Engineering Rock Mechanics

60 In situ stress

3000

e,

1000-

E

I

N

e,

(^2 2) 1500-
VJ
0 e,
g 2000 -
0"
2500 -
T
I I I I I 1 I
N
e,
(^2 2) 1500-
VJ
0
g 2000 -
0"
2500 -
0 Australia
T United States
A Canada
o Scandinavia
Southern Africa
Other regions
Vertical stress uv - MPa
Figure 4.14 Collated worldwide in situ stress data: vertical stress component (after
Hoek and Brown, 1980).
adopted as a generic unit weight). It can be seen that the estimate of the
vertical stress component is basically correct, but only in the sense of a
regression, or best fit, line. In some cases, the measured stress component
is almost exactly as predicted, but in other cases and especially at depths less
than 1000 m, the measured stress component can be dramatically different
to the predicted component. Note that there are cases near the surface
where the measured vertical stress component is about five times the
predicted component. Also, between depths of 500 and 1500 m, there are
cases where the measured stress component is five times less than
predicted. We can conclude, therefore, that whilst the equation provides
a good predictive estimate of the average stress from all the data, it can
certainly not be relied upon to provide a correct estimate at any specific
location. This implies that, if at all possible, it is best to measure rather than
estimate the vertical stress component.
It should be noted that the horizontal axis in Fig. 4.15 is the mean of the
two horizontal stress components, normalized by dividing by the vertical
stress component. In this sense, the ratio on the horizontal axis is equivalent
to the v/(l - v) coefficient calculated earlier: in engineering rock mechanics
it is generally known as k. A particular point to remember is that by taking
the average of the two horizontal stresses, which could well be the major
and minor principal stresses, a large element of the more extreme variability
may have been suppressed. However, the compilers found this was the