Environmental Engineering Reference
In-Depth Information
3.9.1
River channel deposits
The sands, gravels, cobbles and boulders are often highly permeable, particularly in the
horizontal direction. Layers, often thin, of finer or coarser materials cause marked differ-
ences between vertical and horizontal permeability. This is shown diagrammatically in
Figures 3.37
and 10.2. Clean gravels can be interlayered with sands or sandy gravels, giv-
ing overall horizontal permeabilities 10 times to 1000 times the overall vertical perme-
ability. The relative density of such deposits is variable, but the upper few metres which
are most affected by scour and redeposition during flooding, are likely to be loose to
medium dense and, hence, will be relatively compressible and have effective friction angles
in the range of 28° to 35°. Deeper deposits are more likely to be dense, less compressible
and have a high effective friction angle.
3.9.2
Open-work gravels
At the 143 m high, 2740 m long Tarbela Dam in Pakistan extensive deposits of open-work
gravels occur in the 190 m deep alluvium which forms the foundation for the embank-
ment. The alluvium comprises sands, open-work gravels and boulders and boulder grav-
els in which the voids are sand-filled (i.e. extremely gap-graded materials). The design
allowed for underseepage to be controlled by an impervious blanket which extended
1500 m upstream from the impervious core. The blanket ranged in thickness from 13 m
near the upstream toe of the embankment to 1.5 m at its upstream extremity. For several
years after first filling (1974) many “sink-holes” or graben-like craters and depressions
appeared in the blanket, apparently due to local zones of cavitation within the underlying
alluvium. Some of the sinkholes were repaired in the dry and others by dumping new
blanket material over them through water using bottom-dump barges. The local collapse
zones which caused the sinkholes are believed to have formed when excessively high flow
rates through open-work gravels caused adjacent sandy layers to migrate into their large
voids.
3.9.3
Oxbow lake deposits
Where clays, silts and organic soils deposited in oxbow lakes have not dried out they are
near normally consolidated and may be highly compressible. McAlexander and Engemoen
(1985) describe the occurrences of extensive oxbow lake deposits up to 5 m thick in the
foundation of the 29 m high Calamus Dam in Nebraska, USA. These deposits comprised
fibrous peat, organic silty sands and clays and were highly variable in thickness and lateral
extent. Testing showed that the peat was highly compressible. Because of concern about dif-
ferential settlements and cracking in the embankment, the organic materials were removed
from beneath the impervious core and from beneath extensive parts of the shoulders.
3.9.4
Flood plain, lacustrine and estuarine deposits
The clays and silts in these deposits are likely to show pronounced horizontal stratification,
with each flood or period of deposition resulting in an initially relatively coarse layer fining
upwards as the flood recedes. This may result in marked anisotropy in permeability, with the
horizontal permeability being 10 times or even 100 or 1000 times the vertical permeability.
The permeability of these deposits is often increased by the desiccation cracks, sand filled
cracks, fissures and holes left by burrowing animals and rotted vegetation. Where such
defects have been backfilled by clay soils, the permeability of the mass can be decreased.
Where desiccated the clay soils are overconsolidated and their shear strengths are
affected by the presence of fissures which are often slickensided.
