We constantly receive questions on aspects of mound design and installation. One common question is: What soil problems or conditions warrant installation of a mound?
Initial research on mounds began as a solution for slowly permeable soils with high water tables. In Minnesota, there is a long history of using mound treatment systems to overcome specific soil and site limitations. These limitations include slowly permeable soils with percolation rates of 61 to 120 minutes per inch or slower, as well as rapidly permeable soils with percolation rates of 1 to 5 minutes per inch, and shallow soils over bedrock or high water tables, either seasonal or permanent.
While each of these conditions requires a unique set of design criteria, the basic components of a mound system are the same: a clean sand fill over the original, intact, undisturbed soil surface; a pressure distribution system to deliver the effluent; and soil cover over the top of the system that will support vegetative growth.
SIZING WITH ROCK FRAGMENTS
Another common question we get — particularly in more mountainous areas — is whether the presence of rock fragments in the soil can affect mound design and installation. The answer is yes because there is essentially no flow of effluent through the rock fragments. This is the case in any soil with rock fragments, so it is not only a problem in mountainous regions. As the effluent moves around the fragments, there is not much of an opportunity for the effluent to be treated through contact with soil particles. Think of it as the rock fragments taking up space in the soil that would be taken up by soil particles (sand, silt and clay). Opportunity for treatment is reduced proportional to the percentage of rock fragments in the soil.
A common rule of thumb is to require additional absorption area under the mound if the percentage of rock fragments exceeds 50 percent. As a note, this is the number most often seen in state codes that address this issue. Here is an example to illustrate the concern: If the absorption area is 50 feet by 70 feet by 1 foot of soil depth or 3,500 cubic feet, half of the volume is taken up by rock fragments, which means only 1,750 cubic feet are available for treatment of the effluent as it moves through the soil.
Another common set of questions involves characteristics of the sand fill that forms the treatment part of the mound. Often the questions reflect that there are mounds in their area constructed of fill materials that are not clean sand. This is usually followed by comments that these mounds seem to leak or fail very often.
The original recommendation for mound fill under the absorption bed or trenches in contact with the original soil was to use a medium sand without significant amounts of silt- and clay-size particles. In fact, it was suggested that some fine particles were desirable because treatment would be improved. However, the definition of “not significant amounts,” was left to the designer to determine. A variety of soil mixtures or textures were used in other states. In our state, we landed on no more than 10 percent fines (silt and clay), and there were literally thousands of mounds installed with success using this criterion.
A BALANCING ACT
As more research was conducted and a number of the failures mentioned above were looked at in detail, the medium-sand designation remained, but the percentage of fines could not be more than 5 percent. It was also noted that fine sands could also be a problem since they are very close in size to silt particles. So fine sand as a fraction of the medium sand material was limited to less than 30 percent.
The bottom line is that a balance is desired between the treatment occurring in the sand and maintaining permeability in the fill that allows the effluent to infiltrate in the absorption bed, move through the sand, and be distributed over the absorption area of the mound to the original soil surface where treatment is completed. To do this, a material with fewer fines operates better in the long term. Of course, the downside of having fewer fines allowed means sand meeting this criterion is harder to find or the sand needs to be washed. Either of these increases the price of the system.
Another common question is about the proper material to finish over the top of the mound. Filter fabric is placed over the top of the absorption bed to prevent soil from entering the bed and clogging the infiltrative surface. It is then recommended that a cap (soil) be placed over the top to a depth of 6 inches over the outer edge of the absorption bed, crowned in the center to 1 foot of depth. Then another 6 inches of topsoil is placed over the entire mound area to allow for establishing good grass cover.
Cap material and topsoil are recommended to have soil textures such as sandy loam or silt loam. It is recommended to stay away from clay loam or clay textures due to difficulty in placing materials, as well as maintaining the ability for water and oxygen movement through the soil cover.
In coming months, we will continue to comment on additional mound or at-grade system questions; if you have a specific question or area you would like to see addressed, send us an email at ander045@umn.edu.
