Environmental Engineering Reference
In-Depth Information
FIGURE 7.10
Colluvial deposit of boulders in a clayey matrix, originating from residual soils failing in an avalanche,
exposed in a road cut along the Rio Santos Highway, Brazil. Material is called talus in Brazil because of the
large number of boulders.
7.3.3
Engineering Significance
Unstable Masses
When colluvium rests on a slope, it normally represents an unstable condition, and further
slope movements are likely. In slides, the mass is bounded by a failure surface along which
residual (or slightly higher) strengths prevail, representing a weakness surface in the mass
that is often evidenced by slickensided surfaces. The unconformable mass on the slope
blocks the normal slope seepage and evaporation because of its relative impermeability,
resulting in pore-pressure buildup during the rainy season (see
Figure 9.85),
and a further
decrease in stability.
Slope movements of colluvium are common, and before total failure range from the
barely perceptible movements of creep to the more discernible movements of several
inches per week. Movements are normally periodic, accelerating and decelerating, and
stopping completely for some period of time (the slip-stick phenomenon). The natural
phenomena causing movements are rainfall, snow and ice melt, earthquake-induced
vibrations, and changing levels of adjacent water bodies resulting from floods and tides.
Cuts made in colluvial soil slopes can be expected to become much less stable with time
and usually lead to failure, unless retained.
Buried Weak Alluvium
Colluvial deposits overlying soft alluvium in valleys are less recognized but common
phenomenon in hilly or mountainous terrain. An example is illustrated in
Figure 7.11
through
Figure 7.14.
In
Figure 7.11,
the colluvium from a debris avalanche has moved out onto the valley
floor, forming a relatively level blanket extending over an area of several acres, a large part
of which overlies alluvium. If test boring were not deep enough (
Figure 7.12)
,
the collu-
vium (
Figure 7.13)
could have easily been mistaken for residual soil, especially when it has
been strengthened by drying, as indicated at this location by the high SPT values. In this