DRAINAGE FOR YAZOO CLAY

This blog is about residential structures built with slab foundations in central Mississippi but it could also be used for residences with crawl spaces in this area. In this area lies one of the most active expansive clays in the world. It is called Yazoo clay. When this clay surface is above a depth of 10-11 feet of the surface, you should have adequate fall away from the house (positive drainage) on all sides. We often refer to this elevated surface around a residence as a berm. Typically you would need a fall of 8 or 9 inches in a 6 to 10-foot perimeter from the house with a Yazoo clay depth of 8 feet. In addition, the surface water that runs off of this berm must also be properly drained away without ponding. This is so important that it should be part of a local building code. This well-known construction fact is still violated by some home builders. Often the violation occurs on terraced lots where the lot is too small for the house. When this happens it is impossible to construct proper drainage away from the house without expensive retaining walls. These houses are often abandoned by their owners 10 to 15 years down the road when the owners are confronted with the cost of releveling plus a retaining wall and drainage that should have been part of the original construction. 

With less active clays like we find near Dallas and Austin Texas, these same rules would apply except that these less active clays must be closer to the surface than the Yazoo to create this need for good surface drainage. Surface drainage problems often go unnoticed in planting areas next to the residence that are covered with mulch. Typically depressions from erosion occur around the edge of residences that do not have gutters. Sometimes the depressions are inadvertently created when the planting is installed. To discover these areas the mulch must be removed and replaced.

There are 3 ways to improve drainage other than constructing retaining walls. There are two ways to improve surface drainage plus subsurface drainage. Retaining walls can be added to surface drainage to lower the grade and improve slope away from the residence. Surface drainage should be improved when water stands in puddles and maybe when the ground is mushy after rains in the same areas. Surface drainage might be improved by constructing drainage swales through a poorly graded yard. Otherwise, catch basins are installed with drain pipes. There is a significant improvement in quality when smooth pipe and concrete boxes are used. Higher quality systems are required when the ground is flat or there is potential for leaf or pine straw to clog the pipe.

Generally, subsurface drainage in central Mississippi should be designed by a qualified engineer. Subsurface drainage is only installed to remove groundwater. In central Mississippi, this is almost always a perched water table that is created above an impermeable layer like Yazoo clay that should be at least 6 feet deep. If your drain is not as deep as the impermeable layer the groundwater can just pass underneath the drain. Poor subsurface drainage is hard to detect without soil borings. Be very cautious if someone recommends subsurface drainage without the aid of soil borings. If water is not initially detected in the boring then they should be left open for at least 24 hours. You might detect groundwater with hand-dug holes if it has not rained in a week or so. But typically hand-dug holes will not be deep enough. Note that water standing in a hand-dug hole a day or two after a hard rain might just be surface water that has saturated the upper layer of soil and may not be a condition that justifies a subsurface drain. Also if you suspect a leaking water line, you can test the water in a boring for chlorine and sewer bacteria.

I typically recommend drainage repairs when foundation movements are moderate or severe. I also correlate drainage issues with the floor elevations. This kind of evaluation is described in explained in a blog with the title:   Investigating Slab Foundation Failures in Central Mississippi.