Geology Reference
In-Depth Information
decomposed stage, the dry density may be reduced by more than
50% if weathering products have been washed out. The
final stage is
collapse to residual soil and an increase in density. Weathering is
discussed in detail in Chapter 3.
Geotechnical properties at the material scale are linked quite closely
to density empirically and, therefore, degree of deterioration from the
rock
s fresh state. Fresh granite might have a uniaxial compressive
strength of perhaps 200 MPa but by the time the rock is highly
decomposed the strength is reduced to 10
'
-
15 MPa and when comple-
tely decomposed perhaps 10
15 kPa. Where the rock is relatively
strong, then properties and behaviour will be dominated by contained
fractures; for most projects, the point at which material strength begins
to dominate design decisions is where the rock can be broken by hand.
At the mass scale in weathered pro
-
les, strength and deformation
might be affected by the presence of strong corestones of less weath-
ered rock in a weakened matrix, and the problem of characterisation is
similar to that of mixed soils and rock such as boulder clay or boulder
landslide colluvium, as discussed later.
Permeability in fractured rock or in weathered pro
les can be extre-
mely variable and dif
ow
providing high permeability. Elsewhere, accumulations of clay or gen-
eral heterogeneity in the pro
cult to predict, with localised channel
le can prevent and divert water
ow. The
complexities of
ow through weathered rock pro
les and dif
culties in
measuring permeability are discussed in Chapters 3 and 4.
5.3.5.2 Diagenesis and lithi
cation (formation of rock from soil)
As discussed in Chapter 3, soil is transported by water, wind or gravity
from the parent rock. During the process of transportation, the sediment
is sorted in size. Some soils such as glacial moraine and colluvium
remain relatively unsorted. Sediments tend to be continually deposited
over a very long period of time, for example, in river estuaries, and each
layer of sediment overlies and buries the earlier sediment. The under-
lying sediment is compacted and water squeezed out. This is termed
burial consolidation and is a very important process governing the
strength and deformability of sediments. Grains become better packed,
deformed and may form strong chemical bonds with interpenetration
and sutured margins. Voids may be in
lled with cement precipitated
from soluble grains in the sediment (authigenic cement) or from solu-
tions passing through the sediment pile, as illustrated in Figures 5.7
and 5.8. Many clay oozes initially have a very high percentage of voids,
with the mineral grains arranged like a house of cards. With time,
overburden stress and chemical changes cause the
flaky minerals to
align and the porosity (or void ratio) to decrease markedly, as illustrated
in Figure 5.9. Burland (1990) has expressed the rate at which void ratio
is reduced with burial depth as a normalised equation although there are
 
 
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