Environmental Engineering Reference
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gravity. Internal friction strength is generated at the contact points of constituent grains
and minerals, determined by their size, shape and packing arrangement ( texture ). It is
additional to cohesion, and is given as an angle along which shearing would take place;
the higher the angle, the greater the strength. Its significance is seen clearly in a dry scree
or pile of loose dry sand, where the 'angle of rest' equals the friction angle. Mohr-
Coulomb criteria will help us to understand some principal failure conditions and forms
later; meanwhile, typical component strengths are shown in Table 13.1 0
Figure 13.6 Geological cross-section from Anglesey to
Brighton (450 km), linking lithology and structure with relief.
Older, resistant igneous and metamorphic rocks form higher
ground in Wales than younger, less resistant sedimentary
rocks in central and south-east England. Mesozoic sediments
in the Irish Sea basin suggest that Wales may have been
exhumed from beneath a Mesozoic cover.
Table 13.1 Some typical geotechnical parameters
of principal rock types
Type
Typical
lithology
Cohesion
(MN m -2 )
Friction
angle (°)
Residual
angle (°)
Strength (MN
m -2 )
Porosity
(%)
Compressive
Tensile
shear
Plutonic
Granite
56·1
45
35
146·4
20·6
31·5
0·5-
2·0
Volcanic
Tuff
42·2
35
31
123·9
25·2
37·9
0·5-
1·5
Metamorphic Slate
22·9
27
25
79·6
13·3 22·5
0·1-
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