Environmental Engineering Reference
In-Depth Information
weathered in its uppermost 2-5 m. The upper surface of the granodiorite is an eroded and
weathered land surface of Permian Age.
2.6.4.4
Groundwater and topography
Optimum conditions for chemical weathering occur where vertical movement of aerated
groundwater is at a maximum, i.e. beneath elevated flat ground or gentle slopes, above the
main water table. Beneath steep slopes there is less infiltration of surface waters due to
rapid runoff.
Field evidence in many places indicates that very little weathering has occurred beneath the
main or fundamental water table. This is probably due to less oxygen and slower ground-
water movement, below the water surface. In some places it is due to the fact that the main
water table is in fact perched above fresh rock which is effectively impervious. However,
where suitably permeable rocks occur beneath the main water table, rapid groundwater flow
can occur well below the water table, and chemical weathering (or solution, in soluble rocks)
occurs.
It must be appreciated that due to changes in climate, the present water table may be
higher or lower than the water table which dominated when a particular weathered pro-
file was formed.
2.6.4.5
Features of weathered profiles near valley floors
Figure 2.27 is a hypothetical cross section through the lower part of a relatively steep val-
ley in relatively strong, jointed rocks. It shows diagrammatically a number of features
found commonly when excavating foundations for dams.
The river is flowing in an entrenched meander channel and at this site it has been migrat-
ing towards the right bank. As a result of this the left bank slopes are flatter, and show less
rock outcrop, than the right bank slopes. The river channel occupies only half of the valley
floor, the left side being formed by a well established (overgrown) alluvial terrace.
There are three bedrock types: granite (1) and microgranite (2) which are fresh and very
strong at depth, and basalt (3) in the form of an intrusive dyke which is distinctly altered,
weak. The granite contains two faults (4) in which the rock is sheared and partly crushed
to soil properties. Joints at depth (5) are mainly tightly closed, but as the valley floor is
Figure 2.27.
Diagrammatic cross section through the floor and lowest slopes of a steep-sided river valley,
showing the types of feature developed by mechanical and chemical weathering processes.
