Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 533

σ t1 = σ b - p r1 = 4.01 MPa ,

σ t2 = σ b - p r2 = 3.85 MPa .

The orientations of the horizontal stress components were derived from the impression
packer record represented in Fig. 16.7 (left). Orientations of the principal normal stress-
es are represented in a lower hemisphere stereographic projection in Fig. 16.7 (right).
The HF method, like compensation tests, has the advantage that the elastic constants
of the rock mass need not be known. On the other hand, there are several sources of
potential misinterpretation of HF tests discussed in Fairhurst (2003). In addition to
the difficulties of correctly identifying the reopening pressure on the hydraulic pressure
versus time plot (Lee & Haimson 1989) some uncertainties exist with regard to the de-
termination of p r . Ito et al. (1999) have shown that

σ H should be calculated as

σ H = 3p s - 2p r (16.23)

in cases where the fracture is not fully closed and is slightly permeable. If the permea-
bility of the fracture is higher, the water completely penetrates into the fracture and the
reopening pressure is close to

σ h and thus according to (16.18) close to p s (Cheung &
Haimson 1989, Rutqvist et al. 2000).
Due to these uncertainties Equation (16.19) should be used to determine σ H (ISRM

2003). This, however, requires a reliable estimate of the rock mass tensile strength. It is
recommended to determine

σ t , preferably by laboratory tests on hollow cylindrical rock
specimens taken from the testing depth, which are fractured by applying an internal
pressure (ISRM 1987a, ISRM 2003).
Figures 16.6 and 16.17 show the results of tests using a triaxial cell and the HF meth-
od carried out in the same rock formation and in the same borehole at depths of
125.16 m and 94 m, respectively. A good agreement for the orientations of the princi-
pal normal stresses was achieved but the stress ratios σ V determined by

the HF method were considerably larger. From the triaxial cell measurement, stress
ratios of

σ h / σ V and σ H /

σ h / σ V = 0.33 and σ H / σ V = 1.39 were obtained, whereas the HF test yielded

σ h / σ V = 1.6.

This result was characteristic of all the tests carried out in this borehole within the
scope of an exploration program for a tunnel. Due to the above-mentioned uncertain-
ties with respect to the interpretation of data obtained from HF tests the results of the
triaxial stress measurements were regarded as being more reliable than the results of
the HF tests and thus were taken as an estimate of the in-situ stress state in the area
of the tunnel (WBI 2010).
Inclined fractures in vertical boreholes occur if the in-situ stresses are not vertical and
horizontal or when the rock mass is anisotropic. Then p s can be related only to the stress
component normal to the fracture. Inclined fractures are therefore not suitable for stress
evaluation (Haimson 1993).

σ V = 0.9 and σ H /