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

Percentage passing
son of particle size gradings from one area to another is
hampered by a shortage of data in many areas. The
general rule seems to be that the greater the number of
data, the greater its scatter. For example, the scatters for
Areas 1 and 2 data are as large as for all data combined.
However, Area 1 does tend to be finer overall than Area
2. This is also reflected in the higher plasticity results for
Area 1 (section 7.4). Other areas tend to be intermediate
between Areas 1 and 2 in terms of grading.
It has been clearly demonstrated in the database, and in
the literature (Chandler et al., 1968), that a large proportion
of particle size data are misleading; dispersion (or disaggregation)
during sample preparation may have been inadequate
and the fine fractions reported are lower than they
should be. Davies (1967) defined the aggregation ratio, A r
as follows:
A r =
100
90
80
70
60
50
40
30
20
10
0
0.001 0.01 0.1 1 10 100
LKM LKSB MKM Particle size (mm)
Figure 7.2 Particle size grading curves (Area 1, subdivided by formation).
% clay mineral
% clay size fraction
The % clay mineral content is determined
from mineralogical analysis,
whereas the % clay fraction
(particles < 0.002 mm) derives from
particle size analysis. Both percentages
are expressed as weight proportions
of the whole sample.
Sherwood and Hollis (1966),
Dumbleton and West (1966a, b),
Davis (1967), and Chandler et al.
(1968) gave values for A r of
between 1.39 and 9.35. The implication
of these figures is that either the
clay minerals exist as particles
larger than 0.002 mm, or that the
particle size fraction is incorrect.
Chandler et al. (1968) gave a
positive correlation between aggregation
ratio and activity. Clay size
fraction was also introduced in
Skempton’s definition of Activity
Percentage passing
100
90
80
70
60
50
40
30
20
10
36
(Skempton, 1953). This is discussed
in section 7.4.
The problem appears to stem in
part from the inadequacy of the
British Standard (British Standards
Institution, 1990) mixing method and
its duration with respect to the Mercia
Mudstone. This was touched on by
Birch (1966). Experiments carried out
at the BGS have shown that extended
periods (2 to 24 hours) of disaggregation
using the normal chemicals, as
specified in BS1377, in shaking
flasks has resulted in the ‘correct’
gradings. Consistent ‘particle size
analysis’ grading plots with clay-sizes
(%