Civil Engineering Reference
In-Depth Information
Table II.4
Weathering and alteration grades
Grade
Term
Description
I
Fresh
No visible sign of rock material weathering;
perhaps slight discoloration on major
discontinuity surfaces.
II
Slightly weathered
Discoloration indicates weathering of rock
material and discontinuity surfaces. All the rock
material may be discolored by weathering
and may be somewhat weaker externally than in
its fresh condition.
III
Moderately weathered
Less than half of the rock material is decomposed
and/or disintegrated to a soil. Fresh or
discolored rock is present either as a continuous
framework or as corestones.
IV
Highly weathered
More than half of the rock material is
decomposed and/or disintegrated to a soil. Fresh
or discolored rock is present either as a
discontinuous framework or as corestones.
V
Completely weathered
All rock material is decomposed and/or
disintegrated to soil. The original mass structure
is still largely intact.
VI
Residual soil
All rock material is converted to soil. The mass
structure and material fabric are destroyed.
There is a large change in volume, but the soil
has not been significantly transported.
usually do not exceed a few meters and
they often contain no infilling. Section 3.3.3
describes the characteristics of the most com-
mon types of discontinuities, which include
faults, bedding, foliation, joints, cleavage and
schistosity.
discontinuities or joint sets for sliding to
occur.
•
The mutual orientation of discontinuities will
determine the shape of the individual blocks
comprising the rock mass.
F Roughness
•
E Discontinuity orientation
The roughness of a discontinuity surface is a
potentially important component of its shear
strength, especially in the case of undisplaced
and interlocked features (e.g. unfilled joints).
The importance of surface roughness declines
as aperture, or infilling thickness, or the degree
of previous displacement increases.
•
The orientation of a discontinuity in space is
described by the dip of the line of steepest
declination measured from horizontal, and
by the dip direction measured clockwise from
true north. Example: dip (
ψ
)/dip direction (
α
)
(
45
◦
/
025
◦
)
.
•
The orientation of discontinuities relative to
an engineering structure largely controls the
possibility of unstable conditions or excessive
deformations developing. The importance of
orientation increases when other conditions
for deformation are present, such as low
shear strength and a sufficient number of
•
The roughness can be characterized by
waviness, and the unevenness or asperities.
Waviness describes large-scale undulations,
which, if interlocked and in contact, cause
dilation during shear displacement since they
are too large to be sheared off. Unevenness
or asperities describe small-scale roughness.
