Environmental Engineering Reference
In-Depth Information
Where they occur on steep slopes (e.g. as in
Figure 3.39
)
slopewash deposits are often
only just stable. Construction activities which cause such deposits to be over-steepened or
to take up excessive amounts of water can result in landsliding.
3.10.2.3
Landslide debris
Many landslide debris soils have relatively low permeabilities, but their mass permeabili-
ties may be high, due to the presence of cracks resulting from sliding movements. The
shear strength of the colluvium is often reasonably high, but slide surfaces at the base and
within the colluvium will be at or near residual. Almost invariably, the soil at the base of
the slide is not the same as the slide debris, so shear strength tests on the slide debris can
be misleading and usually overestimate the strength. Where the colluvium is derived from
fine grained rocks such as shale, siltstone or claystone, the weathered rock underlying the
colluvium often has higher permeability than the colluvium, an important point when con-
sidering drainage to reduce pore pressures and improve stability.
Landslide debris deposits are often only marginally stable and slope instability may be
initiated by minor changes to the surface topography or to groundwater conditions (see
Section 2.10.1).
3.10.3
Use as construction materials
Landslide debris and slopewash were used for the impervious core of the 161 m high
Talbingo Dam, see Hunter (1982) and Section 2.10.3. The landslide debris, composed
mainly of extremely weathered andesite, was used in most of the core. The slopewash,
higher plasticity material derived from the landslip deposit, was used as core-abutment
contact material.
Slopewash derived from extremely weathered granitic rocks was used for core-abutment
contact material in the following dams, for which the parent extremely weathered gran-
ite formed the remainder of the core.
- Eucumbene (N.S.W., Australia);
- Dartmouth (Victoria, Australia);
- Thomson (Victoria, Australia);
-Trengganu (Malaysia).
Slopewash derived from extremely weathered rhyodacite was used for the core at
Tuggeranong Dam (A.C.T., Australia).
When considering the possible use of landslide debris as earthfill, the critical issues are
the potential variability of the soil and the possible need to remove large boulders and
cobbles. Also the possibility of renewed slope movements must be considered, where the
deposits occur on sloping ground. High groundwater levels and resulting wet conditions
may create further difficulties.
3.10.4
Colluvial soil - list of questions
(a) Scree and talus:
- High permeability and compressibility?
-Timber debris, rotted or preserved?
- Potential for instability or debris-flow?
(b) Slopewash:
-Tubular voids causing high mass permeability?
- Compressible?
-Erodible?
