Geology Reference
In-Depth Information
Figure 3.5 Welded
tuff with
attened
amme that were
plastic when
deposited sub-
aerially, Ap Li
Chau, Hong Kong.
Some of the characteristics of an area intruded by igneous rocks are
illustrated schematically in Figure 3.8. Hot igneous rocks affect the
country rock that they intrude, giving rise to thermal metamorph-
ism, as discussed in Section 3.3.5. Some metamorphosed zones may
be much weaker than the associated igneous body, with obvious
potential consequences if encountered in an engineering project.
Figure 3.9 shows a zone of weak, hydrothermally altered rock
encountered at a depth of about 200m in a tunnel, well below
the level of anticipated weathering and therefore rather unexpected.
It resulted in a minor collapse that delayed tunnelling. The extent
of the zone was investigated with horizontal drillholes. Following
forward probing, ground treatment and support, tunnelling was
able to proceed.
3.3.4 Sediments and associations
soils and rocks
-
3.3.4.1 General nature and classi
cation
Sediments and sedimentary rocks are derived from the breakdown of
older rocks. Detritus is transported by gravity, water, wind and
 
 
 
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