We often get one question from installers who are troubleshooting sewage breakouts at the toe of the down-slope dike of a mound system: Can’t I just haul in a little more soil and cover up the breakout?
The answer is somewhat more complicated than it might first appear, and it often goes all the way back to the original design and installation. There are at least three key items to consider when determining cause of the breakout and deciding how to fix it.
Back to the beginning
The first question is whether the original soil was properly scarified and protected. This means the vegetation was clipped and removed and the soil surface carefully prepared to maintain a good infiltration surface contact with the sand media.
The soil must have been worked when it was too wet so as to avoid compaction or smearing. This is the old KIDD principle of Keep It Dry, D------. In addition, during installation, equipment must be kept off the area under the mound, and any remaining vegetative stubble must be turned green side down.
These actions address the KINN principle (Keep It Natural, N-------) of keeping the soil infiltrative surface as natural as possible. This provides a good infiltrative surface and avoids any potential slime development that can create a short-circuiting pathway for the effluent to run out the toe of the dike.
The second key is to use clean sand as the media to distribute the effluent over the bottom of the mound. Use of dirty sand (more than 5 percent silt and clay) usually results in failure at the rock or media bed, but use of less than clean sand can create problems with the effluent not being distributed evenly across the mound absorption area, creating the opportunity for breakouts to occur.
Enough area?
The third key is whether there is enough absorption area under the mound. You perform this analysis after evaluating the first two keys. The absorption width of a mound is the width of soil under the sand layer that receives the treated effluent. This soil must have the capacity to absorb the full amount of effluent generated by the system.
The total absorption area is the product of the length of the rock bed and the absorption width. The berms located at the short ends of the rock bed layer are necessary for mound construction but are not part of the absorption area calculation.
Adequate absorption width is essential to the successful operation of the mound. The required absorption width depends on the allowable loading rate of the soil under the clean sand layer of the mound. The allowable loading rate in turn depends on the infiltration ability of the soil in contact with the sand.
In Minnesota, we determine this soil-sizing factor based on percolation rate or soil analysis. Most areas are moving toward using soil analysis exclusively. This analysis is based on soil texture, soil structure and consistency. An example absorption width multiplier table is provided above. Check your state and local regulations for your sizing table. By evaluating the soil and the absorption width, you can answer the question of whether sufficient area to absorb the effluent was provided under the mound.
Making calculations
On original soil with slopes less than one percent (level), the absorption width is the sum of the up-slope berm width, the rock bed width and the down-slope berm width. On ground sloping more than one percent, all effluent is presumed to move down-slope, and the absorption width becomes the rock bed width and the down-slope berm width.
If 1.2 gallons per day is the loading rate on a square foot of the clean sand, but the soil under the sand can absorb only 0.24 gallons per day per square foot, then 5.0 times as much absorption area must be available as sand in contact with the rock layer. Since only the side berms are used in determining the absorption area, the term absorption width is preferred to absorption area.
Another way to express the absorption width requirement is to use the absorption width ratio, which is the area of soil required to absorb the effluent percolating downward from one square foot of the rock layer. Since the rock layer is sized in Minnesota on the basis of 0.83 square feet per gallon of wastewater per day, the loading rate is 1.2 gallons per day per square foot. If the soil under the clean sand does not have this absorption capability, then the effluent must be spread out over additional soil area.
For example, a silty clay soil having a moderate to strong structure, and a percolation rate of 61 to 120 mpi, has a soil-sizing factor of 4.2 square feet per gallon per day. Multiplying the soil-sizing factor of the rock layer of 1.2 gallons per day per square foot results in a ratio of soil area to rock layer of 5.00.
So to answer the question we started with when troubleshooting mound failure on the down-slope dike, you need to evaluate the soil infiltrative condition and then decide the appropriate course of action. Simply adding soil and hoping for the best is not enough. You need to determine the width necessary and add sand and topsoil to the width needed. This assumes the soil under the mound has not been compacted or smeared.
