Saturday, August 3, 2013

Extreme Building Damage Caused by Yazoo Clay

Everytime I look at this picture I can't help but compare it with earthquake damage. The end result can be very similar. Geologic deposits like the Yazoo clay are often called "highly active" and the movement is not sudden like an earthquake. You get a little bit every day and some days you get a retraction where cracks close, door that would not close last month suddenly work fine. But when you get heaving (or uplift) in high rise construction, the second floor tries resist the movement and the first floor behaves like it's being squeezed in a giant vice.

Extreme damage to structures can result from expansive soil movements. This photo is a reproduction of a Polaroid that I took before I went to the University of Texas. It shows the first floor of a high rise building in Jackson Mississippi. This might be the most dramatic photograph of interior building damage from expansive soils ever taken. Each floor of the building was a flat plate structural slab (self supporting). The first floor slab was constructed with an undrained 6 inch open void underneath. However, by the time the building was 8 years old, the void had closed and first floor had heaved as much as 8 inches. The problem was that the expansion potential (uplift) of the Yazoo clay was seriously underestimated. 

Expansive soil damage to the first floor of a high rise
building in Jackson Mississippi.

The floor of this room was a structural slab that was supported by the same concrete columns that supported every floor. On the first floor heaving of the slab caused the structural slab to disconnect from the columns in an odd reversed punching shear failure. A punching shear failure is when a structural slab shears away from a supporting column. Normally the slab falls down; but in this case the slab moved up (in reverse) from expansion of the clay. The columns were supported by deep drilled piers and were not damaged. Fortunately, this part of the building structure was stable.

Damage seen in this photo is the result of crushing metal stud partitions (in compression) that were constructed from the first floor to the bottom of the second floor slab. The damage was so severe that most interior doors had been removed on the first floor. In the left side of the photo is a door that has been replaced by tapping sheets of brown wrapping paper over the opening (a common solution). The door near the center of the photo and the one on the right has been removed. In the room in the foreground partitions have failed by rupturing the wallboard. In the background the wallboard buckled and disconnected from the studs.

In Central Mississippi this kind of construction represented the first efforts by engineers to isolate the bottom floor (using structural self supporting slabs) from the expansive soils in the 70's and early 80's. Numerous buildings were built just like this. I doubt that any of them still exist like they were originally built. We now know that a typical rate of heave for a normal soil profile with Yazoo clay is about ¾ inch per year. So a 6 inch void like this building had, couldn't last more than 8 years in a typical setting. In this particular case the expansive clay was exposed at the surface, there was poor drainage, and it potentially heaved at a rate of more than double the amount from a typical condition. That is the highest heave rate that that I know of in Central Mississippi.  One end of the building was built several feet below the surrounding natural grade. So construction required the removal of natural inactive soils above the clay. This created an unloading condition that promoted rebound of the expansive clay. This construction also created a condition of poor drainage around the structure.  Surface drainage that was constructed rapidly deteriorated from heaving that occurred in the ground around the building.

To repair the building, the entire first floor and the slab was completely removed and replaced when this building was about 9 years old. As a result this building is still in service today. The repair involved increasing the void space size to 30 inches. However, clearances under plumbing in the crawl space had to be recently dug out again in part of the crawl space. At the other end of the building the new crawl space was still intact and some shrinkage had actually increased the crawl space by a few inches.

Unfortunately buildings on drilled piers like this one today are still having issues with Yazoo clay in Central Mississippi. My studies of these newer buildings indicate these primary issues:

1. Inadequate clearances specified for utilities or specific elements of the foundation.

2. Inadequate retainage to prevent soil flow into the crawl space or voids under grade beams.

3.  Inadequate drainage of the void spaces or crawl spaces.

4.  Inadequate construction review of void or crawl spaces.

All rights reserved by Britt Maxwell P.E.