How clean does sand need to be for installation of a mound system? This question comes up very often and the answer has changed over the years since the first of what I would call the modern mounds were installed.

When I began designing mounds many years ago, the answer was the mound could have no more than 10% material finer than sand-sized particles — defined in the USDA Soil Classification System as particles between 0.05 and 2.0 millimeters. Anything finer than 0.05 millimeters would be clay- or silt-size particles and would be viewed as unacceptable if they exceeded 10% as determined by a sieve analysis.

In Minnesota, we installed thousands of mounds using the 10% fine criteria. A large number of them have functioned without problems traced to the sand for 30-plus years. A common corresponding question at the time was whether pit-run sand meets the criteria. The answer for the most part was it does not meet criteria because, in general, pit run implies more fines than 10%.

Our contractors at the time said they never knew whether it met these criteria when the material arrived on site despite sieve analysis presented where they obtained the sand. At this time, we developed a quick field test to determine if the sand is suitable. We actually took the idea from a Portland Cement Guide on mortar sand.

THE JAR TEST

Put 2 inches of the material to be tested in the bottom of a quart jar, fill the jar with water, shake the jar and then let it sit. After 30 minutes, if the water above the sand is clear and the material that settled on top of the sand is less than 1/4 inch in depth, the sand is OK. This test has since been modified to less than 1/8 inch because the amount of allowable fine material has changed.

The change in allowable fines occurred after research showed mounds performed better with fewer failures if the sand was coarser and cleaner. Allowable fine material was reduced to 5% with some suggesting it be reduced even further. The 5% allowable fines are now considered the maximum amount. This makes sense since our current mounds are really similar to single-pass sand filters where having a coarser sand reduces the amount of maintenance due to sand-plugging and biomat formation.

Recently a question has come up regarding recommended sieve sizes to define sand and subsequently the finer silt and clay fractions. This is where the criteria run into the differences between soil classification for engineering purposes (Unified or the American Association of State Highway and Transportation Officials) and the USDA System. For our purposes, we use the USDA system because the system better relates particle size and soil texture to water movement in the soil.

While many of the breaks are similar, they are not a perfect match. This can create confusion when looking at sieve analyses from your local quarry or sand pit.

SIEVE STATS

In the USDA system, the No. 10 sieve is looked at as the upper range of sand particles, so any material from a sample collected on the surface would be considered gravel and could be used as a modifier of the textural classification. Below are the sieves used in a USDA sieve analysis.

The Kellogg Soil Survey Laboratory (Lincoln, Nebraska) uses a No. 300 sieve (0.047-mm opening) for the USDA sand/silt measurement. A No. 270 sieve (0.053-mm opening) is more readily available and widely used.

If more than 5% of the sample passes the size 300 sieve or alternatively the No. 270 sieve, the sand would not be considered suitable. Also, remember it is desirable for the sand used to have a variety of sizes in its distribution; so you would ideally see a range in sizes from fine to medium to coarse sand.

Given this array of sizes and the fact that most sand pits and quarries are used to supply an American Society of Testing Materials C-33 coarse aggregate for engineering purposes, this has been suggested as an acceptable, readily available material. The last sieve size used to determine this material is a 100 sieve, so between the 60 and 140 sizes in the USDA system with a percent passing the 100 sieve of 10% allowed. This is probably a practical solution for the quarry or pit to supply for mound installation.

As a final note, if you look through the literature or write-ups on sand analysis, you will see a lot of other sieve numbers and sizes, so it can be very confusing. The bottom line is to keep the silt- and clay-sized particles out of sand used for mound installation.