Environmental Engineering Reference
In-Depth Information
increase the soil mass permeability. Fissured soils are sometimes present in the founda-
tions of dams - either left in place by design, or because they were not removed by those
who built the dam, not realising the effect on the shear strength.
Fissures in soil have a wide range of characteristics relating to the properties of the soil
mass, the depositional and weathering history and climate (when the fissures were formed
and now). It is useful to log, and classify the fissures systematically.
Walker et al. (1987) provide a fissure classification system which is simple and practical to
use. The main components are shown in Figure 6.28.
Figures 6.29
and
6.30
show examples
of fissured soils.
Fissures are caused by one or more of the following:
(a) Stress relief due to erosion or retreat of glaciation;
(b) Freeze-thaw;
(c) Shrink-swell (desiccation) due to seasonal moisture content changes;
(d) Differential consolidation and settlement in sedimentary soils.
It is important to recognise that the fissures are often formed in a different geological
and climatic condition from the present - e.g. in periods of glacial retreat, or when the sea
level was lower than now. The fissured soils may in some cases be overlain by more recent
sediments which are not fissured (e.g. Thorne, 1984).
The shear strength of fissured soils is dependent on:
-
The peak, fully softened and residual strength of the soil substance;
-
The continuity, orientation, shape and spacing of the fissures and the nature of the fissure
surfaces;
-
These are in turn related to the origin of the fissures.
For example, smooth, planar fissures may only reduce the soil mass strength to the fully
softened strength, but polished and slickensided, planar or undulose, continuous fissures,
will reduce the strength in the direction of the fissures to near the residual strength.
1st Number
Continuity
2nd Number
Orientation
3rd Number
Shape
4th Number
Spacing
5th Number
Surface
Number
Planar in two
directions with
maximum
amplitude
across 75mm
sample of 2mm
Individual
fissures
continuous
across
sample
Actual measured angle(s)
to horizontal
Fissure surfaces grooved,
striated or slickensided
Fissures
less than
5mm apart
1
Individual fissures
not continuous
but make up
potential
continuous
failure plane
because of
many intersections
Fissure surfaces polished
Fissures
5 to 10mm
apart
2
Undulose
Individual
fissures not
continuous,
some
intersections
Fissures
10 to 40mm
apart
Fissure surfaces smooth
3
Conchoidal
Individual
fissures not
continuous,
very few
intersections
Fissures
more than
40mm apart
Fissure surfaces rough
4
Irregular
Figure 6.28.
Fissure classification explanation sheet (Walker et al., 1987, after Coffey Geosciences).
