Environmental Engineering Reference
In-Depth Information
approached they are either slightly open (6) due to destressing, or else (7) have been open
but are now partly or wholly infilled with clay which has migrated down from the chem-
ically weathered zone. In the bedrock just below the river, some joints (8) have become
infilled with alluvial silt which has migrated down from the river.
It can be seen that although the valley in profile is V-shaped, the profile of the base of the
main mechanically weathered zone is closer to U-shaped. This is often found to be the case,
and is due to pronounced destressing at the highly stressed base of the V-notch (see
Figure
2.11a
). In this very strong rock there is no obvious bulge or antiform structure across the
valley floor, but the opened joints beneath and next to the river bed (including that part
beneath the alluvial terrace) indicate that significant rebound movements have occurred.
The actual river bed below the alluvium (10) and (11) contains a slot (9), eroded dif-
ferentially by the river, along an infilled joint. Similar eroded slots occur over the altered
dyke (3) and fault (4).
Upslope from the river there are small granite outcrops (12) on the left bank and larger
and steeper granite outcrops (13) on the right bank.
Above the levels of these outcrops the granite is variably weathered. Extremely ranging to
distinctly weathered rock (14) locally contains corestones (15) of fresh and slightly weathered
rock, and extends downwards next to some joints in the dominantly fresh rock. Extremely
weathered rock extends down to about river level, along the fault zone (4).
The microgranite is very resistant to chemical weathering, and forms fresh and slightly
weathered steep outcrops (16) at the ground surface. This rock has closely spaced joints,
however, and is mechanically weathered near the surface. Toppled and fallen blocks of
microgranite have formed the scree (17) which covers the slope below.
On the left bank, slopewash soil (18) derived from extremely weathered granite occurs
to shallow depth beneath the ground surface in most places, becoming deeper locally over
the fault (4) and beneath gullies.
The water table (19) on the right bank, daylights at a seeping, partly infilled joint. Ferns
and swampy type grasses (20) are established along the outcrop of this joint.
The alluvial gravels (10) beneath the terrace are very old (of Pleistocene age, probably).
Gravels of this age are commonly partly weathered. Sands and gravels of the present day point
bar (11) are not weathered - they are deposited during the dying stages of present-day floods.
2.6.5
Complications due to cementation
Chemical weathering does not always result in weakening of rock substances. Detailed
observations at many sites have indicated that cements such as limonite or other iron oxide
minerals have been leached out from one zone, to produce weaker, more porous substance,
and redeposited in another to produce stronger, denser substance. The latter substance in
some cases has been noticeably stronger than the (assumed) fresh substance. Joints and faults
have also been found to have been strengthened greatly by such deposition during weathering.
These effects are seen commonly in all rock types in the deeply weathered mantle of west-
ern and southern Australia, and in many siltstones and sandstones in Victoria and New South
Wales. They are believed to result from weathering during warm, humid, ranging to tropical
conditions.
Similar effects have been observed in siltstones and sandstones containing small amounts
of carbonate minerals in their matrices.
2.7
CHEMICAL ALTERATION
Some igneous rocks have been partly or wholly decomposed by hot waters and gases dur-
ing late stages of their solidification. The hot waters may be derived from the igneous
