The envelopes of Mohr stress circle at failure of soft rocks show generally the non-linearity. The experimental equation to represent this envelopes has been reported by the authors (Yoshinaka and Yamabe 1980).
In this paper, the outline of the power function criterion is described briefly, and then it is investigated that:
applicability of the criterion to anisotropic rocks
estimation of strength of intact rocks under triaxial compression using unconfined compressive strength qu and tensile strength a, obtained from radial compression test
applicability of the criterion to closely jointed rocks and rock masses In this paper, the experimental values of other authors are quoted from their papers, but some of these values has been directly read from graphs. Therefore it seems that these values contain some amount of error of reading.
A failure criterion has been derived from the experimental results of CD triaxial compression tests conducted on undisturbed samples of various kinds of soft rocks in Japan.
For example Fig. 1 shows the strength relation of soft rocks normalized by reference strength. am' and Tm are expressed as follows: σm'=(σ1'+ σ 2'+ σ 3)/3. τm=(σ2- σ3)/2 σ mo' and (τm" are at the case of σ 2= σ3=O. In this figure, qu is also plotted.
Sampling location of Kobe mudstone A and B are at a distance of horizontally 3 km apart each other. These mudstones are a series of sediments belonging to the same sedimentary basin in Miocene period. Strength relation of failure of these mudstones which have different values of qu each other, can be expressed by a same line using the normalized strength as shown in Fig. 1. This may be originated from the identity of geological history, and suggests that qu can be utilized as a good parameter to represent strength relation.
(Figure in full paper)
Fig. 1 also includes the results of London clay (Bishop, Webb & Lewin 1966), Ohya pumiceous tuff and Yokohama siltstone. All these examples show the non-linear relationship. To express this non-linearity, normalized strength can be plotted on a logarithmic representation. These relation can be expressed by the following power function:
Figs.2 and 3 show about Ohya pumiceous tuff and Yokohama siltstone. These figures include the experimental results of CD triaxial compression tests conducted on samples (50 mm diameter, 100 mm length) and larger samples (100 mm diameter, 200 mm length), confined Brazilian tests (Jaeger & Hoskins 1966) and Brazilian tests.
(Figure in full paper)
According to these figures, the power function makes it possible to represent the scale effect on strength by adopting the reduction of quo These figures also show that the criterion can be applied to the tensile stress state produced in Brazilian and confined Brazilian tests on rocks with =1.
It is well known that peak stress at failure of anisotropic rock is greatly influenced by the orientation of the weak planes, with respect to the principal stress axes.
