Clean washed coarse aggregate should be provided to protect the distribution system. A minimum depth of 6 inches of coarse aggregate base is beneath the distribution laterals, 2 inches aggregate depth for pipe depth, and 1 inch of aggregate above distribution laterals as cover. The total aggregate depth should be 9 inches.
A distribution system cover of a 12-inch soil crown will help drain water from the top of the mound. The extra depth also provides additional frost protection.
To ensure proper mound system performance, 6 inches of soil cover is required for the mound side slope cap providing frost and erosion protection. Soil cap can be one of the expensive features among the construction materials. The preferred materials for the soil cap are sand loam, loamy sand, and silt loams. The soil must be adequate to support grass growth to prevent soil erosion. Clay subsoil may be amended with 1/3 compost peat or manure and 1/3 coarse sand to create a 6-12 inch cap suitable for grass growth. Do not compact the mound soil cap because the microbial community treating wastewater in the mound and the grass growing on the mound need air. Natural settling of the soil cover and cap will occur over time.
| Aggregate Depth in Distribution Area | = 9 inches (6 inches of aggregate base, 2 inches for pipe depth, and 1 inch aggregate above distribution system) |
| Distribution System Cover Depth | = 12 inches |
| Mound Cap Depth | = 6 inches |