A soil evaluation is an on-site examination of a lot’s soil, usually through dug test pits, to determine what kind of septic system, if any, the site can support. It looks at soil texture and structure, depth to the seasonal high water table, depth to any restrictive layer like bedrock or dense clay, and slope, and the results directly drive both system type and drainfield size.
A licensed soil evaluator or designer typically digs or bores test pits deep enough to see the soil profile below the surface, then classifies each layer by texture and structure, which relates to how well it will transmit and treat effluent. This is a different, generally more informative test than a simple percolation test, though many states use one, the other, or both depending on the code.
Depth to the seasonal high water table and depth to any restrictive layer (bedrock, a dense clay pan, or a fragipan) are critical because effluent needs unsaturated soil beneath the drainfield to be treated properly before reaching groundwater. A shallow water table or a restrictive layer close to the surface limits how deep a conventional system can go, and can rule one out entirely in favor of a raised system like a mound.
The evaluation is what determines whether a lot gets a conventional gravity system, needs a pressure-dosed or raised alternative, or in the worst case cannot support a system at all without significant site modification. It is the single most consequential step in the design process, because every later decision assumes its findings are correct.
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Soil texture and structure, depth to the seasonal high water table, depth to any restrictive layer such as bedrock or dense clay, and slope, usually assessed through dug test pits.
A soil evaluation examines the full soil profile and structure through test pits, which generally gives more design information than a percolation test's simple water-drop timing, though many state codes use one, the other, or both.
Yes. A shallow water table, a restrictive layer near the surface, or unsuitable soil texture can rule out a conventional system, and in the most limited cases can mean the lot cannot support any on-site system without significant site work.
Guide last reviewed 2026-08-31.