Environmental Engineering Reference
In-Depth Information
TABLE 7.7
Characteristics of Colluvial Deposits
Origin
a
Depositional Extent
b
Movement Rate
c
Material Characteristics
d
Falls
Along slope to
Very rapid
Rock blocks and fragments heterogeneously
beyond
assembled (talus, see
Figure 6.70)
Creep
Few centimeters to
Very slow
Original structure distorted but preserved
meters
Rotational
Few meters to tens of
Slow to rapid
Original structure preserved in blocks but
slides
meters
planar orientation altered. Debris mass
at slide toe
Translational
Few to hundreds of
Slow to very
Original structure preserved in blocks which
slides in rock
meters
rapid
are dislocated: or,in major movements, a
mass of mixed debris at and beyond the
toe of slope
Lateral
Few to many tens of
Slow to very
Same as for translational slides
spreading
meters
rapid
Avalanche
Many tens of meters to
Very rapid
Completely heterogeneous mixture of soil
kilometers
and rock debris; all fabric destroyed
(see
Figure 7.10)
Flows
Many tens of meters to
Very rapid
Completely heterogeneous mixture of
kilometers
e
soil and rock, or of only soil. All fabric
destroyed
a
Origin refers to type of slope failure: see
Chapter 9.
b
Significant to the size of the deposit at final failure.
c
Ranges refer to failure stage (initial to total). See
Section 9.1.2
slope activity.
d
Relate to the materials of the parent formation prior to failure.
e
The Achocallo mudflow near La Paz, Bolivia, extends for 25 km (see
Section 9.2.11).
Materials can be grouped into four general categories based on their structure and fab-
ric. Talus is the heterogeneous assembly of rock blocks and fragments at the toe of steep
slopes (see
Figure 6.67).
Distorted surficial beds result from creep. Displaced and reori-
ented intact blocks with some mixed debris at the toe are common to rotational and trans-
lational slides. Heterogeneous mixtures in which all fabric is destroyed (Figure 7.10) are
common to avalanches and flows. These latter mixtures can be difficult to detect when
they originate from residual soils with a heterogeneous fabric.
Unconformities
In slides, the colluvium will be separated from the underlying material by a failure
surface, which may demarcate a material change or a change in fabric or structural ori-
entation.
In avalanches and flows, the underlying material will often be completely different in
texture and fabric from the colluvium, especially where large displacements occur.
Properties
Distinguishing colluvium from residual soils is often difficult, but, in general, the collu-
vium will be less dense with higher moisture contents, lower strengths, and completely
without structure (if of avalanche origin), whereas residual soil may have some relict
structure. A few typical properties are given in
Table 3.38.
In some cases, seismic refraction surveys will detect the contact between colluvium and
residual soils, especially if the contact is above the saturated zone. Typical velocity ranges
for soils derived from gneiss are: colluvium,
V
p
1000 to 2000 ft/sec (300-600 m/sec);
residuum,
V
p
2000 to 3000 ft/sec (600-900 m/sec).
