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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
15, and 20 percent, respectively. Data and diffractograms for
the mixed-layer clay determinations are shown in Birchmier
(2005). These values fall within the data compiled by Schultz
(1978) for mixed-layer clay minerals in Pierre Shale. Overlays
of diffractograms of bulk samples for the wet/dry cycles of the
70.3 meter depth showed little change over the weathering
cycles except for the reduction of gypsum particle size during
the initial cycles. The combination of the decrease in the
gypsum peak intensity at 7.63 Å, and the constant 4.28 Å and
3.8 Å peak intensities during cycling suggests the breakdown of
the gypsum particles.
4 DISCUSSION
Residual friction angles for the wet/dry cycles decreased from
6.8° to 5.4°, 6.1° to 5.2°, and 6.7° to 5.4° for the depths 63.0,
63.6, and 70.3 m, respectively. A one degree drop in a material
with an initial residual friction angle of 6.5° is significant. For a
given normal stress and negligible residual cohesion, the factor
of safety would be reduced by a ratio of 1.2. Exposure and
removal of confining stresses during construction activities in
Pierre Shale could cause wetting and drying to occur and lead to
slope failures.
The residual friction angle for each cycle for the three depths
was determined from ring-shear tests. 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.
Residual friction angles for the wet/dry cycles decreased from
6.8° to 5.4°, 6.1° to 5.2°, and 6.7° to 5.4° for the depths tested.
A degree drop in a material with an initial residual friction angle
of 6.5° is significant. For a given normal stress and negligible
residual cohesion, the factor of safety would be reduced by a
ratio of 1.2. Exposure and removal of confining stresses during
construction activities in Pierre Shale could cause wetting and
drying to occur and lead to slope failures.
5 CONCLUSIONS
Minor mineralogical changes were observed in the wet/dry
cycles similar to the weathering occurrences in Pierre Shale.
Gypsum concentrations decreased initially in the wet/dry cycles.
The low residual friction angles of 6.1° to 6.8° decreased an
additional 0.8° to 1.4° during the wet/dry cycles. A significant
fabric contrast was apparent after three cycles as the material’s
structure became more massive. The most noticeable difference
in the cycles was the particle settlement rates. Excessive
cycling caused particles to stay up in suspension for weeks to
months longer than the un-cycled material. This observation
indicates clay aggregates are becoming smaller and going
towards their unit-cell size. The reduction in size increased the
clay fraction, contributing to the residual strength decrease.
The mechanical behavior varied for the samples analyzed.
The material with larger amounts of illite in the mixed-layer
clay mineral showed a decrease in residual strength following
Stark and Eid's (1994) curves. The material with higher
amounts of montmorillonite in the mixed-layer clay mineral
showed little change in the liquid limits, a contrast to the other
sample. A decrease in residual strength was observed for the
first two cycles but increased thereafter. The contrasting
behavior shows the heterogeneity of the material and the
difficulties in determining design parameters. The
mineralogical changes and the disintegration of aggregates
during wetting and drying are concluded to be more influential
than physico-chemical effects.
6 ACKNOWLEDGEMENTS
The authors thank the engineers of the U.S. Army Corps of
Engineers Project Office in Pierre, SD for their help in
obtaining the samples of Pierre Shale. Ashley Schwaller,
undergraduate student at Iowa State University, is thanked for
conducting the XRD tests. This material is based on work
supported by the National Science Foundation under Grant Nos.
CMS-0201482 and CMS-0227874. This support is gratefully
acknowledged. Any opinions, findings and conclusions or
recommendations expressed in this material are those of the
authors and do not necessarily reflect the views of the National
Science Foundation.
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