Environmental Engineering Reference
In-Depth Information
Their engineering characteristics vary with the nature of the original formation and the
form of slope failures, as described in
Chapter 9.
They typically represent an unstable
mass, often of relatively weak material, and are frequently found burying very weak allu-
vial soils. Their recognition, therefore, is important.
Mode of Occurrence
Colluvium is found on slopes, or at or far beyond the toe of a slope. Displacements from
the origin can vary from a few inches to feet for creep movements, to tens of feet for rota-
tional slides, and many feet or even miles for avalanches and flows.
7.3.2
Recognition
Terrain Features (see Also Chapter 9)
General Criteria
Terrain features vary with the type of slope failure, its stage of development, and the time
elapsed since failure occurred. At the beginning of failure, tension cracks appear along the
upper slope. During long-term gradual movements, tree trunks tilt or bend.
After slope failure, rotational slides result in distinctive spoon-shaped depressions read-
ily identifiable in aerial photographs (see
Figure 2.12,
Figure 9.26,
and
Figure 9.27).
Slides,
avalanches, and flows can result in hillside scars often devoid of vegetation (see
Figure
9.51),
also readily identifiable in aerial photos. The failure mass (colluvium) of slides, ava-
lanches, and flows results in an irregular, hummocky topography that is distinctly differ-
ent from the adjacent topography of stable areas (see
Figure 9.26
and
Figure 9.27).
Vegetation is an indicator of colluvium in some climates: banana trees, for example,
favor colluvial soils over residual.
Slope Failure Development Stages
Terrain features will vary with the stage of the slope failure:
1.
Initial stage
: The early stage is characterized by slow, often discontinuous move-
ments, tension cracks, and tilted and bent trees. Recent movements are character-
ized by tilted but straight trunks; older, continuing movements result in bent trunks.
2.
Intermediate stage
: Significant movement occurs, often still discontinuous, accom-
panied by large displacements. This stage is characteristic of slides, especially in
the progressive mode. It is during the intermediate stage that the soils may be
considered as colluvium, although the displaced blocks are often intact.
3.
Final stage
: Total displacement has occurred, leaving a prominent failure scar,
resulting in a mature colluvial deposit. Additional movement can still occur.
Normally only falls, avalanches, and flows constitute the final stage because fail-
ure is usually sudden.
Time Factor
Erosion modifies and generally smoothens and reduces the irregular surface of a fresh fail-
ure mass. At times it may make detection difficult.
Depositional Characteristics
Materials
The composition of the failure mass varies with the type of slope failure and the extent of
mass displacement as summarized in
Table 7.7
as well as with the original materials.
