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

7 Geotechnical database — results of analysis
7.1 GENERAL
The geotechnical properties of Mercia Mudstone, as collected
for the database, are described in this section. Geotechnical
data are derived in the main from routine laboratory testing
using either British Standards (British Standards Institution,
1990), American Standards (ASTM) or recommended international
procedures (e.g. International Society for Rock
Mechanics, 1988). In addition, a small number of tests have
been carried out in the British Geological Survey laboratories
and are referred to in the text.
In general, research data are not included unless stated
otherwise. Geotechnical tests on soils and rocks may be
broadly subdivided into ‘index’ and ‘mechanical’ property
tests. The term ‘index’ implies a simple, rapid test, the
equipment and procedure for which are recognised internationally
(e.g. point load index) and which can be repeated in
any laboratory; or a test which measures a fundamental
physical property of the material (e.g. density). A mechanical
property test may be more complex, time consuming, and
measure a particular behaviour of the material under certain
imposed conditions (e.g. a triaxial strength test). If these conditions
are changed the result of the test will be different.
Equipment and methods for these tests tend to vary internationally,
and note should be taken of the test methodology,
particularly where no standard exists. Mechanical property
tests tend to require carefully prepared specimens. Index tests
tend to be used to characterise a formation and to plan further
testing, whereas mechanical property tests may be used for
design calculations. For mechanical properties where little or
no data are available (e.g. swelling, shrinkage, durability),
index tests are often used as a guide if correlations have been
established elsewhere. In some cases, however, such correlations
may not be appropriate.
Geotechnical tests may also be subdivided into ‘soil’ and
‘rock’ tests. This distinction is solely a function of the
equipment used and the method of specimen preparation.
Larger stresses are generally required when determining
mechanical properties of rock, compared with soil. The
Mercia Mudstone is a material that spans the interface
between ‘soil’ and ‘rock’ by geotechnical definition, and as
such may have both ‘soil’ and ‘rock’ type tests applied to
it, depending on its condition and weathering state.
Mechanical property tests on rock usually require
machined specimens. If the material is incapable of being
machined the test cannot be carried out. Test data in the
database may be biased towards strong material in the
case of the ‘rock’ tests and weak material in the case of the
‘soil’ tests, purely as the result of the need to match the test
method to the sample. Test data in the database consist
mainly of clay and mudstone lithologies, rather than
sandstone and evaporite. An assessment of the general
geotechnical properties of Permo-Triassic anhydrite and
gypsum is found in Bell (1994).
7.2 DENSITY AND MOISTURE CONTENT
The database contains few values for bulk density (452).
The density and natural moisture content of the Mercia
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Mudstone are affected by cementation, aggregation, weathering
and other lithological changes with depth.
Considerable variations on a scale of metres and millimetres
are seen. These changes also affect strength. Bulk
density ( b or BD), or wet unit weight, for undisturbed
samples is recorded in units of Mg/m 3 . Values of bulk
density range from 1.47 to 2.47 Mg/m 3 . The trend of bulk
density with increased weathering is negative in the case of
Areas 4 and 5 (that is, bulk density reduces with increased
weathering) but indeterminate in other areas. The median
value of bulk density for Area 4 (2.27 Mg/m3) is notably
higher than for other areas. Individually, each of Area 4’s
weathering zones is high except for 4a. Overall median
values for the areas lie between 1.96 and 2.27 Mg/m 3 . The
highest values were obtained for Area 4, Weathering Zone
1/2, at 2.47 Mg/m 3 . The lowest value was obtained for
Area 2, Weathering Zone 4a at 1.47 Mg/m 3 .
The dry density, or dry unit weight, d is the density of
the oven-dried soil, i.e. with no ‘free’ water contained in
the voids. Dry densities in the database comprise 536
values representing Areas 1, 2, 3, 8, and 10, and for which
the overall median dry density is 1.68 Mg/m 3 . Areas 1, 2,
and 3 have medians close to the overall median. In Area 8
the Edwalton Formation has a distinctly low median dry
density and the Sneinton Formation a distinctly high
median dry density.
There are 3429 natural moisture contents in the database.
The overall natural moisture content medians for Areas 1
to 11 are within the range 18 to 20% with the exceptions of
Area 5 (23%), Area 9 (26%), and Area 10 (12%). The
Edwalton Formation (Area 8) has a high median value of
25%. This compares with 17% common to other formations
in Area 8. The full range of values is very large, as
might be expected. However, some very high values (w >
w L) are possibly erroneous. These may have been obtained
from poorly executed or flooded cable percussion
boreholes or from highly reworked material. These result
in unfeasibly high values for liquidity index (section 7.4).
Particle density is poorly represented in the database.
Area 2 gives an overall particle density median of 2.69
Mg/m 3 . The Cropwell Formation (Area 8) gives a median
value of 2.74 Mg/m 3 . Maximum and minimum values are
2.86 and 2.42 Mg/m 3 , respectively. The particle density of
gypsum is 2.32 Mg/m 3 .
Under SEM examination Zone 4 Mercia Mudstone
samples from Gringley-on-the-Hill (Area 8) were found to
have more porous, less compact textures than Zone 2
material from Cropwell Bishop (Area 8). Deformation
structures are observed in the vicinity of gypsum bodies
(Pearce et al., 1996). Meigh (1976) notes that total
carbonate content may have an important effect on density,
and hence on mechanical properties.
7.3 PARTICLE SIZE
Particle size data for soils are usually obtained by
combining the results of sieving and sedimentation
analyses (British Standards Institution, Part 2, 1990; Head,
1992). The coarse fraction (>0.060 mm) is determined by