Environmental Engineering Reference
In-Depth Information
Many of the sinkholes occur in lines above steeply dipping defect zones in the peri-
dotite. The collapses appear to have occurred progressively into voids formed where sil-
ica and silicates have been removed by both dissolution and erosion and carried away in
near-horizontal and near-vertical flow paths (widened defects) within the defect zones.
The authors suggest that, if such features are evident, investigations should be carried
out to establish the mechanism and the potential effect on the dam or its storage.
3.11.5
Recognition and interpretation of silcrete layer
As shown in Figure 3.40 , the silcrete formed by deposition of silica in the pallid horizon
can contain “relic” structural features, e.g. bedding or foliation which were present in the
original rock before it was weathered. It is important that this is understood, to avoid
errors in interpretation of drill cores and exposures in excavations. For example, a typical
silcrete layer is very strong rock, similar to quartzite and is near-horizontal. It is usually
underlain by weathered rock with soil or very weak rock properties. If the silcrete was
formed within weathered material which was previously vertically bedded rock, it will
appear to show vertical bedding. It might then be misinterpreted as part of a vertical layer
of quartzite bedrock, continuing to depth.
This type of situation was found during the raising of Hinze rockfill dam in Queensland
(see Fell et al., 1992, Page 122). Steeply dipping beds of open-jointed and cavernous
“quartzite” were found to grade downwards into dolomite and limestone. It is believed
that silica released during lateritic weathering of adjacent metamorphic rocks had pro-
gressively replaced the uppermost 40 m or so of the dolomite and limestone, to form a
cavernous silcrete horizon.
3.11.6
Lateritic soils and profiles - list of questions
-Variable laterally and vertically?
- Deeply weathered?
- High in situ permeability, vertical and horizontal?
- Low in situ density at depth?
- If sinkholes present, their mechanism and effect?
- Fine soils suitable for earth core?
- Gravelly ferricrete or alcrete suitable for pavements?
- Cemented material in crust suitable for rockfill or rip-rap?
- Silcrete horizon or quartzite bed?
3.12
GLACIAL DEPOSITS AND LANDFORMS
During the Pleistocene period, large parts of the earth's surface were covered by sheets of
ice, similar to those which occur today in Greenland, Antarctica, parts of Europe and
North and South America. The ice moved across the landscape, eroding and reshaping it,
and when it melted it deposited the eroded materials. Similar “ice ages” occurred earlier
in the earth's history, but with rare exceptions glacial deposits formed at those times have
been so modified and strengthened by diagenesis or metamorphism that they are now
rocks not greatly different in engineering properties from the surfaces on which they were
deposited. The discussion here is therefore limited to effects of the Pleistocene glaciation
and of valley glaciers such as still occur commonly in alpine regions ( Figure 3.41 ).
Gaciated landscapes usually have complex histories of erosion and deposition, includ-
ing for example, the following sequence shown on Figure 3.42 .
 
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