Sizing Mounds for the Poorest Soils

Infiltration rates, mound design are critical factors in designing for successful treatment in heavy clay

We have really tough soils in our area so you don’t understand our problems and why we do the things we do.”

Over the years we have encountered this statement every time we have gone to a new state or area. This has even happened here in Minnesota! In our recent column on design factors and how they are impacted by organic loading, we reviewed early research looking at a “tough soil” and the recommendations for design from the study.

In this case, the “tough soil” was an Ontonagon clay, which has a clay content of 65 percent or higher. The soil was initially described near the town of Ontonagon, Michigan, on the south shore of Lake Superior. We mention this because it means this soil or something very similar occurs throughout the old Lake Superior lake plain. Everyone who lives in these areas has “tough soils” that are difficult to deal with.


Previously we said that in clay soils we are not so worried about whether the effluent gets treated — because of how slow water moves through these soils — but getting water to move into them. A typical design number in state codes for clay soils is 0.2 gallons per day per square foot (gpd/sq ft). This is a conservative design number based on research that had a clay soil design number of 0.24 gpd/sq ft, which was equivalent to a saturated hydraulic conductivity in clay soils of 1 centimeter per day. Plus, the math is easier with the 0.2 value.

Since the research we looked at was done in the early 1970s and soil treatment mounds were just coming on the scene, the system installed was two alternating seepage beds. The beds were both 15 feet by 40 feet, or 600 square feet of bottom area. The plan was to alternate the beds on an annual basis to control biomat development and provide resting time. To provide protection to the infiltrative surface of the beds, loamy sand and clean sand layers were provided before the distribution rock was placed.

Total bottom area for the two beds was 1,200 square feet. If we used the 0.24 gpd/sq ft loading rate, we would estimate the two beds would accept an average daily flow of 240 gpd/sq ft. That is less than the estimated daily flow from a three-bedroom residence of 450 gpd.

A lot went on during the three years of the study, and it was well-documented in the research paper. One of the interesting things to us was that over the period the study was conducted and using what we would recognize today as not “good” practice, the beds accepted on average 0.187 gpd/sq ft or about 224 gpd for the two beds.

This number is less than the design number found in state codes for clay soils. But it does indicate that while movement through clay soils is slow, there is movement. And if it can be defined or estimated, it can be used to size systems. In the conclusions of the study, the authors indicated that for soils such as Ontonagon clays, the design loading rate should be 0.175 gpd/sq ft.


For comparison using an estimated daily sewage flow of 450 gallons per day for a three-bedroom house and a design loading rate of 0.24 gpd/sq ft, the bottom area required to accept the effluent would be 1,875 square feet. Using the recommended 0.175 gpd/sq ft, the required area is 2,571 square feet, a difference of almost 700 square feet. The bottom line is a bottom area 37 percent larger is required to accept the average daily flow. In low-precipitation areas where seasonal water tables do not occur, these numbers may be useful to set criteria for soils high in clay content.

As indicated above, there was a lot of management including switching back and forth between the beds due to precipitation events and other occurrences. As a result, some other conclusions came from the study that are worth remembering when thinking about system design.

They attempted to drain water from around the bed area. In the heavy clay soil, this was ineffective. This is instructive for those with “tough,” slowly permeable soils high in clay content. Just as the researchers concluded, building an elevated sewage treatment mound system to get away from seasonal high water as opposed to drainage is the way to go. They suggested that the mounds be located on the crests of slopes and surface water be directed away from the system.

They also recognized when mounds were just coming onto the scene that with slow infiltration rates, the toe of the mound dikes would have to extend far enough to allow the required area to infiltrate. So choosing a longer, narrower mound over a short, square mound was preferable to help move the water away from the system. We have seen a number of cases where adequate absorption areas were not provided under the mounds, leading to surfacing around the toe of the mounds. This caused some to abandon the use of mounds all together when they were just not made large enough in the first place.

Another important recommendation from the study was that if the surface of the Ontonagon clay was left undisturbed, the absorption area needed under the mound could be determined using the 0.24 gpd/sq ft figure. This recommendation was based on the surface of the soil being permeable and, if left intact, would accept more effluent.


We think you should now begin to recognize the origin of some of our guiding principles, such as “keep it shallow,” “keep it dry,” and “keep it natural.” They are not something we made up to be amusing, but principles based on sound research projects and experience over 40 years of working with systems. 


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.