Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 420

Figure 14.9 Determination of the shear parameters associated with peak and residual strength from
stress-strain curves obtained in triaxial compression tests (Wittke 1990)

The determination of the shear strength of intact rock with planar grain structure by
means of triaxial compression tests requires the conduction of tests with different con-
fining stresses

of the isotropic plane with respect to the
specimen's end faces. The shear strength then can be expressed, by Equation (3.29),
as a function of

σ 3 and inclination angles β

φ IR describing the
shear strength of the rock matrix and of the shear parameters c S and

σ 3 and β in terms of the shear parameters c IR and

φ S describing the

reduced shear strength on the isotropic plane (Section 3.2.2).
The subsequently described testing procedure referred to as the “multiple failure state
test” proposed by Kovari & Tisa (1975) allows the determination of the shear para-
meters associated with peak and residual strength in one single test (Kim & Ko 1979,
ISRM 1983, DGEG 1987, Blümel 2000).
The test is normally started by applying the lowest confining stress level pi. i . After hydro-
static loading, the axial stress σ 1 is increased, keeping the confining stress constant until

the corresponding peak strength is reached or approached (point A in Fig. 14.10). Sub-
sequently the confining stress is increased (point A'). Then, the axial stress is increased
using the procedure described above (point B). This procedure is continued stepwise up
to the highest confining stress level p max to be investigated (point C). At p max , failure
is initiated and the axial stress will drop to its residual value (point D). In the state of
residual strength the confining stress and the axial stress are continuously or stepwise
reduced until the specimen is completely unloaded (ISRM 1983).