Environmental Engineering Reference
In-Depth Information
3.1.3
Stability of slopes in granitic rocks
Active landsliding, or evidence of past landsliding, is relatively common in steep country
underlain by weathered granitic rocks, particularly in areas with high rainfall. Brand
(1984) describes the widespread occurrence of natural and man-induced landslides in
weathered granitic rocks in Hong Kong.
Although some landslides in weathered granitic rocks no doubt occur by failure
through the fabric of extremely weathered material, it is probable that in many landslides
failure occurs wholly or partly along relict joints or other defects (see Chapter 2, Section
2.10 and Figure 2.28).
The landsliding at Tooma and Wungong dams (Sections 2.10.3.2 and 2.10.3.3)
occurred along localised weathered zones along pre-existing defects and past movements
had occurred well into rock masses which were dominantly slightly weathered.
3.1.4
Granitic rocks: check list
- Concealed sheet joints?
-Fresh rock outcrop; does it extend down into fresh rock?
- Chemically altered zone(s)?
-Fresh granite “boulders” within extremely weathered materials?
- Extremely weathered materials, suitable for impervious core? road pavements? highly
erodible? low density
in situ
?
- Past landsliding? stability of extremely weathered materials in cuts?
3.2
VOLCANIC ROCKS (INTRUSIVE AND FLOW)
The common rocks in this group range from basalt (basic) through andesite, dacite, tra-
chyte, to rhyolite (acidic). Basalt is the most common. All are formed from molten magma
and are very fine grained, usually very strong to extremely strong when fresh. In this fresh
condition the rocks generally are also very durable and are used commonly as sources of
materials for filters, concrete aggregates, rockfill and road base courses. However vol-
canic rocks, particularly basalts and andesites, often show subtle alteration effects, which
in some cases render them unsuitable for some or all of these purposes. This matter will
be discussed further in Section 3.2.3. Also most volcanic rocks have initially contained
some glass. In rocks of Mesozoic age and older the glass has usually “devitrified” or crys-
tallized. However, in rocks of Tertiary and younger age the glass is usually still present
today and, if the rock is used as concrete aggregate, it may react with alkalis in the cement
and cause the concrete to deteriorate (see also
Section 3.2.6
).
The shape and other field characteristics of a body of volcanic rock depends upon the
circumstances in which it solidified, i.e. as a plug, dyke, sill or flow.
3.2.1
Intrusive plugs, dykes and sills
In these types of bodies the magma has been confined within other rocks (or soils), and
has flowed against them and eventually solidified against them. As a result of this mode
of formation, any of the following characteristics shown in
Figure 3.2
are commonly seen:
(a) The host-rock (or soil) close to the contacts may be stronger and more durable than
elsewhere due to being subjected to very high temperatures;
(b) The intrusive rock has “chilled” margins, i.e. it is extremely fine-grained or even glassy,
close to its contacts, due to a faster rate of cooling than in the interior of the mass;
