I receive quite a few questions about using soil fill to overcome site limitations. Sometimes these questions come before fill is used, but more often the questions are about fill that was placed at or around the time the house was built. When I receive these questions about using fill for septic system installation, I point out that our designs and installation guidelines are based on the use of natural and undisturbed soil.

Usually the question revolves around why engineered fill shouldn’t or can’t be used when my recommendation is for an aboveground mound system to overcome shallow soils over a limiting layer (bedrock, water table or dense soil layers). In a mound system, we use a specified clean sand as the fill. During installation, we are careful not to overly compact the sand in order to maintain its permeability.

What is engineered fill? Fill is a material used to fill in a depression or hole in the ground or to create mounds or otherwise artificially change the elevation of the ground. These may include earthworks such as infill, raising or leveling ground, embankments, foundation pads, road bases and landscaping. Typically, the material is placed in lifts of 12 inches in depth while compacting the material. This is the way house pads and roadbeds are created. This results in a compacted material with reduced ability to accept water, and more important, predicting the exact loss of permeability is difficult.

PERCOLATION RATES

When soils with textures other than clean sand are moved to a new location, natural soil structure is destroyed, resulting in less pore space and smaller pores available for water movement. It makes it difficult to determine what soil sizing factor to use to design a system. For instance in a soil with a loam texture, percolation rates can vary widely from 10 to 200 minutes per inch. In addition, rates measured in one spot may change dramatically over a short distance.

Since the fill was applied in lifts or layers, there are usually problems with moving water from one layer to another. Moving water from one layer to another requires saturation where the two layers meet. With a lack of structure, movement is restricted between the layers. This reduces oxygen available for treatment and often results in a more resistant biomat, further restricting water movement and causing surfacing and system failure.

Aside from the permeability problems, other issues can be observed in fill. As I looked at engineering definitions of fill for this column, one stated use of fill was to fill in areas of “soft” soil to make the lot buildable. My interpretation of soft soil is high water table soil where the soil is also high in organic matter. Not necessarily wetland soils, but soils that are wet.

When the low areas containing these soils are filled — changing the surrounding topography — water tables in the fill will be higher than the natural soil condition. Identifying depth to saturated conditions in the fill can be challenging. Since almost by definition the fill is a mixture of materials, redoximorphic features are not well developed or are absent. Unless there is information from open boreholes or piezometers (devices used to measure water levels), the prediction of water levels is difficult.

CLUES FROM COLOR

An additional problem with fill colors is they are characteristic of where the fill was excavated so it may not match with natural soil colors in the area. They can be read as indicating saturated soil conditions when they are not. Correctly identifying the limiting conditions requires evaluation of the natural topography, soils and vegetation in the vicinity to help with identification of problems.

If there are questions, several keys can be used to identify fill soils. When the fill has been placed in lifts, the material has abrupt boundaries. Identifying these conditions requires observing the material in soil pits instead of soil augers. One natural soil condition that can be confused with fill are soils in valleys and near streams where soil materials are deposited by flooding. These materials can have varying colors and a wide range in soil textures, from coarse to fine. This should not be confused with fill soils.

Important landscape features can be used to indicate fill areas. These include short, steep linear slopes; unusually flat areas in a rolling topography; higher areas adjacent to wetlands; the presence of man-made structures (roads, buildings); sparse or different vegetation; and many rocks on the surface.

The only material that can be picked up, moved and redeposited as fill while maintaining permeability is clean sand, which is why this is the material used in sewage treatment mounds, because soil sizing is predictable and the original soil surface is left in place to be part of the treatment process.