Environmental Engineering Reference
In-Depth Information
up of joints due to destressing and some fragmentation of exposed rock surfaces due to
extreme temperature changes. At the other extreme, under hot, humid (e.g. tropical or sub-
tropical) conditions chemical weathering proceeds relatively rapidly, due to the ready
availability of water containing oxygen, carbon dioxide and organic acids derived from the
vegetation. Regardless of the composition and structure of the parent rock mass, the near-
surface weathered profile is usually of the lateritic type, the upper part of which may consist
almost entirely of oxides of iron and aluminium. Figure 3.40 shows a typical near-surface
profile this type, and the processes believed to be involved in its development. It must be
appreciated that the weathered profile at a particular site may not have been developed
under the present climatic conditions. Throughout some arid areas of Australia and South
Africa there are deep weathered profiles which developed under tropical or semi tropical
conditions, largely during Tertiary time.
2.6.4.2
Rock substance types, and defect types and pattern
Many fresh rock masses are relatively complex in their composition and structure. They
may contain several different rock types, with widely differing substance strengths and sus-
ceptibilities to chemical weathering, and the rocks may be folded and intersected by defects
such as joints and faults. The material in the fault zones may be crushed rock which has
essentially “soil” properties in the fresh (unweathered) state. Chemical weathering generally
proceeds from the joints and faults, which act as groundwater conduits. Because of this, the
distribution of intensely weathered rock is usually governed as much or more by the pattern
of occurrence of these defects, than by the depth below the ground surface.
Figures 2.17
to
2.24
show a range of weathered rock profiles, illustrating these effects.
Figure 2.17 is a profile developed under tropical conditions in a gneiss rock mass with
very simple structure - sheet joints parallel to the original (sloping) ground surface. The
uppermost 13 m comprises a laterite soil profile overlying extremely weathered gneiss. The
extremely weathered gneiss is a gravelly clay (CL-GC). Its upper boundary with the pallid
horizon of the laterite profile is gradational, but its lower boundary with slightly weathered
gneiss is sharp, coinciding with Sheet Joint (1). It is likely that several other sheet joints were
present initially at shallower depth, but all trace of them has been obscured by the extreme
weathering or by disturbance of the exposed surface of the cut. Between Sheet Joints (1) and
(2) the gneiss is slightly weathered except for a narrow extremely weathered zone which sur-
rounds Sheet Joint (2). Below this zone the gneiss is fresh. It is clear from this simple profile
that chemical weathering proceeded both from the ground surface and from the sheet joints.
Figure 2.18
shows a more complex profile developed under tropical conditions in the
same region as the profile in Figure 2.17. The profile steps downwards (i.e. deepens) to
the east because of the presence of three different types of granite which are progressively
less siliceous and hence are more susceptible to chemical weathering. The dolerite dyke
has a high resistance to weathering and so forms a prominent ridge in the upper surface
of the dominantly fresh rock zone. The contact between the dyke and quartz feldspar
granite is sheared, and weathering of the sheared rock has resulted in a deep, narrow slot
of highly to extremely weathered rock. A local depression in the fresh rock zone occurs
also along a sheared zone near the eastern side.
Figure 2.19
shows an idealised weathered profile through granitic rocks in Hong Kong
described by Ruxton and Berry (1957). Broadly similar profiles are found in many other
areas underlain by granitic and other igneous rocks, and this profile has been widely
accepted as typical for such rocks. It is clear that the main controls on the distribution of
weathered materials have been the depth below the ground surface and the pattern of the
joints in the rock mass.
The corestones shown on Figure 2.19 usually display spheroidal weathering effects, that is,
they comprise fresh or slightly weathered rock surrounded by concentric shells of rock which
becomes progressively more weathered away from the core, as shown on
Figure 2.20
.
