THE SOUTH SHALL RISE (AND SHRINK) AGAIN: MINERALOGY AND ENGINEERING PROPERTIES OF THE EXPANSIVE YAZOO CLAY, CENTRAL MISSISSIPPI

The Yazoo Formation of the Tertiary Jackson Group (informally know as the 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 nearshore marine environment and is the formation from which the primitive whale Basilosaurus, the Mississippi state fossil, was collected. Based on over 240 XRD analyses, the average composition of the Yazoo Clay is 28% smectite (probably montmorillonite), 24% kaolinite, 22% quartz, 15% calcite, 8% illite, 2% feldspar, and 1% gypsum.

Surface exposures of Yazoo are weathered to an average depth of 30 to 40 ft. Weathered Yazoo has a distinctive yellow/brown color while unweathered Yazoo is blue/gray. In most wells the amount of smectite decreases and the amount of kaolinite increases with depth through the weathered horizon. It is unclear if these mineralogic changes are due to the physical segregation of the minerals in the shallow samples or if there is actual growth of smectite at the expense of kaolinite. Regardless of the cause, the mineralogic changes correlate well to the engineering properties of the samples, which are a decrease in the liquid limit, plastic limit, and shrinkage limit with depth through the weathered zone.

The volume increase of the Yazoo Clay can be more than 200% and is almost always greater than 130%. The volume increase is greatest in the smectite-rich weathered Yazoo. The general “rule-of-thumb” for building on the Yazoo calls for removal of at least 3 ft of the weathered rock. Nevertheless, because of its expansive nature the Yazoo Clay is associated with cracked foundations, cracked walls and ceilings, and “rollar coaster” roadways throughout central Mississippi and the whole southeast.