Geology Reference
In-Depth Information
effect. As illustrated in
Box 6-2,
in overconsolidated clays such as
London Clay, where the rock has been buried to considerable
depth before uplift, erosion and unloading, then the earth pres-
sure at rest can be up to three times the vertical stress. In
tectonically active regions, stresses can be higher or lower than
lithostatic. Horizontal: vertical stress ratios as high as 15 have
been measured in areas where tectonic or thermal stress has been
locked in as the overburden has been eroded (Hoek & Brown,
1980). These stresses can adversely affect engineering projects,
resulting in deformation in tunnels, rock bursts and propagation
of fractures (e.g. Karrow & White, 2002). In mountainous ter-
rain, principal stress trajectories will follow the topography so
that the maximum principal stress runs parallel to steep natural
slopes, and this leads to spalling off of the rock parallel to the
natural slope (
Chapter 3)
and valley bulging at the toe of the
slope.
Whereas in many areas of the Earth
s crust, stress conditions can be estimated reasonably well by
calculating the weight of the soil/rock overburden to give vertical stress and taking account of Poisson
'
s
effect for horizontal stress, considerable variation is found (Hoek & Brown, 1980). In particular, hor-
izontal stresses can be higher or lower than anticipated.
'
Example 1 Overconsolidated clay
Soils and weak rocks that have gone through a cycle of burial, partial lithi
cation and then uplift
and erosion are termed overconsolidated. They typically have lower void ratios (percentage of
pores) and are stiffer than would be expected for normally consolidated soils at similar depths of
occurrence. They are also sometimes partially cemented, as described in
Chapters 1
,
3
and
5.
Under compression, they demonstrate high moduli up until the original maximum burial stress,
at which point they revert to the normal consolidation stress curve, as described in soil
mechanics textbooks (e.g. Craig, 1992). Because the stress level has been much higher in
geological history, the horizontal stress may have become locked-in as a residual stress and
may be much higher than the vertical principal stress, as illustrated in
Figure B6-2.1.
Craig
quotes earth pressure at rest K
0
values up to 2.8 for heavily overconsolidated London Clay.
Further discussion of earth pressures and how they relate to geological history is given by
Schmidt (1966).
Example 2 Active and ancient tectonic regions
Deviations from lithostatic stress conditions can be anticipated at destructive plate margins, as along the
western margins of North and South America where high horizontal stresses are to be expected.
Conversely, in extensional tectonic zones the horizontal stresses can be anticipated to be tensile.
Variations can also be expected in ancient mountain chains or areas of igneous intrusion where relict
horizontal stresses can be very high, resulting in rock bursts and large deformation of structures
(e.g. Holzhausen, 1989).
