Geology Reference
In-Depth Information
Too strong
intact failu
(by defini
Too strong for
intact failure
(by definition)
SOIL
SOIL
Failure through
Failure through
ROCK
ROCK
'intact'
material
'intact'
material
Failure fo
pre-existin
joints
Failure follows
pre-existing
joints
Potential slip surface with
lowest, calculated 'Factor of
Safety' against failure
Potential slip surface with
lowest, calculated 'Factor of
Safety' against failure
'Soil' vs. 'Rock' slop assessment: different requirements for
investigation, testing and analysis
Figure 1.3 Distinction between soil and rock at a pragmatic level for slope stability analysis. Soil
FoS for the given strength pro
le. The rock slope is in a limestone quarry, UK, and failure is totally
controlled by pre-existing geological structure (bedding planes and joints).
The compartmentalisation of soil and rock mechanics is quite
distinct in geotechnics, with separate international societies, which
have their own memberships, their own publications and organise
their own conferences. Details and links are given in Appendix
A. Textbooks deal with soil mechanics or rock mechanics but not
the two together. In reality, this is a false distinction and an
unsatisfactory situation. Engineering geologists and geotechnical
engineers need to appreciate that in nature there is a continuum
from soil to rock and from rock to soil. Soil deposited as soft
sediment in an estuary or offshore in a subsiding basin is gradu-
ally buried and becomes stronger as it is compressed by the weight
of the overlying sediment, and strong bonds are formed by cemen-
tation, as illustrated in
Figure 1.4.
Conversely, igneous rock such
as granite is strong in its fresh state but can be severely weakened
by weathering to a soil-like condition, as illustrated in
Figure 1.5,
so that it might disintegrate on soaking and even
flow into exca-
vations below the water table.
An engineering geologist must be familiar with the full range
of geological materials and understand the principles and methods of
