Geology Reference
In-Depth Information
filling pore spaces and from inter-granular bonds such as those
formed by pressure solution (Tada & Siever, 1989). The strength
and deformability of soil is also a function of the closeness of
packing of the mineral grains. Densely packed soil will be forced
to dilate (open up) during shear at relatively low con
ning stresses as
the grains override one another and deform, and the work done
against dilation provides additional strength. The same principles
apply to rough rock joints or fractured rock masses. Different miner-
als may also have fundamentally different properties
some are
more chemically reactive and may form strong chemical bonds in
the short term, some are readily crushed or scratched, whilst others
are highly resistant to damage or chemical attack. Some, such as talc
and chlorite, are decidedly slippery and if present on rock joints can
result in instability.
The huge range of properties in soil and rock and how these
evolve with time is illustrated by a single sample in
Figure 5.1.
The
left-hand picture shows a graded series of sediments. The sand
horizons become
-
finer upwards, as is typical of sediments deposited
from a river into a lake. At the top of the sample, there is a second
sand horizon that has been deposited onto the underlying sediment.
This has deformed the underlying sediments, producing a loading
structure, which shows that the soil was in a very soft state at the
time of formation. Contrast this with the rear of the same sample
showing conchoidal fractures in what is actually extremely strong
rock. The conversion from soft mud to rock has occurred over a
long time but has occurred naturally and, in practical geotechnical
engineering, we encounter and need to deal with the full range of
materials, transitional between these end members.
a)
b)
Figure 5.1
(a) Graded,
probably seasonal
bedding with clear
evidence of soft
sediment
deformation. (b)
Rear of the same
sample with
conchoidal
the strength of this
rock (probably of
the order of
300 MPa).
