1183
Mechanisms of Strength Loss during Wetting and Drying of Pierre Shale
Mécanismes de la perte de force pendant
h
umidification et séchage de
Pierre Shale
Schaefer V.R.
Iowa State University, Ames, Iowa USA
Birchmier M.A.
Soil Nail Launcher, Inc., Grand Junction, Colorado USA
ABSTRACT: The physio-chemical and morphological role on the residual strength of Pierre Shale during wetting and drying cycles
was investigated. Ring-shear tests were conducted on intact and cycled material to assess the residual strength. The mineralogy and
chemistry were determined from x-ray diffraction and x-ray fluorescence results. Minor mineralogical changes were observed during
the cycling process. The material degraded from a firm, dense shale to a massive, clayey material after three to four cycles. Gypsum
concentrations decreased during the wet-dry cycles. The low residual friction angles of 6.1 to 6.8 degrees decreased an additional 0.8
to 1.4 degrees during the wet-dry cycles. A significant fabric contrast was apparent after three cycles as the material’s structure
became more massive. The material with higher amounts of montmorillonite in the mixed-layer clay mineral showed little change in
the liquid limits with cycling, in contrast to the illite materials. A decrease in residual strength was observed for the first two wet-dry
cycles, but little change for successive wet-dry cycles. The results suggest the disintegration of particles during slaking is the main
determinate of strength loss. The initial mineralogy was also observed to be a factor on the slaking rate and the residual strength
behavior.
RÉSUMÉ : Le rôle physio-chimique et morphologique sur la résistance résiduelle de
Pierre Shale
au cours des cycles de mouillage et
de séchage a été étudié . Les essais de cisaillement annulaires ont été effectués sur ce matériau intact et recyclé pour évaluer sa
résistance résiduelle. La minéralogie et la chimie ont été étudiées à partir de diffraction des rayons X et de rayons X de fluorescence.
Des modifications minéralogiques mineures ont été observées au cours des cycles hydriques. . Le schiste dense a subi une
dégradation après 3 à 4 cycles et a été transformé en une argile. La concentration en gypse a également été diminuée. Les faibles
angles de frottement interne résiduels de 6,1 à 6,8 degrés ont diminué d’environ 0,8 à 1,4 degrés supplémentaires au cours des cycles
de mouillage-séchage. Une modification importante de la structure interne et une densification ont été notées après trois cycles. Les
limites de liquidité du matériau avec un pourcentage élevé de montmorillonite n’ont pas été modifiées contrairement au matériau
contenant de l’illite. Une diminution de la résistance résiduelle a été observée pour les deux premiers cycles hydriques, mais peu de
changement a été observé pour des cycles ultérieurs de mouillage-séchage. Les résultats suggèrent que la désintégration des particules
pendant l’humidification est la principale cause de perte de résistance. La minéralogie initiale a également été considérée comme un
facteur important influençant cette désintégration et la résistance résiduelle.
KEYWORDS: Shale, weathering, strength, clay mineralogy, residual friction angle
1 INTRODUCTION
The tendency for clay shales to weather, soften, and slake upon
drying and rewetting has been well documented. The
degradation causes the material to soften and lose strength,
possibly leading to slope failures. Skempton (1964) noted
strength losses of up to 80% in some deposits after softening.
The slaking rate has been observed to be dependent on the
mineralogy and physico-chemical behavior, especially in
materials with high activity clay minerals (Perry and Andrews
1984). While the mineralogy mechanisms are well known, very
few studies on clay shales have been conducted to analyze the
role of physico-chemical occurrences on the strength loss.
Weathering in overconsolidated clays and clay shales has
been observed to be a significant process due to the mode of
deposition and the bonding from diagenesis, especially in
outcroppings. One of the largest and most problematic clay
shales in the United States is the Pierre Shale Formation
(Fleming
et al
. 1970). The drop in strength from the peak to
residual strength is a source of the stability problems associated
with these materials. Initial fissuring results from rebound in
clay shales and leads to clay swelling, strain softening and
weathering (Brooker and Peck 1993). Much of the past work on
residual strength has focused on the mechanical aspects with
less emphasis on the role of the more dynamic, physico-
chemical effects on residual strength.
This paper reports the results of tests conducted to relate the
strength of a series of laboratory wetting and drying cycles on
unweathered Pierre Shale to its chemistry, mineralogy, and
micromorphology. The study analyzes the role of the initial
mineralogy, breakdown of particle size, and the overall changes
in the mineralogy.
1 MATERIALS AND METHODS
1.1
Materials
As described by Bjerrum (1967), the behavior of
overconsolidated materials is strongly correlated to their
geologic history. Pierre Shale is a heavily overconsolidated
clay shale formed from a marine/non-marine environment
sedimentation during the Cretaceous Period approximately 60 to
80 million years ago (Fleming
et al.
1970). The formation
extends throughout Canada and as far south as the Gulf of
Mexico. Significant slope failures have been observed
throughout the formation, but are mainly focused in the upper
Missouri and South Saskatchewan River basins. The
