Samples of a fine-grained sandstone were tested under conventional (CTC) and true triaxial compression (TTC) conditions in an attempt to reveal the effect of confining pressure, intermediate principal stress and minimum principal stress on the mechanical behavior of rocks. Under CTC conditions, an increase in confining pressure resulted in a strong increase in pre-dilatant compaction, in the threshold of dilatancy, in pre-peak ductility and in the ultimate strength. Under TTC conditions, both the intermediate principal stress and the minimum principal stress cause some increase in the ultimate strength and the threshold of absolute dilatancy. However, the effect of these stresses on the deformational properties is different: an increase in the intermediate principal stress arrests the process of microcracking and causes the rock to behave in an increasingly brittle manner, an increase in the minimum principal stress causes an increase in ductility. Mogi's empirical failure criterion has been found the most appropriate to fit all the triaxial strength data.
This paper is a follow-up to a paper that was presented last November at the 4th Asian Rock Mechanics Symposium in Singapore (Kwa'sniewski&Takahashi 2006). In that paper the effect of, independently, confining pressure (p), intermediate principal stress (σ 2) and minimum principal stress (σ 3) on the ultimate strength and post-failure behavior of a fine-grained sandstone was discussed. In the present paper, results of conventional triaxial compression and true triaxial compression tests will be presented in more detail with special emphasis on the effect of p, σ 2 and σ 3 on the volumetric deformation mode of the rock in both the pre-and post-failure range (Fig. 1). Koide andTakahashi of the Geological Survey of Japan, having built a slightly modified version of Mogi's apparatus, tested samples of three different sandstones, a shale and a marble (Takahashi & Koide 1989). In the 1990s Haimson and Chang of the University ofWisconsin in Madison built another, highly sophisticated version of aMogitype apparatus and studied the behavior of a KTB amphibolite and a Westerly granite under high σ 2 and high σ 3 conditions (Haimson&Chang 2000, Chang&Haimson 2000). However, in all these [those] studies it was only the onset of dilatancy that was investigated in detail. Moreover, no attempts were made to study the effect of not only σ 2 but also the effect of σ 3 on the volumetric deformational mode of the rocks tested. As is shown in Figure 1, the so-called onset of dilatancy (C') or the threshold of relative dilatancy (OD) is the stress level at which the volumetric strain – differential stress characteristic starts to deviate from the straight reference line typical for linear elastic materials that undergo compaction when under compression. At the threshold of absolute dilatancy (TD) the compactant volumetric strain attains a maximum value, which means that the volume of the loaded rock body becomes minimum and the instantaneous Poisson's ratio assumes a value equal to 0.5.
