Environmental Engineering Reference
In-Depth Information
Figure 3.1.
Granitic cliff showing 3 sets of joints approximately at right angles to one another.
Table 2.6 is a practical descriptive classification scheme for weathered granitic rocks.
When extremely weathered (i.e. soil properties) most granitic materials are silty or clayey
fine gravels or sands (GM, GC, SM or SC).
In situ
these materials are usually dense to
very dense, but in some tropically weathered areas, where quartz has been partly or
wholly removed, they are more clay rich and of low density. Somerford (1991) and
Bradbury (1990) describe low density, extremely weathered granitic materials at Harris
Dam, Western Australia.
The extremely weathered materials often make good core or earth fill materials and
where the parent rock is very coarse grained the resulting gravels can make good quality
road sub-base for sealed roads or base course for haul roads.
The silty nature of some extremely weathered granitic rocks often causes them to be
highly erodible, when exposed in excavation and when used in fills. At Cardinia earth and
rockfill dam near Melbourne extremely weathered granite is dispersive and where
exposed in the storage area shoreline has required blanketing with rockfill to prevent ero-
sion and subsequent water turbidity problems.
Lumb (1982) describes engineering properties of granitic rocks in various weathered
conditions.
Typical weathered profiles in granitic rock masses are shown on Figures 2.17 to 2.22.
The chemical weathering is initiated at and proceeds from the ground surface and from
sheet-joints, tectonic joints and faults, causing the roughly rectangular joint-blocks to
become smaller, rounded and separated by weathered materials. Thus the profile grades
usually from residual granitic soil near the surface to fresh rock at depth, with varying
amounts of residual “boulders” of fresh or partly weathered rock occurring at any level.
Fresh outcrops or large fresh boulders at the ground surface may or may not be underlain
by fresh rock. It is not uncommon to find that weathering has occurred beneath such out-
crops, along sheet joints, gently dipping tectonic joints, or within previously altered
granitic rock (Figure 2.22). Understanding of this potential for variability in weathered
granite profiles is important not only for dam foundations, but also when planning and
operating either a quarry for rockfill, rip-rap, filters or aggregate, or a borrow pit for
earth fill or core materials.
