Environmental Engineering Reference
In-Depth Information
periods during which weathering of the deposit has been proceeding at a faster rate than
accumulation of new rock fragments.
It is not uncommon to find timber embedded in scree, either rotted or in a preserved
condition.
3.10.1.2
Slopewash soils
Slopewash soils are admixtures of clay, sand and gravel which have been moved downs-
lope by the combined actions of soil creep (due to gravity forces) and erosion by water.
The thickest deposits are developed in depressions or gullies as shown on
Figure 3.39
.
Near the base of steep slopes slopewash soils often overlie or are intertongued with allu-
vial deposits (also shown on Figure 3.39).
In cold climates (see
Section 3.12
) freezing and thawing of the ground can be a major
contributing factor in soil creep and deep slopewash deposits are common.
Slopewash soils sometimes show indistinct bedding parallel or non parallel to the ground
surface. Slopewash usually has low density and often exhibits tubular voids left by rotted veg-
etation or roots or burrowing animals or caused by erosion of fines from within the deposit.
3.10.1.3
Landslide debris
Landslide debris can range from high plasticity clay through to silty sand from ash flows
or sand/gravel/boulder soils resulting from avalanches. In most cases the soils are very
variable, vertically and laterally, and it is not uncommon to find large boulders embedded
in a clay matrix.
Timber (the remains of trees) is often present in modern landslide debris. It may be well
preserved or rotting. Voids left by rotted timber are sometimes found.
Open cracks and irregular voids are often found in landslide debris, particularly where
the debris has resulted from or has been affected by modern slope movements.
At sites where landslides have dammed and diverted pre-existing rivers it is common to
find landslide debris overlying river alluvium.
Deposits of landslide debris are often underlain by a sheared or slickensided zone (the
slide surface) and there may be several sliding and shear surfaces at other levels within the
debris. In many cases the main slide surface may be in a zone of material which appears
to be residual soil or extremely weathered rock and is characterised by a higher clay content
than that of most of the debris.
High groundwater tables are common in landslide debris, but this is not always the case.
3.10.2
Properties of colluvial soils
As for alluvial soils, it is difficult to generalise because of the extremely wide range of soil
types. Some general characteristics which may be present are:
3.10.2.1
Scree and talus
These materials are likely to be highly permeable, and compressible. As they are sorted
they are likely to be poorly graded.
As these materials occur close to their natural angle of repose, excavation into scree or talus
slopes usually causes ravelling failures extending upslope. Entry of excessive water (e.g. by
discharge from roads) into talus materials can cause them to develop into debris-flows.
3.10.2.2
Slopewash
These soils may be more permeable than expected from their soil classification, reflecting
the presence of voids and loose structure. They are also likely to be relatively compressi-
ble. Many slopewash soils are highly erodible.
