Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 550

So far, the WSM compilation has focused on orientations of horizontal stress measure-
ments neglecting their magnitudes. Recently, however, a second database, the so-called
“Quantitative WSM” (Q-WSM), accounting for also stress magnitude was initiated.
Figure 16.35 shows a preliminary data evaluation of the Q-WSM database for crystalline
and sedimentary rocks coming mainly from Asia and Europe (Zang et al. 2012). Mean
stress gradients, that is, mean rock mass unit weights, of = 26 MPa/km = 26 kN/m3
and = 22 MPa/km = 22 kN/m3, respectively, were derived for both rock groups (Fig.
16.35). An earlier data compilation of Brown & Hoek (1978) with data mainly taken
from Australia, North America and South Africa with no assignment to rock types
yielded a mean rock mass unit weight of = 27 kN/m3.

Figure 16.35 Vertical in-situ stress versus depth (Zang et al. 2012)

The ratio K of horizontal and vertical in-situ stress components σ V , here re-

ferred to as “stress coeffi cient”, obtained from stress measurements in various conti-
nents as compiled by Brown & Hoek (1978) and Zang et al. (2012), follows hyperbolic
equations of the following type:

σ H and

(16.24)

where h is the height of overburden and Δσ H is a horizontal stress component due to

tectonics and/or geologic pre-loading and subsequent unloading (Sections 9.3 and 9.4).