Geology Reference
In-Depth Information
In some soil such as alluvial sand, the material permeability could be
similar to that of the mass, so laboratory testing might be relevant, but
for many ground pro
les water
flow will be localised and involve
natural pipes,
fissures and open joints or faults. Field tests are then
generally necessary to measure mass-scale permeability, as outlined in
Chapter 4. Large-scale pumping tests from wells with observational
boreholes at various distances can give reliable parameters for aquifer
behaviour but localised testing in boreholes, as speci
ed in BS 5930
(BSI, 1999), can be unreliable (Black, 2010). As discussed in
Chapters 3 and 6 and illustrated in examples in Chapter 7, localised
geological features often control
flow through the soil or rock
mass, so testing must be linked to relevant geological and hydrogeo-
logical models.
fluid
5.5 Soil properties
5.5.1 Clay soils
As Skempton (1970) showed ( Figure 5.9), for clay soil deposited off-
shore at rates of perhaps 2m per thousand years, consolidation beha-
viour due to self-weight is fairly well de
ned. As the porosity diminishes
and water is squeezed out, so strength increases and deformability
reduces, even in the absence of other diagenetic processes. Hawkins
et al. (1989), for example, show a consistent linear increase in shear
strength with depth over 20m at a test site in Bothkennar, Scotland,
based on vane tests. Cone test data from the same site are very similar to
other sites in the UK, con
rming the trend. Similar results have been
achieved from other sites worldwide, with a typical relationship:
s u ¼ 10 þ 2:0d
Where s u = undrained shear strength, kPa and d = depth below ground, m.
Elsewhere, values can be somewhat lower; for the Busan Clay in
Korea, the gradient is closer to 1.0 times depth (Chung et al., 2007).
Nevertheless the trend is similar so for design in soft to
firm clay it is
usual practice to carry out a series of vane tests down boreholes or cone
penetrometer soundings, and then try to de
ne a relationship of increas-
ing strength with depth that can easily be input to numerical simula-
tions. Relationships are published both for shear strength and modulus
of clay interpreted from SPT tests, and these are reviewed in Clayton
(1995) although the SPT is less appropriate for clay than for granular
soils. Most of the values obtained from
eldtestsarenecessarily
undrained and expressed as a value of apparent cohesion with no
frictional component. Undrained shear strength of clay can also be
obtained from undrained tests in the laboratory and is estimated
during
eld description using index tests like resistance to
nger pressure
 
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