resistance effective.

Zusammenfassung

Als Ergebnis auf unserem ausgedehnten Studium auf dem mechanischen Gebiet bei zusammen gesetzten Block-Methoden (Felsbloecke), sind die Autoren zu dieser Ueberzeugung gekommen, dass Fehlschlaege erscheine, andere als die, die in der allgemeinen Literatur beschrieben sind, welche eine tiefere Wirkung beim Zusammenbruch von Felsmassen (Steinmassen) haben. Solche Fehlschlaege sind von Zeit zu Zeit bei vorherigen entwickelten und wirksamen Druck- Ausgleichungen beschrieben und besproehen worden. Besondere Achtung ist zu dem wichtigen «Spalten-Zusamrnenbruch» auf niedrigem, eingeschraenktem und begrenztem Druck gegeben worden, wo diese Erscheinungen bei einer bestimmten Zeitdauer unter Wirkung auf einem gleichmaessigem Druck auftreten.

INTRODUCTION

Studies of the influence of jointed systems on the behaviour of rock masses are assuming increasing importance in engineering works. As a contribution to this work an extensive investigation of idealised block-jointed masses is being conducted in the authors' laboratory and, while the experimental work to date has been restricted to conditions of uniaxial compression, it is considered that sufficient information has been accumulated on modes of failure to warrant discussion. Further it should be emphasised that the case where at least one of the principal stresses is close to zero is a most important one in that it relates to a number of often occuring engineering situations concerning, for example, the foundations and abutments of dams and tunnels. For a block-jointed system, it is convenient to sub-divide the observed modes of failure as follows:

  1. Failure along continuous joints - by sliding

    • in a single sample

    • in a block-jointed mass

  2. Failure within the rock materials

    • under effective compressive stress

    • under effective tensile stress

  3. Composite failure involving rock material and joints

    • where failure is influenced by, but does not necessarily occur along, joint systems

    • where failure occurs partially along joints and partially through the rock material Before proceeding. to a detailed description of these cases, it is necessary to consider the definition of effective stress as it is to be understood in this paper.

2. The concept of effective stress

It is well established in the field of soil mechanics that the strength and deformation characteristics of granular materials are dominantly influenced by an effective stress component which was first introduced by TERZAGID(1925) and subsequently elaborated by him in 1936 and 1945. It is the authors view however that these arguments neglect a most significant component of effective stress that derived from intermolecular forces. Recognition of the role of intermolecular forces derives from the assumption that all «contact» in the condensed state (both fluid and solid) is achieved through interaction of proximate molecules. The case where only intergranular (or interparticle) behaviour is to be considered has been dealt with previously (TROLLOPE, 1960) where the role of intermolecular forces transmitted across «junctions» (TERZAGHI,1925; BOWDEN & Tabor, 1950) is described. In this paper, it is emphasised that the intermolecular stress component (p) is a major factor in determining the intra granular strength of polycrystalline aggregates such as rock (and indeed of the crystals themselves). From a study of the properties of brown coal, the authors * * have further suggested that there are two components of ρf, one (ρp) derived from primary bonds arising from the direct chemical linking of molecular units and the other (ρs) derived from secondary bonds arising from, for example, van der Waals and electrostatic forces (TROLLOPE, ROSENGREN and BROWN,1965). It is considered that the definition of effective stress given above leads to a more rational statement of material behaviour and is free of the restriction imposed by the assumption of plane Coulombic friction which is inherent in Terzaghi's and Skempton's analyses.

3. Theoretical failure criteria

The appropriate failure criteria may be.illustrated by considering the disc model of Figure 1.

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