Information to Build On
Expansive and Yazoo Clay
What is Expansive Soil: Expansive soil is comprised of clay soil with a high degree of potential volumetric change as its soil moisture content changes. A microscopic view of the clay particles reveals a thin, flat, plate-like shape. This shape results in the clay particles having a relatively large surface area. The clay particles carry an electrical charge and are bonded to one
consisting of greater than 50% of the soil particles passing the No. 200 sieve, a Liquid Limit (LL) of 50 or higher, and a Plasticity Index (PI) of 30 or higher. Expansive clay is denoted in the USCS with the two-letter symbol, C (Clay) and H (High Plasticity). Thus expansive clay defined by USCS is noted as CH.
As can be seen from the map to the left indicating the presence of expansive soils across the United States, the majority of the state of Mississippi has the potential for the presence of expansive soils to some degree.
Expansive Clay in Mississippi: Expansive clays are located throughout the state of Mississippi in both small and large deposits. Anyone who has ever heard of "Gumbo" in the Mississippi Delta, well that is expansive clay. However, the Mississippi Department of Transportation
another not unlike a magnet. The charge is not very strong, thus can be broken relatively easily by water molecules inserted between the clay plates. During saturation, as more water is wedged between the clay plates, the plates are pushed apart. The net result is an overall increase in volume of the soil matrix that we recognize as expansion, or clay heave. This process works in reverse during drying, resulting in shrinkage. Expansive, or High Plasticity clay, has the physical properties defined by the Unified Soils Classification System (USCS)
(MDOT) SOP has identified certain types of soils as having a high volume change. These are the Yazoo, Porters Creek, Zilpha, and Hattiesburg/Pascagoula Formations. Again there is the potential for localized deposits of expansive clay to be present at pretty much any location within our state, but the formations listed by MDOT contain predominately expansive materials. The figure below present general locations of the problematic formations.
Zilpha and Porters Creek Formations in North Mississippi
Jackson Group, Zilpha and Porters Creek Formations in Central Mississippi
Jackson Group, Zilpha, Porters Creek Formations in Central/North MS
Hattiesburg / Pascagoula Formation in South Mississippi
What is Yazoo Clay: Generally the most problematic expansive clay in the greater Jackson Metro and Central Mississippi region is that of the Yazoo Formation of the Jackson Geologic Group, or Yazoo clay.
The figure below detailing and mapping the location and extent of the Yazoo clay was provided in State Study 151 perfromed by the USACE. The Yazoo clay is identified as the yellow shaded areas with city boundaries identified as red shaded areas. The formation runs entirely beneath the cities of Jackson, Flowood, Madison, Ridgeland, Flora, Bentonia
Canton, Sandhill, Morton, and Forest, and beneath most of the cities of Clinton, Flowood, Richland, and Yazoo City. The formation generally tapers off to the north just south of Eden and just north of Canton and Sandhill, to the west just east of Sartartia and Yazoo City, to the south about half way between Richland and Byram, and extends to the southeast in a narrow band across Morton, Pelahatchie, Forest, Lake, Pachuta, Shubuta, and Matherville, across the Mississippi state line into the western portions of Alabama beginning to taper offaroudn Melvin and Gilberton and near Cullomburg and
State Study 151 and 236
Frankville. The Yazoo clay is an Eocene Age (56 million to 33.9 million years ago) deposit formed in a marine environment. The Yazoo clay mineralogoy consists of smectite (probably montmorillonite), which is considered to be a very active type of clay. The thickness of the Yazoo Clay is about 400 feet at its maximum and reduces to less than 12 inches along the northern edge of the outcrop. Yazoo Clays are over-consolidated and generally deposited in three zones. The uppermost zone consists of highly weathered silty clay that extends to depths that range from less than
12 inches to about 10 to 15 feet. The silty clay is considered minimally expansive with a relatively low shrink/swell potential and is not present in some locations. Beneath the silty clay is highly plastic weathered clay, or "Yazoo Clay". This is the zone/material that is responsible for most local shrink/swell related foundation issues. The depth of the weathered clay generally extends to about 30 to 40 feet below the ground surface. Underlying the weathered clay is unweathered, blue-gray clay, sometimes referred to as "Blue Yazoo Clay". This deepest zone has not been exposed to the effects of natural weathering and extends to a depth of about 400 feet.
How Can Expansive Clay Affect My Foundation: If you are looking to build or buy a home, especially in the cruicial areas outlined above, expansive clay is a concern. Shallow expansive clay beneath a residential foundation can be detrimental to the long term performance of the structure. Once source has cited that of the approximate 250,000 new homes built on expansive soil each year, approximately 60% experience damage, and 10% experience significant damage. Another source has cited the annual damage to homes in the US from expansive soils is estimated to be on the order or $1,000,000,000.00.
As noted, expansive Yazoo clay exhibits a particularly high degree of volumetric change as its soil moisture content increases or decreases, relative to other expansive clays in the region. These changes in soil moisture are generally caused by but are not limited to; prevailing seasonal weather, irrigation, utilities, and vegetative root systems. The expansion/shrinkage will be realized by new grade-supported foundations as upward and downward movement, or heave and settlement.
Once a slab-on-grade has been constructed, the presence of the slab generally limits fluctuation in soil moisture in the more central areas of the slab. The critical zone generally extends from the foundation perimeter to a distance of about 8 to 10 feet inward from the slab edge. Most of the soil moisture fluctuation, and thus movement, will occur within this zone. As the soil along the perimeter of a foundation wets (expands) and dries (shrinks), the edges of the foundation will move accordingly. As the perimeter soils dry, foundation edges drop resulting in a pattern of movement termed “center lift”. As the perimeter soils wet, the foundation edge rises resulting in a pattern of movement termed “edge lift”. The result of one or a combination of these patterns of movement can be distress to the supported elements of the home that can lead to cracks and un-level floor slabs.
If the presence of shallow expansive clay is not discovered and addressed properly prior to new construction, it will likely be felt by the structure throughout its serviceable life. Once movement is activated, very little can be done to permanently arrest it. The local Yazoo clay can experience volume increases of over 200%, and generate up to 25,000 pounds per square foot of swell pressure. Given the typical house generally weighs between 200 to 375 pounds per square foot, depending upon the number of stories, the net result is the expansive Yazoo clay under swelling conditions can exert as much as 50 times the upward pressure on a foundation as the load of the foundation imparts to the ground. This is similar, though not to nearly the same degree, as other expansive clays in Mississippi. In other words it is very easy for the Yazoo clay, or other expansive clays, to lift and shift the foundation elements of a house. True to form the Yazoo clay can undergo significant shrinkage if subjected to drying as well. Cases I have seen locally have exhibited foundation movement up to 10 to 12 inches and more in extreme cases, with movement of 3 to 5 inches not uncommon. A lot of money can be spent on "foundation repair" and leveling slabs, however despite what you may be told, there is very little that can be done to arrest the shrink/swell movement of the clay once it has started. This is primarily because the movement is typically cyclic with changing seasonal weather and it is very difficult at best to prevent moisture fluctuation in soil.
The problems associated with the presence of expansive clay can be addressed, and effective engineering can significantly reduce the potential impact of the expansive soils on foundations to tolerable levels. However, the presence of the expansive material must first be verified by effective subsurface exploration.