Geology Reference
In-Depth Information
or in a rather more controlled way using a hand penetrometer, as
discussed in
Chapter 4.
Undrained strength is useful for assessing the
fundamental behaviour of clay empirically, for example, in designing
foundations
(Table 6.1
). It is also used for numerical analysis in soils of
low permeability immediately after or during construction. Conversely
drained conditions apply where excess pore pressures have dissipated
following construction or where they dissipate relatively rapidly during
construction. For design of structures in clay under drained conditions,
effective stress parameters are required
friction and possibly some
cohesion where there has been some geological bonding. These para-
meters are generally obtained from triaxial testing, in which pore pres-
sures are monitored and corrected for throughout the test (e.g. Craig,
1992). Effective stress parameters can also be interpreted from in situ
piezocone penetrometer soundings (
Chapter 4).
Laboratory tests are relied upon for characterising natural clay far
more than for any other soils, because reasonably undisturbed samples
can be taken and the small grain size relative to testing apparatus means
that scale effects are not evident. An exception is in settlement analysis,
where it is found that standard oedometer tests give lower stiffness than
larger-scale plate load tests or are evident from back analysis of the
construction of a structure. Specialised testing is necessary to simulate
low strain deformation (e.g. Atkinson, 2000).
As noted earlier, for some active and ancient landslides, the strength
along the slip plane through clay/mudstone is reduced below the
critical state friction angle to a residual friction angle well below
20 degrees, even for clay of relatively low plasticity such as kaolinite
or illite (Skempton et al., 1989). Such low values can be measured in
the laboratory using ring shear boxes and back-analysed from land-
slide case histories.
Clays include some groups of very problematical soils. Quick clays
are clay and silt size but mostly detrital materials (rock
-
our produced
by glacial scour), weakly cemented by salt, which can become dis-
turbed and then
flow, sometimes to disastrous effect. The Rissa,
Norway, landslide in 1978 was
at
ground, indicating the sensitivity of such materials. Other clays such
as black cotton soils swell and shrink dramatically with changes in
moisture, which causes damage to roads and other structures. The clay
mineral group smectite (montmorillonite/bentonite) is most commonly
associated with volume change and is typically identi
filmed,
flowing rapidly across
ed by X-ray
testing. Its presence is also indicated from high liquid limits and high
plasticity indices in Atterberg limit tests (
Chapter 3).
These clay miner-
als can have very low shear strengths. Starr et al. (2010) describe a
creeping major rock slope failure where the rock is smectite-rich and
for which the operating residual friction angle was only about 7
degrees as established by numerical back-analysis and con
rmed
from laboratory tests.
