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Weather pattern’s impact on Yazoo clay often delayed

The dry weather experienced in Mississippi since Hurricane Katrina soaked the state last summer has had little effect on Yazoo clay, but possible weather pattern shifts could have a more serious impact.

“We really haven’t had any noticeable increase in consultations on Yazoo clay due to the dryness the first half of the year,” said Tommy Dunlap, PE, RPG, a principal with Burns Cooley Dennis Inc., a Ridgeland-based firm that provides geotechnical engineering services. “With expansive clays, it usually takes more time for problems to begin to occur. It’s a cumulative longer-term issue that causes problems with the clay. It isn’t a matter of a few months of dryness, but you have to think in terms of a long-term drought or rains over a couple of years that would cause contraction or expansion.”

Many other factors impact the qualities of Yazoo clay besides dry or wet weather, said Dunlap.

“We usually find more problems with such things as clay being close to the ground surface, or poor drainage around a structure or leaking water lines under or near a foundation,” he explained. “One of the concerns we’re seeing is where there was once a large tree — or trees — on a site that was removed for a house or commercial structure and the underlying clay was in a drier condition than you would see in, say, an open pasture. Trees influence the soil moisture to a significant depth by taking the water out of the soil, and when you remove those trees, the clay becomes ripe for taking on water. When a house is built over the area and drainage patterns change or people begin to water their lawns, then we see trouble.”

Corey Ray, PE, an engineer with Ewing & Ray Foundation Services, said the firm has received more calls than usual concerning foundation work in the past few months “but it’s mighty hard to tell why.”

“(Having a long dry spell) has little effect on current construction in progress other that scheduling challenges and possible compaction problems,” said Ray. “When you lay the foundation, the footings will create a kind of Ziploc bag effect and make it difficult for the water to raise back up under the foundation. It wants to go down due to gravity. It takes a lot of rain over a long period of time for the water to travel back up underneath the foundation.”

During long dry spells, soils surrounding a building dry out, which in turn causes them to shrink, said Ray.

“This shrinking causes settlement to occur on the structure,” he said. “A good example of this is to think about the soil like a sponge. When the sponge dries out, it shrinks, and when it’s placed in water, it swells. The challenge of the soils in the Jackson area is to maintain a constant moisture content in the soils so the foundations do not go through the moisture changes.”

David Dockery III, RPG, who works throughout Mississippi, explained that the weathered portion, a surficial section named the regolith, of the Yazoo clay causes all the problems with shrinking and swelling.

“This section is usually about 20 feet or more in thickness, is brown in color, and grades downward into the blue-gray unweathered clay, which makes a stable foundation for foundation pilings as long as it remains unweathered,” he explained. “A friend who is both a geologist and an engineer and has made a career repairing foundations on Yazoo clay recommended a cover of foundation-quality dirt/sand of 14 to 16 feet on the Yazoo clay before building a conventional foundation.”

Two other options include “floating slab” foundations constructed of a concrete slab with extra concrete and steel, and foundations consisting of pilings anchored into the unweathered clay, said Dockery.

“The latter is how my family’s house was built on Manila Drive in Jackson around 1950,” he said. “It never had any foundation problems. However, many structures on the Yazoo clay save on construction costs by building inadequate foundations. For these structures, a dry spell can cause problems.”

Soil movement in dry or wet weather, Dockery pointed out, may also damage water lines.

“Dry spells may set the stage for damage to come when the rains return this winter and the dry clay begins to swell,” he said. “To see damage caused by the shrinking and swelling regolith on the Yazoo clay, check out the road around the Metrocenter Mall parking lot or take a look at the sidewalks around the stadium on North West Street.”

Ray pointed to new methods for new construction that are gaining popularity with engineers and architects in the south.

“AB Chance helicals are a building code-approved product,” he explained. “Helicals use an exterior power unit to screw the piers into the soil as opposed to pushing the pier into the soil. Helicals allow the loads to be transferred from the building to the stable soils. In the Jackson area, the zone of stable soils (unweathered Yazoo clay) is typically found around 25 to 35 feet deep.”

For residential and light construction, helical piers are being added into the new foundation design to transfer the loads for both tension and compression, said Ray.

“An example of a project that used this type of foundation design is the Fed Ex Ground building we did in Gulfport,” he said. “We installed helical piers in the foundation design to be used as hurricane anchors.”

For heavy construction, the patented AB Chance Helical Pulldown Micropile can be installed, said Ray.

“They use a helical pier, plus a grout column is installed around the shaft for sites with weak surface soil conditions,” he said. “This works like the common Micropile, but the helical at the end acts similar to a bell on the bottom of a concrete pile. An example of a project that used Pulldown Micropiles in the foundation design is the renovation of the MSU Riley Center we did in Meridian.”

Contact MBJ contributing writer Lynne W. Jeter at Lynne.Jeter@gmail.com.

About Lynne W. Jeter

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