Environmental Engineering Reference
In-Depth Information
In environments where the water table has remained near the surface, e.g. coastal estu-
aries, the flood-plain soils may be very soft clays, which are near normally consolidated
except for an overconsolidated upper 0.5-2 m. In these cases the soils at depth may have
a very low undrained shear strength and very high compressibility.
3.9.5
Use of alluvial soils for construction
The filters and concrete aggregate for many dams are obtained from alluvial sand and
gravel deposits. In many cases the strict grading requirements and need for low silt and clay
content (usually less than 5% or 2% passing 0.075 mm) necessitate washing, screening
and regrading. The source of the sediments will have a marked effect on their durability.
For example, sand and gravel in a stream fed from areas partly underlain by siltstone is
likely to have gravel size particles which will break up readily, rendering the gravel unsuit-
able for filters or concrete aggregates, whereas those originating from areas underlain by
granite, quartzite or other durable rocks are more likely to be suitable.
Alluvial clays, sandy clays and clayey sands (including a proportion of gravel in some
cases) can be suitable for earthfill zones in dam construction. Because of their likely vari-
ability, the deposits need careful investigation to delineate suitable areas. Borrowing with
a shovel and truck operation is sometimes necessary to ensure adequate mixing. Some
Australian dams in which alluvial materials have been used for earthfill core zones include
Blue Rock, Cairn Curran, Buffalo, Eildon (Victoria), Blowering (N.S.W.), Bjelke-Peterson
and Proserpine (Queensland), Hume (NSW - Victoria).
3.9.6
Alluvial soils, list of questions
-Vertical and lateral variability related to deposition conditions?
- Lenticular deposits of open-work gravels with extremely high permeability?
- Anisotropy due to layering?
- High kH : kV ratio?
- Oxbow lake deposits, compressible organic soils?
- Cracks, fissures, holes after rotting vegetation or burrowing animals, all either open or
backfilled?
- Cemented layers?
- Buried timber, rotten or preserved, large voids?
3.10
COLLUVIAL SOILS
3.10.1
Occurrence and description
Included under this heading are all soils which have been eroded and deposited under
gravity forces, often with the aid of water flow. They include slopewash, scree (talus), and
landslide debris. The soils range from high plasticity clays through to boulder talus deposits,
but are characterised by being mixtures of particles of contrasting sizes e.g. clays with
embedded gravel and boulders in landslide colluvium and clayey gravelly sand slopewash
deposits. They are also commonly variable within each deposit.
Figures 3.38
and
3.39
show typical environments in which scree and slopewash deposits are formed.
3.10.1.1
Scree and talus
These are deposits of rock fragments which detach from cliffs or areas of steep outcrops
and fall by gravity and roll/slide downslope. The upper scree slopes are composed of
smaller rock fragments and usually are at slopes of 35° to 38°; the toe of the slope usually
