Reasons for high horizontal stresses 634.8.7 Erosion
The possibility that erosion of the ground surface causes an increase in the
k-value is discussed by Goodman (1989). The basic idea is that the formula
for v/(l - v) is valid for the initial rock mass. On erosion of the ground
surface, however, the removal of the overburden and the consequential
effect on both the vertical and horizontal stresses will cause an increase in
the k-value, including values above unity. Also, if the horizontal
stresses become 'locked in', naturally, dividing by a lower vertical stress
component will result in a higher k-value. This subject is complex
because of the prograde and retrograde modes of geological deformation,
the time-dependent effects during this phenomenon, followed by erosion
of a possibly uneven ground surface. Suffice it to say that the processes are
certain to affect the magnitudes of the horizontal stress components.
4.8.2 Tectonic activity
We discussed under the Geological Setting heading in Chapter 2, that there
is significant current stress activity due to tectonic plate movement.
Certainly, the data in Fig. 4.16 would indicate that some form of tectonic
activity was responsible for a reasonably consistent trend of the maximum
horizontal principal stress over such a large region. From the 1906 and 1989
earthquakes along the San Andreas fault in California, USA, we know that
Figure 4.17 Subduction zone off the west coast of South America causing high
horizontal in situ stresses (from Barros et al., 1983).