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Mr. Smarty Plants - Expanding clay soils near rain garden

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Monday - May 11, 2009

From: Kyle, TX
Region: Southwest
Topic: Rain Gardens
Title: Expanding clay soils near rain garden
Answered by: Nan Hampton and Chris Caran

QUESTION:

I want to put a rain garden in my yard in central TX (Kyle). My subdivision architectural review committee expressed concerns about the expansive clay soils becoming saturated and possibly shortening the anticipated lifespan of a sidewalk and alleyway near by. How do I find information about setback distances for clay soils? I have found that most rain garden plans recommend 10' from foundations, but no comment or mention about distances form sidewalks and alleyways. I found that in Seattle they have rain gardens in the planting strip between the roadway and sidewalk without concern for damage to road or sidewalk. How or where can I get help with this concern? Thanks

ANSWER:

First of all, and I'm sure you know this, Seattle soils are not like Kyle soils.  I've thought about your question a great deal and, although I wasn't able to find any exact recommendations for distances from sidewalks or streets,  I did get some insights from consulting a geologist friend who knows a lot about the expansive clay soils of Central Texas.  This is a summary of what he says about your problem:  When the weather has been dry, these expansive clay soils shrink and become permeable with small channels running through them much like the honeycomb limestone of the area.  When it rains, the water runs through them vertically until they absorb enough to expand and become impermeable.  They expand vertically as well as horizontally.  When they absorb enough water to become impermeable, the water runs off the surface.   The soil can't expand further at this point.  The water you catch in your rain garden pond is going to permeate the clay soils underneath it until they become saturated and almost impermeable.  Then, the pond will hold the water until it evaporates or seeps slowly down into the ground.  The soils under and perhaps adjacent to the rain garden WILL expand considerably once they are wet, so any structures (e.g., pipelines, walkways, etc.) in the immediate proximity of the garden can be expected to shift.  The adjoining clay soil in the yard is also going to expand just from the rain that falls on it and the soils near the sidewalk will expand after a rain regardless of whether there is a rain garden nearby or not.  If 10' is the recommendation for foundations, that would seem an adequate distance for sidewalks as well.
 

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