Yazoo clay: Old problem getting new look from researchers
by Wally Northway
Published: June 16,2008
Oh, that devilish Yazoo clay. It continues to be the bane of builders in Central Mississippi, making shambles of the best-laid plans — not to mention foundations. The water-loving formation expands when moist and contracts when dry, making it a moving target and a disruptive force in the construction industry.
There is good news, bad news and yet more good news on the Yazoo clay front. One piece of positive news is that many experts say the recent drought and subsequent heavy rains will have little impact on most Yazoo clay, though the portion of the formation that might be affected would not show movement for some time to come.
The bad news is that some of the formation, as always, will cause problems, and there are no new techniques available to deal with the problem.
However, engineers may soon have a good bit more information at hand, as the Mississippi Department of Transportation (MDOT) has funded a research project aimed at taking a closer look at Yazoo clay.
What is it?
The Yazoo Formation of the Tertiary Jackson Group, or Yazoo clay, is a “calcareous fossiliferous mudrock” that outcrops in a northwest-southeast belt across much of Mississippi and in adjacent states. The Yazoo was deposited in a near-shore marine environment and is the formation from which the primitive whale Basilosaurus, the Mississippi state fossil, was collected.
Surface exposures of Yazoo are weathered to an average depth of 30 to 40 feet. Weathered Yazoo has a distinctive yellow/brown color while unweathered Yazoo is blue/gray.
The average composition of the Yazoo clay is 28% smectite, a water-bearing mineral, with the remainder made up of six other minerals. The volume increases are most pronounced in smectite-rich regions of the clay.
The clay’s ability to expand is incredible. Research has found the volume increase can be more than 200%, and is almost always greater than 130%.
Because of its expansive nature, the clay has been associated with cracked foundations, cracked walls and ceilings and “roller coaster” roadways in Holmes, Hinds, Madison, Rankin, Smith, Scott, Newton, Jasper, Clarke and Yazoo counties.
There are multiple factors that determine whether a certain “well” will be highly expansive or not. Depth, coverage and mineral make-up are contributors. The bottom line is not all Yazoo clay is the same.
What to do?
The best way to handle Yazoo clay is, well, not to handle it, if possible. It is exposure to moisture that causes the problems. So, leaving the clay covered is the best bet.
That is easier said than done in the construction business. Dirt work will expose the clay, which means some of it must be removed. A report written by faculty from Mississippi State University (MSU) and a member of the Materials Division of MDOT and presented to the Geological Society of America said the “rule of thumb” for building on Yazoo clay is to remove a minimum of three feet of the weathered rock.
Charlie Furlow, senior consultant at SoilTech, a sister company of the engineering firm Neel-Schaffer, says the key is depth and cover. “The more cover you have, separation of the foundation from the clay, the better off you’re going to be. To beat Yazoo clay, you either need a buffer or a deep foundation.”
Dr. John Mylroie, professor of geology at MSU, points to the aforementioned “roller coaster” roadways as a good example of how moisture can affect Yazoo clay. He says moisture is able to come in through the seams of the pavement, causing the underlying clay to expand. However, the clay underneath the roadway segments remains relatively dry, thus causing the roadway to buckle at the seams.
Because of all the variables affecting whether Yazoo clay will be a problem or not at a particular location, it is a hard substance to understand. And, what research is out there is some 30 years old. However, that is about to change.
The Department of Geosciences at MSU is conducting research into Yazoo clay. Funded by MDOT, Mylroie says the research’s aim is to get a better handle on where and how thick Yazoo clay formations are, to “know what we can’t see.”
Specifically, 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, using technology developed since the last Yazoo clay study.
The 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 clay samples with depth and spatial separation.
The project is broken into three tasks. Task 1 is to identify and define the chemical and structural composition of the minerals in the Yazoo clay. A specific objective in Task 1 is to separate Yazoo samples, 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.
Task 2 is the development of an XRD-based quantitative mineralogic analysis. 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.
Furlow said the findings are greatly anticipated. When asked if this research could make a difference at future construction sites, he said, “most certainly.”
Contact MBJ staff writer Wally Northway at firstname.lastname@example.org.
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