Environmental Engineering Reference
In-Depth Information
2.3.2
Sheared and crushed zones (faults)
Regardless of whether its displacements have been normal, reverse or transcurrent, a fault
may be a sheared zone, a crushed zone, or some combination of these two. In crushed zones
the rock in the displacement zone has suffered brittle failure. In sheared zones the rock fail-
ure may have ranged from brittle to pseudo-ductile, where the shear displacements have been
microscopic in size. Rarely, a single slickensided joint may prove to be a continuous and sig-
nificant fault. This same fault could be a thin crushed seam (gravelly clay, CL-GC) where it
passes through a shale bed, and a wider crushed zone (fine gravel GP) where it passes
through a sandstone bed. Faults are important in dam foundations because they contain
material which is usually
- of low strength and stiffness in shear;
- compressible and erodible, and
-of large extent.
Some faults are more permeable than the average rock, or have open-jointed zones of
high permeability next to them. Some clay or gouge filled fault zones are less permeable
than the surrounding rock, and may act as low permeability barriers to groundwater
movement. Often, the rock in and next to faults is weakened further by weathering or
alteration.
Because of these characteristics, if present in foundations but undetected or inade-
quately treated or allowed for in designs, faults have the potential to seriously disrupt
dam construction or operation, or even to cause or contribute to dam failure. A thin
crushed seam contributed to the failure of Malpasset Dam in 1958. The role of this seam
is discussed in Gosselin et al. (1960), Terzaghi (1962), Jaeger (1963), Londe (1967),
Stapledon (1976) and James and Wood (1984).
The nature of treatment applied to a fault in a dam foundation will depend upon the
designer's assessment of the capacity of the fault to adversely affect the behaviour of the
foundation and its interaction with the dam, during operation. Examples of treatments of
faults are described in Chapter 17.
2.3.3
Soil infill seams (or just infill seams)
Soil infill seams are formed by soil which has gravitated, washed or squeezed into slots.
Most of the slots would have been gaping joints in rock masses which have undergone
either appreciable dilation due to stress relief (
Figures 2.1
and
2.6
)
or disruption due to
creep or landsliding (see
Sections 2.10
and 3.10). In soluble rocks (carbonates or evapo-
rates) infill materials occur mainly in irregular cavities, but also as seams in slots resulting
from widening of joints or thin beds, by solution (see Sections 3.7.1 and 3.8).
The infill materials are commonly clay (CH), and often contain roots or tubes
remaining after the rotting of roots. Although individual infill seams are rarely of large
lateral extent, they are likely to have low shear strength and to be compressible and
erodible.
2.3.4
Extremely weathered (or altered) seams
Extremely weathered (or altered) rock is material which was once rock but has been con-
verted by weathering (or alteration), in place, to soil material (see
Sections 2.6.3
,
2.7
,
2.8.1
and
Tables 2.3
and
2.6
)
. The extremely weathered seam (a) in Figure 2.1
has formed by the weathering of a thin bed of shale next to its boundary with a sand-
stone bed. Some such seams in this type of situation show slickensided surfaces, which
