[GSBN] Below grade clay mixes for slowing movement of water and gophers?

Art Ludwig/ Oasis oasis at oasisdesign.net
Mon Jan 25 09:24:04 CST 2016

This is kind of a random topic…but has anyone from this list explored the use of natural-building type mixes as a multi-functional substitute for concrete in stone retaining walls?  

My application: stone walls to hold landscape terraces designed to infiltrate stormwater. There is up to 6" of standing water on top of the terraces. (Note: I realize this contravenes the usual convention to make the walls porous, but people make water tanks with gravity stone walls that don't topple and we seem to be OK on toppling so far. I'm going with 25% lean in the walls.)

behind the wall: slow the movement of water through the wall
under the wall: support the stones with gravel if water finds a way under the wall (usually gravel w/o clay is used. The idea is that the gravel would stay if the clay is flushed away by a leak.
extending over the previous soil surface behind the wall: discourage gophers and water from tunneling under the wall. (Gophers are allegedly stopped by 1" gravel)
attract roots to infiltrate the mix and make a long-term, living reinforcing mesh around the gravel and stones….similar to those flying rocks one sees held by roots on the sides of eroded creek banks

 Here's my draft specification:

Stormwater Terrace Retaining Wall Construction Specifications
Under the wall:  bigger than 1" minus gravel (every size from say, 1.5 or 2" down) mixed with clay(just enough to fill spaces between gravel, no more; weight of stones should be bearing on gravel not clay)  and a small amount (2-%?) of slow release fertilizer/ compost. Mix to the consistency of concrete and use like concrete under the wall. Can also add chips from working stones. Make sure spaces between the large gravel and chips is too small for gophers to pass. SHould be something like 1-4" thick. Predicted effects:  
•                Gravel provides initial and permanent support for the stone, filling any voids between the stone and earth. 
•                Clay stops water from going under the wall.
•                Large gravel (over 1") blocks gophers (too big for them to move, allegedly). Spaces between 1" gravel too small for gophers to pass.
•                Roots will find the compost/ fertilizer and colonize the clay, gradually replacing the compost mass with root mass. This root mass will be like living rebar/ mesh that holds the whole thing together for eternity. 
•                If there is a leak under the wall, the clay, sand and small gravel can wash through, leaving the large gravel to support the wall so it doesn't collapse into a void.
On the surface of the dirt behind the wall;  The same mix, as gopher, water barrier, extending along the ground 6"-24"
On the back side of the wall It will hold if it's sandwiched between the soil and the wall:  The same mix or substitute sand for gravel, as a water barrier (presumed that gophers won't go through the wall)
In between rocks of the wall:  The same mix, with gravel sized to the space between the rocks (might be more like 3/4" than 1.5")
In between rocks on the top course of the wall, above the soil level:  Cement stucco, to keep water from going between the rocks. 
In low spaces on top of the wall: Small decorative rocks or shells mortared in place to maintain the line for the top water level.
Minimmum batter (lean) of wall suggested for <3' wall in well-draining soil:  25%

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