The behaviour of dense sand, a rough joint, and models representing closely jointed media lead to the observation that, well before failure is reached, these materials increase in volume, i.e. the onset of dilatancy occurs before the peak strength is reached. The purpose of this paper is to investigate, in a rather simple manner, the effect of this prefailure dilatancy on the stress distribution in a rock mass beneath a surface loading and adjacent to a pile.
Le comportement d'un sable dense, d'une fissure grossière et de modèles representant des milieux fissures de façon dense conduit à observer que le volume de ces materiaux croît bien avant que le point de rupture soit atteint, c.a.d. que le debut de la dilatation se produit avant que la force maximum soit atteinte. Le but de cet article est d'etudier, d'une façon assez simple, l'effet de cette dilatation d'avant rupture sur la distribution de l'effort dans une masse rocheuse placee sous une charge de surface et adjacente à un pilotis.
Das Verhalten von dichtem Sand, rauhen Trennflachen und Modellen, die eng geklueftete Felsen darstellen, fuehrt zu der Beobachtung, daß sich das Volumen betrachtlich vor dem Eintreten des Bruchs vergrößert, d.h. Dilatation tritt vor Erreichen der Höchstfestigkeit ein. In dem Aufsatz wird der Einfluß der Dilatation auf die Spannungsverteilung im Gebirge, infolge einer Oberflachenlast und in der Umgebung eines Pfahls in einer vereinfachten Form untersucht.
The stress-strain behaviour of dense granular soils, models representing closely jointed media, and the shear load-displacement behaviour of a single rough joint surface, all indicate a net increase in volume before the peak shear resistance is mobilised. This phenomenon, volume change on the application of shearing stress, is commonly referred to as dilatancy. The effect of dilatancy on the behaviour of a rock mass is of some interest. Although the effect in the post-failure region is well known the effect of pre failure dilatancy seems to have received little attention. This paper presents the results of some preliminary studies of the topic. Unfortunately no field evidence giving a direct indication of prefailure dilatant behaviour is known to the authors. This is not surprising in view of the difficulty of field measurement in a jointed rock mass. However there is good evidence from carefully controlled laboratory studies on model materials representing a closely jointed rock mass. For example Brown (1976) presented volumetric strain curves for an assembly of accurately fitting 'bricks' of various shapes. Some results are reproduced in Fig. 1. It is clear that the volume change behaviour is dependent on the arrangement of the separate blocks relative to the direction of loading. However in at least one of the cases (H30) there is a very pronounced increase in volume right from the start of loading. As another example Gerogiannopolous and Brown (1978) reported triaxial test results on granulated marble (Rosengren and Jaeger (1968)). These results are reproduced in Fig. 2. At the start of the loading process the material decreases in volume, but before half the peak strength has been mobilised the incremental volumetric strain gives a volume increase. As is well known this volume increase continues well past the point at which the peak strength is mobilised. Thirdly Barton (1972) performed an interesting series of tests on slabs of model rock material which were intersected by a series of approximately orthogonal rough joints. The shear and normal displacement results for some of these tests are reproduced in Fig. 3. Once again it is evident that well before the peak shear strength is mobilised there is an opening of the joints. In the three examples quoted above there is no constraint on the volume change or normal displacement. The boundary conditions are such that the confining stress remains constant. However if there is some deformational restraint the dilatancy is suppressed by an increase in the confining pressure. This point is made by Goodman (1974). Thus it might be expected that in a dilatant medium subject to deformational restraint, such as the rock mass surrounding the shaft of a socketed pile, the confining pressures would be higher than in an equivalent non-dilatant medium. A limiting case in the spectrum of possible jointed rock masses is a randomly jointed medium. If the joint spacing is small in relation to the scale of the structure of interest then it is possible, at least as a first approximation, to idealise the rock mass as a homogeneous isotropic continuum. This is the viewpoint adopted in this paper.
The dilatant volume change is calculated herein by relating it to the change in octahedral shear stress. As has been explained in the introduction the objective of this paper is to explore in a simple manner the consequences of pre failure dilatancy.