Geology and ground models - Practical Engineering Geology - page 63

3.4 Geological structures

3.4.1 Introduction
Where plates collide, large compressional stresses are generated, as
along the western coast of North and South America. The con-
sequence is uplift of the Rocky and Andes mountains and earth-
quakes on active faults such as the San Andreas in California and
on the subducting plate beneath Chile and Peru. The rocks are
squeezed and are either deformed plastically (see Figure 3.2),
where temperatures and con ning pressures are high, or fractured

or both ( Figure 3.20). Folding may control the disposition of the
various rocks at a site and a speci

c geotechnical hazard involved
with folding is intra-formational slip. As the rocks are folded,
different layers slip relative to one another, possibly resulting in
highly polished planes of low shear strength (Salehy et al., 1977;
Kovacevic et al., 2007). Such highly polished intra-formational
shear surfaces are common in the Coal Measures in the UK and
have been responsible for large landslides.
For geotechnical engineering, the geological structures that are
of prime importance are called discontinuities. These are funda-
mentally important to the mechanical properties of rock and some
soil masses and how they perform in engineering projects.

Figure 3.20
Severely folded and
thrusted sandstone
of the Table
Mountain Group,
Cogmanskloof,
South Africa.