1185
Technical Committee 106 /
Comité technique 106
3 RESULTS
3.1
Engineering Properties
Samples from three different depths below 60 meters were
cycled through wetting and drying. Table 1 shows the index
properties for the cycles. The material at the 63.0 and 70.3-
meter depths consisted of approximately 2% sand, 50% silt, and
48% clay sized particles. The 63-meter samples exhibited a
substantial increase in the liquid limit, as the material was
wetted and dried. Minor increases in the liquid limit were
observed in the 63.6-meter sample. The 70.3-meter sample
showed insignificant changes in the Atterberg limits. The
plastic limit increased by ten and five percent for the 63.0 and
63.6 meter cycles. The plastic limit remained almost unchanged
for the 70.3 meter depth.
Table 1. Weathering cycle Atterberg limits.
Depth
(M)
Depth
(Ft)
Weath-
ering
Cycle
Liquid
Limit
(%)
Plastic
Limit
(%)
Plasticity
Limit
(%)
63.0
208
0
79
33
46
63.0
208
1
85
35
50
63.0
208
2
88
36
53
63.0
208
3
100
40
60
63.0
208
4
113
40
73
63.0
208
5
125
43
82
63.6
210
0
63
32
31
63.6
210
1
78
34
44
63.6
210
3
79
35
44
63.6
210
5
86
36
50
70.3
232
0
72
31
41
70.3
232
1
72
30
41
70.3
232
2
72
31
41
70.3
232
3
74
32
42
70.3
232
4
68
29
39
70.3
232
5
71
31
40
The residual friction angle for each cycle for the three depths
was determined from ring-shear tests. The formulated residual
strength-normal stress plot for each sample was constructed
using a spreadsheet program and analyzed for both the trend
line through the origin and with a cohesion intercept. The
residual friction angle (
′
r
) and residual cohesion are
summarized in Table 2. The residual friction angle dropped
almost 1.5 degrees for the 63.0 meter depth, nearly one degree
for the 63.6 meter sample, and 1.3 degrees for the 70.3 meter
depth during the wet/dry cycles. An unexpected increase in
strength was observed in the later stages of the 70.3 meter
cycles. Similarities in the 63.6 and 70.3 meter residual friction
angle plots are apparent for the first two cycles.
3.2
SEM Images and Visual Observations
The unweathered material below 60 meters showed dense, high-
laminae fabric. The three depths analyzed had similar fabrics.
Various other fabrics and particle shapes were observed over
the depths including burrows of pyrite framboids and cemented
calcite accretions. Weathering created a more homogenous
fabric. Weathering led to a more open fabric.
Visual observations of yellow, sulfuric, sand particles scattered
over the sample between the laminae were made. Fabrics with
large amounts of yellow particles showed a significant amount
of degradation after one to two wet/dry cycles as well. After the
third cycle, the material turned into a soil-like material. As the
material dried, large cracks were observed, an indicator of the
shrink/swell potential of the clay minerals in the material.
Table 2. Wet/dry cycle residual friction angles
Depth
(M)
Depth
(ft)
Weather
-ing
cycle
r
´
()
c
r
´
(kPa)
r
´ for
c´=0
()
63.0
208
0
6.8
0.7
6.9
63.0
208
1
6.5
~0
6.5
63.0
208
2
6.5
~0
6.3
63.0
208
3
6.0
0.5
6.1
63.0
208
4
5.6
0.1
5.6
63.0
208
5
5.4
~0
5.3
63.6
210
0
6.1
0.2
6.1
63.6
210
1
5.7
0.8
5.9
63.6
210
3
5.4
1.1
5.6
63.6
210
5
5.2
1.3
5.5
70.3
232
0
6.7
0.2
6.7
70.3
232
1
6.4
0.2
6.4
70.3
232
2
6.3
0.3
6.4
70.3
232
3
5.4
1.2
5.7
70.3
232
4
6.3
1.9
6.6
70.3
232
5
6.6
1.0
6.8
Following each weathering cycle, the sample was fractionated
in a sedimentation cylinder to obtain the clay fraction for XRD
analyses. Observations of the time rate of settlement of the
particles during this process provide insight into the soils’
behavior. Settlement analyses showed a contrast in the
suspension time for wet/dry samples. The fine fraction of the
wet/dry cycled samples stayed in suspension for many weeks
longer than the un-cycled samples. A sample after five wet/dry
cycles continued to be in suspension after three months.
The elemental composition of the materials was inferred
from elemental maps of the EDS analyses. Elemental maps of
the unweathered Pierre Shale showed K- and Na-ions are
dispersed over the material, a possible indicator of the adsorbed
cation on the montmorillonite mineral. The material has high
aluminum, silicon, and oxygen suggesting clay minerals. The
particles with high reflection were determined to be pyrite due
to the occurrence of iron and sulfur in the bright particles.
3.3
Mineralogy and Chemistry
Bulk mineralogy was determined from powder samples. Sand,
silt, clay, and very fine clay fractions were divided to determine
the mineralogy of the fractions. Sand and silt mineralogy was
determined by random-mount samples and clay mineralogy was
found from oriented-mount samples. Bulk mineralogy indicated
quartz (SiO2), gypsum (CaSO4
2H2O), and pyrite (FeS2). The
mineralogy was similar over the depths except for higher
concentrations of gypsum at the 70.3 m depth.
The sand fraction was observed to be highly heterogeneous,
with a significant amount of quartz. Large calcite accretions
were observed visually in the bulk sample and were found in the
sand fraction mineralogy. These observations were verified by
EDS analyses on SEM specimens. Pyrite, gypsum, and
bassanite (CaSO4
1/2H2O) were also observed in the sand
fraction. Gypsum is a hydrated form of bassanite. Quartz and
pyrite were found in the silt fraction with minor amounts of
feldspars including orthoclase (KAlSi3O8) and albite
(NaAlSi3O8).
The clay minerals were primarily mixed-layer clays as well
as minor amounts of illite clay mineral. Shultz (1978) provides
an outline for the determination of mixed-layer clay minerals in
Pierre Shale. The 63.0 meter depth showed a mixed-layer clay
mineral with montmorillonite, illite, and beidellite
concentrations of 20, 45, and 35 percent, respectively. The 70.3
meter depth had a mixed-layer clay mineral closer to bentonite
with montmorillonite, illite, and beidellite concentrations of 60,
