Environmental Engineering Reference
In-Depth Information
can indicate either:
-
The seam was, or has formed next to, a bedding-surface fault (Figure 3.19), or
-
It is simply a weathered bed within which some displacement has occurred since it
became weathered.
Extremely weathered seam (b) has developed along or next to a joint.
Extremely weathered (or altered) seams can be of large or small extent. Their shear
strengths, shear stiffnesses, compressibilities and erodibilities, will depend largely on the
compositions and fabrics of the parent rocks. Weathered seams normally occur within or
not far below the near-surface weathered zone (see Section 2.6.3 ), but altered seams can
occur at any depth.
2.3.5
The importance of using the above terms to describe defects in rock
Use of the terms defined in Figure 2.1 and discussed above can enable site investigators to
compile geotechnical models in an efficient and economical manner, due to the following:
- Individually, they are based on geological understanding, and used together they allow
the history of the site to be understood;
-They form more reliable bases for correlation between data points (exposures in
boreholes or pits) than purely engineering descriptions of the materials which form
them;
-
Some of their engineering properties, and their probable shapes and extents, can be
predicted from the defect names;
-
They can provide evidence that a rock mass has been mechanically loosened, e.g. by
mechanical weathering, landsliding, or volcanic explosion.
ISRM (1985) use the general terms “infilling” or “filling” to describe any materials (usu-
ally with soil properties) which form, or occur within the boundaries of, defects (or dis-
continuities) in rock masses. This generalization may have been a useful shorthand for
some engineers, or those involved in mathematical modeling, but its use in engineering for
dams is not recommended because
- It provides none of the above-listed benefits of the terms on Figure 2.1, and
- t can be confused with the “infill” or “soil infill” term of Figure 2.1.
The authors recommend that if a seam or zone cannot be confidently identified in terms
of Figure 2.1, then the material in it should be described as “seam material” rather than
“infilling”.
2.4
DEFECTS IN SOIL MASSES
Soils formed by the extreme weathering of rocks can inherit any of the fabrics described in
Section 2.2. Also they may contain remnant joints, crushed zones or sheared zones. The con-
trast between the strengths of these remnant defects and that of the soil (extremely weathered
rock substance) is usually lower than in a comparable mass of less weathered or fresh rock.
Soils of sedimentary origin can contain individual thin beds which behave effectively as
defects. For example, a thin silty or sandy bed in a clay deposit can represent a leakage path
with permeability several orders higher than that of the adjacent soil. Also, a thin clay bed in
a sandy soil can form a local barrier to water flow, and a surface of low shear strength.
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