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Research on the Yazoo Clay
General Information
The Yazoo Formation of the
Tertiary Jackson Group (informally knows as the Yazoo Clay) is a
calcareous fossiliferous mudrock that outcrops in a northwest-southeast
belt across much of Mississippi. The Yazoo Clay is composed in large
part of expansive or “fat” clay minerals that have the
capability to increase in volume (swell) when saturated with water.
Building on the Yazoo Clay is problematic. Because of its expansive
nature the Yazoo Clay has been associated with cracked foundations,
cracked walls and ceilings, and “rollar coaster” roadways
in Holmes, Hinds, Madison, Rankin, Smith, Scott, Newton, Jasper,
Clarke, and Yazoo counties.
Objectives
The goal of this research
is to define the variability in the mineral composition of the Yazoo
Clay in order to better understand the physical properties, especially
swelling, of the rock. Much of the previous mineralogic work on
the Yazoo is over 30 years old. Since that time the understanding
of clay mineralogy and X-ray diffraction analysis (the primary method
of analysis of clays) has increased dramatically. The specific objectives
of this research are to develop a method of mineralogic analysis
specifically for the Yazoo Clay and to use this technique to investigate
the mineral content and physical property differences in Yazoo Clay
samples with depth and spatial separation. This research project
is being funded by the Mississippi Department of Transportation
(MDOT) and is the topic of Angela Taylor's M.S. thesis.
Approach
Task 1 in this project is
to identify and define the chemical and structural composition of
the minerals in the Yazoo Clay. Preliminary semi-quantitative analyses
of three unweathered samples indicate that the Yazoo is composed
of the clay minerals kaolinite, illite, and a swelling-clay, and
lesser amounts of quartz, calcite, and other phases. These preliminary
analyses also show that the swelling-clay present in the Yazoo is
not montmorillonite, as has long been accepted, but rather a more
complex “mixed-layer” mineral with unusual swelling
and structural properties. A specific objective in Task 1 is to
separate Yazoo samples into monomineralic isolates which can be
analyzed by X-ray diffraction (XRD) and chemical techniques. These
analyses will provide insight into the chemical (and other) control
on the swelling capability of the expansive clay in both weathered
and unweathered Yazoo. The samples used for this and all other tasks
in this project will be provided by MDOT.
Task 2 is the development
of an XRD-based quantitative mineralogic analysis procedure specific
to the Yazoo Clay. Incorporation of the chemical and mineralogic
analyses determined in Task 1 is critical to the development of
this procedure. The goal of Task 2 is to test and improve different
laboratory, data acquisition, and data analysis techniques in order
to identify those that work best on Yazoo Clay samples. In effect,
the semi-quantitative techniques employed in the preliminary analyses
will be “fine-tuned” to the Yazoo Clay in order to provide
the most accurate and precise mineralogic data possible. Mineralogic
data from Task 1 will be used to improve the quantitative techniques
developed in Task 2. Those techniques will then be used to better
define the mineralogic data obtained in Task 1. Because of this
fundamental and iterative relationship, Tasks 1 and 2 will be performed
concurrently.
Task 3 is to apply the techniques
developed in Task 2 to determine the spatial- and depth-related
mineralogic differences in the Yazoo Clay. The data obtained during
Tasks 1 and 2 will serve as the preliminary data for Task 3 and
will be used to define future areas of study and major sampling
sites. The mineralogic data obtained in all three tasks will be
correlated with physical tests accomplished by MDOT.
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