3104
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
greater depths than would be anticipated in the present climate.
Commonly the brown LCF extends to some 6-8 m depth and in
rare cases, as recorded by Chandler (2000), to 15 m. Between
the saturated (grey) and aerated/oxidized (brown) material there
is a mottled grey/brown zone. In the upper brown LCF much of
the pyrite has decomposed. In the mottled zone the pyrite
experienced a wetting/drying environment such that the iron
sulphides were oxidized to produce sulphuric acid and ferrous
sulphate (Eq. 1).
FeS
2
+ H
2
O
2FeSO
4
+ 2H
2
SO
4
(1)
Although in stronger, more calcareous materials the ferrous
sulphate may produce a significant initial expansion, in the
weaker LCF this is less apparent. However, the formation of
sulphuric not only changes the pH of the soil but reacts with
calcium carbonate to produce calcium sulphate (Eq. 2). Detailed
chemical reactions are given in Hawkins and Pinches (1987)
and Hawkins (2013).
H
2
SO
4
+ CaCO
3
+ H
2
O
CaSO
4
.2H
2
0 + CO
2
(2)
Gypsum can grow within the Clay in various forms including:
a) Euhedral, monoclinic prisms, often known as selenite
b) Stellate (star shaped) or “rosette”-like, crystals (Figure 3)
c) Void infill, with irregular shapes
d) Apparently amorphous “sucrose” crystals
Figure 3. Stellate gypsum from the LCF at Camberwell (6-7 m depth).
The stellate form is typical of crystals which have developed
over relatively long periods of time. As such crystals are
common between 6 and 7 m depth, it is clear the pressure of
crystallisation must have exceeded 100 kPa.
Gypsum is often associated with sandy horizons in the LCF
(Bessey and Lea, 1953). The porous and relatively permeable
nature of these bands will encourage water movement in wet
periods and facilitate the ingress of oxygen during dry periods.
As a consequence, oxidation takes place in the adjacent Clay
and gypsum may be precipitated from the sulphate-rich solutes.
The development of gypsum on exposed clays can occur in
a matter of weeks. Grey LCF borehole arisings from a site
investigation in Camberwell left open to weathering showed
fine (< 2 mm) white crystals after only four weeks while in an
oven at 105 °C they formed within 1-2 days. Microscopic
examination confirmed that the crystals were gypsum.
4 CHEMICAL TESTING
Although from a geotechnical point of view the sub-division
between the grey Clay and the brown Clay is useful, rarely is
there an explanation highlighting the importance of the
chemistry of the mottled zone, which frequently contains
enriched acid soluble sulphate with a corresponding lower pH.
Hawkins (2013) reports results obtained from the
Camberwell site investigation in February 2012. Two boreholes
only 35 m apart were tested for total sulphur (TS), acid soluble
sulphate (AS) and water soluble sulphate (WS). Table 1 also
shows the calculated total potential sulphate (TPS), oxidisable
sulphides as sulphate (OS) and equivalent pyrite according to
the Canadian Standard CTQ-M200 (Comité Technique, 2001).
Table 1. Sulphur chemistry for Camberwell boreholes, February 2012.
Borehole 1
Depth
(m)
AS (%
SO
4
)
WS
(mg/l
SO
4
)
TS
(% S)
TPS
(%
SO
4
)
OS
(%
SO
4
)
Eqv.
Pyr.
(%)
2.6
0.02
118 0.01
0.03
0.01
0.01
3.6
0.02
10 0.01
0.03
0.01
0.02
4.4
0.33
116 0.11
0.33
0.00
0.20
5.5
0.91
10 0.38
1.14
0.23
0.71
6.4
0.21
906 0.21
0.63
0.42
0.34
7.6
0.25
10 0.54
1.62
1.37
1.01
8.5
0.28
685 0.48
1.44
1.16
0.85
10.6
0.24
501 0.50
1.50
1.26
0.90
12.5
0.26
505 0.56
1.68
1.42
1.02
14.6
0.24
621 1.53
4.59
4.35
2.82
16.6
0.23
405 0.67
2.01
1.78
1.23
17.4
0.17
366 0.57
1.71
1.54
1.04
Borehole 2
Depth
(m)
AS (%
SO
4
)
WS
(mg/l
SO
4
)
TS
(% S)
TPS
(%
SO
4
)
OS
(%
SO
4
)
Eqv.
Pyr.
(%)
3.1
0.05
110 0.05
0.15
0.10
0.09
4.1
4.94
10 2.40
7.20
2.26
4.49
5.1
1.12
1850 0.38
1.14
0.02
0.60
6.0
0.62
1410 0.15
0.45
-0.17
0.19
7.1
0.73
1550 0.70
2.10
1.37
1.21
8.0
0.28
715 0.61
1.83
1.55
1.10
9.1
0.31
1050 0.63
1.89
1.58
1.11
10.1
0.31
898 0.63
1.89
1.58
1.12
12.0
0.28
970 0.44
1.32
1.04
0.76
14.0
0.34
10 1.88
5.64
5.30
3.51
16.0
0.23
10 1.19
3.57
3.34
2.22
18.0
0.20
10 0.52
1.56
1.36
0.97
18.9
0.14
10 0.45
1.35
1.21
0.84
At the Camberwell site, the brown-grey junction is at
approximately 6-7 m depth while the groundwater level is at 1-2
m depth. To a depth of around 3 m the LCF is depleted of
sulphates due to leaching, while a number of elevated values
were recorded from between 4 and 8 m. Below 8 m the AS
content is relatively uniform at 0.25-0.3 % SO
4
. The TS in both
boreholes attains a high value of between 1.5 to 2 % S at around
14 m depth, approaching the base of the LCF. The sulphur at
these depths is likely to be related to pyrite nodules. With the
exception of 4 m depth in BH 2, the equivalent pyrite is highest
towards the base of the boreholes. Several pyrite nodules from
this depth were noted in nearby borehole cores. Despite being
only 35 m apart, there are stark differences between the two
boreholes. For example, at 4 m depth, the AS is 0.3 % and the
TS almost 0.1 % in BH 1 compared with an AS of almost 5 %
and a TS of 2.4 % in BH 2.
The high SO
4
values at 4 m depth in BH 2 are probably due
to the presence of coarse gypsum in the sample. As the sample
mass used in these tests is less than 5 g, it is quite possible for a
crushed gypsum crystal to occupy a large proportion of the
sample and hence contribute to anomalously high sulphur and
sulphate test results. This may in part be related to the presence
of two mature trees within < 3 m of BH 2 (Hawkins 2013).
As a consequence of this significant variation, in September
2012 a new set of samples was taken at 0.2 m intervals close to
the original BH 2. The sulphate and pH of these samples was
determined (Figure 4).
