Geology Reference
In-Depth Information
Uplift and erosion (the clay is
now 'over consolidated')
Clay deposited in
subsiding basin
millions of years
depth,
z
2
σ
v
=
γ .
z
2
σ
h
=
γ .
z
2
.
K
0
depth,
z
1
K
0
for over consolidated clay (London
Clay) might be up to about 3 (Craig, 1992)
so,
σ
v
=
z
1
γ .
σ
h
is now
σ
1
and up to 3 times
σ
v
σ
h
=
γ .
z
1
.
K
0
K
0
might be about 0.6 for normally consolidated
clay (Craig, 1992)
so,
σ
v
is also
σ
1
and
σ
h
is
σ
2 & 3
and = 0.6
σ
v
Figure B6-2.1
Stress conditions in overconsolidated soil. Uplift and erosion will result in a reduction in the
vertical stress on the soil element but some residual horizontal stress may be retained from its burial history.
Example 3 Topographic stresses
Stress conditions may be strongly affected by local topography exacerbated by geological conditions. At
an extreme scale, large-scale mountain structures are ascribed to gravity gliding (e.g. Graham, 1981) and
certainly large landslides have ample evidence of compression and tensile zones. Other key examples of
the effect of localised topographic stress are sheeting joints (Hencher
et al
., 2011) and valley bulging
(Parks, 1991).
Stress conditions have been measured across the world from ins-
truments, by interpretations of breakouts in deep drillholes for oil
and gas exploration, or analysis of earthquakes, and many
such data are compiled centrally and are freely available at
http://
speci
cally for projects (
Chapter 4)
but this is expensive and can
be inconclusive because of the small scale and localised nature of
tests.
Where stress assumptions prove wrong, the consequences can be
severe, as at Pergau Dam, Malaysia, where it had been anticipated
that stresses would be lithostatic (i.e. caused by self weight). During
construction, open joints and voids were encountered in tunnels
together with high in
ow of water (Murray & Gray, 1997). It was
established that horizontal stresses were much lower than had been
expected and this necessitated a complete redesign of shafts and high-
pressure tunnels and their linings, at considerable cost. Low horizontal
stresses can occur in the proximity of valley sides. Further examples are
given later in this chapter.
