This month we continue to address some issues we have seen with installation of mound systems, causing some installers to question whether these systems are truly effective in dealing with problem soil conditions.
After proper surface preparation and treatment of the natural soil (described in last month’s column), the next area where we see many mistakes made is in the selection of the sand used to distribute septic tank effluent over the soil infiltrative surface.
Over the years, the definition of “clean, washed sand” has been clarified, as experts recognized that many mistakes were being made in the sand selection. Additional research studies pointed toward sand specifications as a significant factor in mound system performance.
The original definition of proper sand for mounds was clean sand containing no more than 10 percent silt- and clay-size particles. Research showed that the amount of fine silt and clay should be reduced to no more than 3 percent. This finding recognized that fine soil particles can migrate as effluent is applied and then collect in areas within the sand, or at the sand-soil infiltrative surface, restricting movement of water through the system.
This has led most permitting authorities to call for washed sand to ensure that the fines have been removed. Sand is characterized by a sieve analysis, performed using a set of stackable mesh trays starting large and getting successively smaller. The sieves have numbers that correspond to the number of holes per inch in the mesh.
The soil sample is placed on the top sieve and run through a shaker to separate the different size grains. Material collected from each sieve is weighed, and a spreadsheet is set up with the sieve size/number and the weight of the soil retained on each sieve. The cumulative percent retained and passing each sieve is calculated and recorded. The cumulative passing is the most important because it tells how much of the sample is particles smaller than the sieve size.
Here is where life becomes a little interesting and confusing. The research on mounds and other soil treatment systems has been conducted using the USDA soil textural classification system to describe soil conditions. This is because the USDA system overall closely relates to the ability of water to move into and through the soil.
Use of this system began with characterizing natural soil conditions but also extends to the definition of clean sand for mound systems. This is key because the sieve sizes and the soil particle size classifications do not match with either the Unified or AASHTO engineering particle size classification systems; either as size or definition. It is important when doing a particle size analysis to use the correct sieves that correspond to the sand specifications required for a mound system.
Knowing the definition
The definition now used to specify clean sand suitable for use in mounds systems is a soil texture composed of:
At least 25 percent very coarse, coarse and medium sand varying in size from 2.0 to 0.25 mm.
Less than 50 percent fine or very fine sand ranging in size from 0.25 to 0.05 mm.
No more than 3 percent particles smaller than 0.05 mm.
Particles smaller than 0.05 mm in the USDA system are considered silt- or clay-size. Here are the particle names, sizes and sieve numbers used in the USDA classification system.
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Anything passing the 270 sieve is a silt- or clay-size particle, so that number needs to be less than 3 percent for the sand to qualify as clean. Reaching that level usually requires the sand to be washed.
In addition, the sand should have a low uniformity coefficient – that is, it should contain a wide range of sand particle sizes, and in particular there should not be a large percentage of fine sands. Systems constructed with highly uniform fine sands have been shown to be less able to accept and transmit septic tank effluent.
Checking in the field
As an installer, you should work with your supplier to make sure the correct sand specifications are used. There is a field test you can use to check whether the sand being delivered has too many fines.
Place two inches of the material in the bottom of a quart jar and fill the jar three-fourths with water. Cover the jar and shake until the contents are suspended. Allow the jar to stand for about an hour and observe whether there is a layer of fine material on top of the sand after it settles to the bottom of the jar.
If the layer is more than 1/8-inch thick, the sand is probably not suitable to use for mound construction. Refusing the sand may be costly in the short run; but in the long run, you will prevent system failures, and that will pay dividends to your reputation as a quality installer.