Environmental Engineering Reference
In-Depth Information
Figure 2.14.
Diagrammatic cross section showing hydrothermally altered zone near a granite intrusion,
with the uppermost part of the altered zone being weathered subsequently.
with faults. In the case of more porous rocks, e.g. some sandstones and limestones, ground-
water can also enter through intergranular pores. Most chemical weathering occurs at
extremely slow rates, such that the changes to the strength of high strength, non-porous
rocks (e.g. granite) are likely to be insignificant during the operating life of most civil engi-
neering projects. There are however some minerals and rocks which decompose, weaken
or disintegrate within a few months or years of exposure. These effects will be discussed
separately under Section 2.9, Rapid weathering.
Chemical weathering involves the more or less continuous operation of all or most of
the following:
1. Chemical reactions between the minerals in the rock, and water, oxygen, carbon diox-
ide, and organic acids. These reactions cause decomposition of the minerals to form new
products, some of which are soluble;
2. Removal of the soluble decomposition products by leaching;
3. Development of microcracks in some rocks, probably due to some decomposition prod-
ucts having larger volumes than the original minerals, or to destressing, or capillary or
osmotic suction effects;
4. Deposition of some decomposition products, in pores or microcracks.
Processes 3 and 4 were illustrated by Dixon (1969) who made microscopic studies of gran-
ite and schist in a range of weathered conditions. He found that in slightly weathered sam-
ples, i.e. rocks which showed only slight discolouration in hand specimen, the first
weathering effects visible microscopically were slight discolouration of the felspars and
mica minerals, and the presence of many microcracks, some open and others filled with
an opaque mineral assumed to be limonite or clay (
Figure 2.15
).
Baynes and Dearman (1978a) describe microfabric changes which occur in granites
during various stages of weathering, using a scanning electron microscope. In Baynes and
