Site and Soil Evaluation
What the inspector is actually deciding
Site and soil evaluation answers one question: does this lot, as graded and drained, have a soil profile that can accept and treat the design wastewater flow? Work from the lot outward - setbacks, slope, drainage patterns, floodplain, and the location of wells and surface water - then inward to the profile.
Concepts to master
- Profile description: horizon depth, texture, structure, consistence, color, and mottling read from a properly dug pit, not an auger boring alone.
- Limiting conditions: a restrictive layer, seasonal high water table (judged by redoximorphic features), bedrock, or a fragipan that shortens the effective depth.
- Slope: steeper grades shorten the usable absorption area and raise the risk of downslope seepage.
- Separation distances: vertical separation to a limiting layer and horizontal setbacks to water supplies and property lines.
Traps in questions
Questions love to swap depth to a limiting layer with depth of the pit, or to describe a soil as suitable when the water table evidence contradicts the texture. Watch for answers that treat a single auger refusal as proof of bedrock, or that ignore a compacted or smeared pit face.
Worked example / rule of thumb
A pit shows 45 cm of loamy sand over dense clay with common distinct mottles starting at 40 cm. The mottle depth, not the clay contact, governs the vertical separation. If the code requires 45 cm separation, this profile fails - the evidence of saturation sits above the required depth. Rule of thumb: the shallowest limiting evidence controls the design.