Engineering geology of British rocks and soils Mudstones of the ...

emoulds by repeated forced extrusion of the soil paste) for
extended periods. This usually resulted in significant
changes in the value of liquid limit obtained. In the case of
one soil type (halloysitic) the liquid limit increased, and in
the other (allophanic) it decreased. The standard procedure
for the liquid limit test (British Standard Institution, 1990)
employs hand-mixing of the soil sample ‘for at least 10
minutes’ prior to the test. This level of hand-mixing is
adequate for most temperate climate soils, but becomes
unreliable for highly aggregated soils. The BS1377 Part 2,
Note 4.4.3.2 (British Standard Institution, 1990) says that
care should be exercised when testing ‘residual and highly
plastic soils’ and that these soils should be hand-mixed for
40 minutes, and that comparisons should be made between
the two preparation techniques. Haider (1989) applied
extended hand-mixing times (up to one hour) to Mercia
Mudstone and found that there was no change from the
standard ten minutes. He concluded that whilst the Mercia
Mudstone was an aggregated clay its aggregations were
broken down during the normal mixing time, unlike those
of some tropical clays. He attributed this to differences in
the cementing agent. The above means that the determination
of activity potentially becomes doubly unreliable, i.e.
from both the plasticity and the particle size term.
The plasticity of the Mercia Mudstone ranges from
‘low’to ‘high’ in the Casagrande classification, but with the
majority of data falling within the ‘low’ and ‘intermediate’
groups. Some silty Mercia Mudstone falls below the
Casagrande A-line. Arithmetic means for liquid and plastic
limit for the Coventry area were found to be 35% and 20%,
respectively (Old et al., 1989). Weathering tends to
increase the measured plasticity of the Mercia Mudstone
from ‘low’ to either ‘intermediate’ or ‘high’. Chandler
(1969) suggested a boundary between weathering Zones 3
and 4 at a liquid limit of 38%. However, this was not borne
out by Old et al. (1989).
Variations in natural moisture content within the Mercia
Mudstone may be wide. Chandler (1967) quoted a range of
22 to 25% for a Mercia Mudstone profile at Kings Norton,
Birmingham. Chandler et al. (1968) indicated that moisture
content has a particularly important influence on all
physical properties including geophysical, strength and
compaction. The concept of ‘critical degrees of saturation’
(points of inflection obtained from plots of resistivity vs.
saturation) (Chandler et al., 1968) is related to effective
grain size, hardness, compaction, or cementation.
4.5 CONSOLIDATION
Generally, the Mercia Mudstone is described as having low
compressibility and a high rate of consolidation, but is very
variable (Birch, 1966). The consolidation settlement of
unweathered Mercia Mudstone, under normal engineering
loads, has been described as ‘negligible’ (Birch, 1966). The
compressibility of the Mercia Mudstone, based on
oedometer consolidation data for the Coventry area, was
described as ‘very low’ to ‘medium’, and the rate of consolidation
as ‘low’ to ‘high’ (Old et al., 1989). There is
unlikely to be a clear relation between consolidation
behaviour and maximum previous overburden (pre-consolidation
stress) because the Mercia Mudstone has undergone
some form of diagenesis. However, Chandler (1967)
suggested that a likely ‘pre-consolidation load’ was equivalent
to an overburden of between 230 and 610 m (assuming
an overburden density of 1.12 Mg/m 3 ). Chandler (1967,
1969) described the Mercia Mudstone as ‘heavily overcon-
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solidated’. Standard laboratory consolidation testing
equipment does not provide adequate stress levels to characterise
fully the consolidation behaviour, including the
over-consolidation ratio, of hard, unweathered Mercia
Mudstone.
4.6 STRENGTH AND DEFORMABILITY
A considerable amount of work on the strength of Mercia
Mudstone was carried out during the mid-1960's in the
Midlands, particularly at Birmingham University. Chandler
(1967) and Chandler et al. (1968) described the results of
drained and undrained triaxial tests on specimens of ‘Kings
Norton marl’ obtained from block samples in a brick pit in
Birmingham, and discussed in general terms the geological
factors influencing the strength of the Mercia Mudstone.
Chandler (1967) saturated triaxial specimens by increasing
the back pressure to around 275 kPa. The Mercia Mudstone
is usually described as a heavily over-consolidated soil,
showing considerable dilation at low effective stresses.
Some stress-strain plots at low effective stresses appear to
indicate a distinct yield point at around 1% axial strain that
Chandler (1967) attributed to ‘structural breakdown’ and
‘structural rigidity’. The yield point, which is not universally
observed, coincides with the onset of dilation. This is
reflected in load vs. settlement results for pile tests
(Chandler et al., 1968). Results of tests carried out over a
range of effective confining stresses showed the convexupward
shape of the Mohr envelope; the effective friction
angle changing from 40.5° at low stresses to 20.9° at high
stresses. A ‘strength sensitivity’ of 4.25 was quoted.
Chandler (1967) attributed this to clay aggregations and
calcite crystals. An increase in cohesion and a decrease in
friction angle are noted with disaggregation (destructuring?)
associated with decreasing moisture content at
failure. In the Cheshire area the removal of gypsum in
solution has resulted in what Marsland and Powell (1990)
described as a broken fabric.
Strength, based on unconsolidated undrained (UU or
QU) triaxial tests, standard penetration tests, and rock penetration
tests, for the Mercia Mudstone in the Coventry
area, are described by Old et al. (1989). Hobbs et al. (1994)
obtained values for Mercia Mudstone near Gainsborough
for c’ and ø’ from isotropically consolidated, undrained
(CIU) triaxial tests of 14.7 kPa and 20.9°, respectively.
These results agreed with those of Chandler (1967). The
tendency to obtain unreliable results and positive values for
ø u in the UU triaxial test, due to partial saturation, was
highlighted by Chandler et al. (1968).
The rate of increase of strength with depth for the
Mercia Mudstone was quoted as 37.5 kPa/m (Cripps and
Taylor, 1981). This relatively high rate reflects the age and
diagenetic changes of the Mercia Mudstone. Results from
Marsland and Powell (1990) indicated an increase in
ultimate bearing pressure with depth, derived from pressuremeter
and plate bearing tests, of approximately 1.0
MPa/m for Mercia Mudstone near Warrington, Cheshire.
Weathering tends to reduce the shear strength of
mudrocks to a common value irrespective of lithostratigraphy
(Cripps and Taylor, 1981). Unweathered Mercia
Mudstone (Zone 1) tends to exhibit brittle failure at low
strains, whereas the weathered material (Zones 2 to 4)
tends to exhibit a more plastic failure with a lower elastic
modulus. The effect of weathering on strength and
deformability has been investigated by Chandler (1969).
Deformation behaviour, ranges from brittle for Zone 1