Environmental Engineering Reference
In-Depth Information
2.8.2
Limitations on classification systems for weathered rock
Quite often, particularly in sedimentary rocks, it is difficult or even impossible to determine
which, if any of the rock substance at a site is fresh (unweathered). This is because such rocks
can vary greatly in colour, porosity and strength due to past processes involved in their for-
mation, rather than weathering processes.
Some rocks which are weak or very weak when fresh (e.g. some shales and poorly
cemented sandstones) assume soil properties (i.e. they classify as “extremely weathered”)
when only slightly affected by chemical weathering. Intermediate weathered conditions
cannot be defined in these cases.
Weathering products of carbonate rocks and rocks cemented by gypsum or anhydrite
range from cavities or soils (due to solution) to very strong rock substances (due to rede-
position of material dissolved elsewhere in the mass). Such weathering products usually
cannot be described adequately using the system on
Table 2.3
. Site-specific classifications
are preferable in such cases.
2.9
RAPID WEATHERING
Rapid weathering processes are defined here as those processes which cause exposed
rocks to be significantly weakened during periods of only days, months or years. In gen-
eral the rocks most affected by rapid weathering are relatively porous and not of high
strength. However even some very high strength rocks can be affected and so careful
observational studies and testing are needed whenever rocks are to be used as construc-
tion materials.
2.9.1
Slaking of mudrocks
The weakest claystones and shales, which can barely be described as rock, have usually
been strengthened only by consolidation. When such “compaction shales” are exposed in
excavations they usually develop fine cracks at the exposed surface due to destressing and
drying out. Swelling, further cracking and sometimes complete disintegration back to clay
occurs, when the destressed shale or claystone is allowed to absorb water.
Stronger claystones, siltstones and shales (i.e. in the weak to strong rock range) have
usually received some strength by cementation and/or recrystallisation of clay minerals to
form micas, as well as by consolidation. These rocks also generally develop fine drying
out cracks when exposed by excavation but the cracking is much less severe and occurs
more slowly than in the “compaction shales”. Once the fine cracks appear on the rock
surface, addition of water causes relatively rapid deterioration of the rock. The reasons
for this are believed to be as follows. Water is absorbed rapidly into the cracks and adja-
cent rock substances, by capillary suction. The water compresses the air trapped in the
cracks and allows clay minerals in the adjacent rock to swell slightly. As a result the cracks
widen and propagate.
The mechanisms of slaking are discussed in more detail by Taylor and Smith (1986),
Taylor (1988) and Olivier (1990).
Many strong siltstones develop cracks eventually with repeated wetting and drying.
However in constant humidity environments such as in rockfills or earthfill, such rocks
have been found to have suffered little or no breakdown over periods of up to 90 years.
The weathering of mudrocks and its significance in dam construction is discussed in
Chapter 3, Sections 3.5.1 and 3.5.4.
Some poorly cemented sandstones which contain a high proportion of clay minerals in
their matrices, show similar slaking properties to mudrocks.
