by Britt Maxwell P.E.
The Yazoo clay is an Eocene epoch expansive clay deposit that outcrops in a tilting band through Central Mississippi. There is an interesting cosmic connection with the Yazoo clay that I think you will enjoy, but I will leave that for the end of this blog. There are a lot of ideas about construction and Yazoo clay, so I will use the most reliable information available (data subject to peer review). I start with four myths about slab foundations built on Yazoo clay.
The first myth is a very understandable, because 7 to 8 feet of fill is commonly specified (locally) for the construction of slabs for residences and small buildings. Locally, a layer of soil above the Yazoo that is not expansive is called a “buffer.” It is also called “the cover.” If the cover is manmade then it is select fill, otherwise it is just a natural deposit, usually silty clay. Based on foundations designed in Mississippi the 1950's and the 1960's that I have seen, some engineers of that era also misunderstood the real effect of the buffer. But there was very little data to make this determination. Foundation designs have definitely evolved in response to an improved understanding of the real behavior of Yazoo clay.
The buffer has a purpose but it does not remove the threat of Yazoo clay unless it extends much deeper. The actual purpose of the buffer is to reduce the differential movements in a slab foundation created by the expansive soil to within acceptable limits. When I say a slab foundation, I mean a typical stiffened slab (with beams built into the slab) without deep drilled piers or piles. The use of a buffer is a legitimate strategy for the design of shallow foundations on expansive soils. Locally the depth of the buffer that engineers have specified has changed. In the 1970's and early 80's we typically saw 6 feet as the required thickness of the buffer. In the last 15 years or so we now often see 7 or 8 feet.
If you want to negate the primary effect of Yazoo clay you need about twice the depth of the typical buffer that we use today. The depth of buffer material needed to remove heave potential is called the active zone. At one site the depth of the active zone was 12.7 feet. At two other sites, it was deeper than that, because related movements were detected where the clay was at that depth. There are different ways to determine the active depth of an expansive soil, but I like to rely on field measurements of movements as opposed to theoretical methods.
An important fact is that you can not always count on a buffer of 7 or even 9 feet to keep foundation movements in an acceptable range. Do we accept a small percentage of buffer failures or do we analyze these failures and try to prevent them? I wish I could report that this issue has been completely solved, but it has not. But we are beginning to get a handle on these failures and it is clear that there is a higher risk for foundation movements when the buffer strategy will be used in the following circumstances:
Field measurements made so far have given us a hint of the kinds of movements that are possible with Yazoo clay where there is an established water table. Unfortunately there is not enough solid data for us to understand how much variability exists. We recently measured 17 ½ inches of differential heave in a slab that was constructed during the mid 1970's. In this case the Yazoo clay was only about 4 feet from the top of the slab and there had been apparent problems with plumbing leaks for years. The historic Manship House was releveled in1980 but by 2010 it was 13 1/2 inches out of level. At another site we were able to compare the rate of heave over 5 years over a large area where the depth of the clay varied from about 7 feet to about 14 feet. The rate of heave at 7 feet was just over ¾ inch per year using regression on the data. The rate of heave when the buffer was 9 feet was ½ inch per year. If we apply this movement at a single spot to a residence and we define a failure to occur when it is 5 inches out of level, then failure occurs at about 6 ½ years with 7 foot buffer and failure occurs in 10 years with a 9 foot buffer. In these scenarios there is the assumption that there is only one point under the structure where water gets to the Yazoo clay. Fortunately this is rare, but it does happen. This shows us how critical a plumbing leak or poor drainage can be.
RECOMMENDATIONS FOR HIGHER RISK STRUCTURES:
1. Make at least 3 soil borings roughly inline along each segment or wing of a structure to see how much variability occurs in the geology. With only 2 borings we assume a steady transition between the borings and we never see the real transition.
2. If the expansive clay is found above the active zone in a boring, 1 or 2 borings need to be left open at the low elevations of the site (but in the footprint of the structure) for a long enough period to determine if a natural water table exists. Some of the local geotechnical engineers already do this.
3. Constructed surface drainage can severely degrade due to expansive soil movements, erosion and other factors. This means that you should compensate for this with more initial runoff slope in your drainage system. We already knew this but there is added emphasis.
Taylor, Angela C.(2005) "Mineralogy and Engineering Properties of the Yazoo Clay Formation, Jackson Group,Central Mississippi." Mineralogy and Engineering Properties of the Yazoo Clay Formation, Jackson Group, Central Mississippi. Mississippi State: Mississippi State University.