42 In situ stress
0u2
0003 OI(a) (b) (C)
Figure 4.1 (a) Principal stresses acting on a small cube. @) Principal stresses
expressed in matrix form. (c) Principal stress orientations shown on a hemispherical
projection.4.3 Methods of stress determination
Clearly, any system utilized for estimating the in situ stress state must
involve a minimum of six independent measurements. There are methods
of ’direct’ stress measurement and there are methods of estimating the
stresses via various ‘indirect’ or ‘indicator’ methods. In this book, we will
concentrate on the four main methods recommended by the International
Society for Rock Mechanics (ISRM), while recognizing that there are a
multitude of direct and indicator approaches available.
The four direct methods recommended by the ISM (Kim and Franklin,
1987) are:
(a) the flatjack test;
@) the hydraulic fracturing test;
(c) the United States Bureau of Mines (USBM) overcoring torpedo; and
(d) the Commonwealth Scientific and Industrial Research Organization
(CSIRO) overcoring gauge.
Use of the overcoring method is shown in Fig. 4.2.
Some of the indicator methods are:(a) borehole breakouts-damage to a borehole indicating principal stress
orientations;
(b) fault plane solutions-back analysis of principal stresses causing faults;
(c) acoustic emission-the rock emits low-intensity ’noise’ when it is
stressed;
(d) anelastic strain relaxation--core exhibits expansiodcontraction on
removal from the borehole;
(e) differential strain analysis-pressurizing a piece of rock indicates its
previous stress state through differential strain effects;
(f) core discing-geometry of stress-induced core fracturing indicates
stress components;
(g) observations of discontinuity states, e.g. open discontinuities are not
transmitting stress across the gap.
The four direct ISRM recommended methods are described below; for