Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 535

determination requires a minimum of six different tests on fractures with different dip
and strike directions. However, a minimum of eight tests are recommended to obtain
an over-determined equation system that can be solved using the least square method
(ISRM 2003).
A detailed description of the interpretation of hydraulic pressure versus time records
considering pre-existing fractures is given in Baumgärtner & Rummel (1989), Cornet
(1993) and Valinezhad et al. (2008).

16.6 Methods of Large-Scale In-situ Stress Determination

Stress measurement methods according to Sections 16.2 - 16.5 allow only pointwise de-
termination of the in-situ stress state. Thus, for the evaluation of the large-scale in-situ
stress state and its spatial variation, other methods are required.
One way is to deduce the large-scale in-situ stress state from local stress measure-
ments by means of numerical methods (Kiehl 1991b, Li et al. 2009).

As an example, Fig. 16.18 shows the interpretation of triaxial cell measurements car-
ried out in a clay slate (Kiehl 1991b), the structural model of which is represent-
ed in Fig. 2.37. These tests were conducted using SST cells (Section 16.2.1) in three
boreholes located in the middle and on both slopes of the valley at the location of a
planned concrete dam. Because of the pronounced anisotropic deformability of the
existing clay slate the results of these measurements needed to be evaluated account-
ing for rock mass anisotropy. Since the test locations within each borehole lie only a
few meters apart, the stress components resulting from all tests in one borehole were
averaged.
To interpret these measurements with respect to the large-scale in-situ stress state at
the dam site, a pseudo-three-dimensional finite element analysis was carried out using
the program FEST03 (Section 10.7.1). The computation section consists of a vertical
slab through the valley. In this analysis the stress state resulting from gravity, account-
ing for the topographic situation, was evaluated (Fig. 16.18, upper). The calculated
stresses were subtracted component by component from the measured stresses and
subsequently transformed onto principal axes. This stress state with principal normal
stresses Δσ 3 was interpreted as a tectonic stress state existing in the

foundation area of the dam. As a result,

Δσ 1 , Δσ 2 and

Δσ 2 are directed almost horizontal.
At the sides of the valley the maximum principal normal stress

Δσ 1 and

Δσ 1 is oriented ap-

proximately in the W-E direction. In the middle of the valley Δσ 1 runs approximately

from N to S. The steeply dipping minimum principal stress Δσ 3 is approximately zero

(Fig. 16.18, lower).