Site Evaluation and Soil/Site Conditions
Why site evaluation drives design
A sewage system succeeds or fails on the site. Evaluation establishes the vertical separation to a limiting layer, the soil's capacity to accept and treat effluent, the depth to seasonal high groundwater or bedrock, slope, and the available area for the dispersal field and its reserve. Candidates must understand that soil texture and structure are the primary indicators of permeability, and that a percolation or permeability test must be paired with a soil profile from a test pit. A percolation rate alone, without knowing what lies beneath, is a classic trap.
Reading the profile
Describe horizons, colour, mottling, and consistency. Mottling and greyed colours suggest periodic saturation. Restrictive layers such as clay or bedrock limit depth. The most restrictive condition governs the design, and the required separation distance is measured from the bottom of the distribution trench, not from ground surface. Slope affects both setback and construction method; steep sites may require special designs or be unsuitable.
Traps in questions
- Measuring separation from grade instead of trench bottom.
- Treating a fast percolation rate as always good; excessively rapid soils may transmit effluent without adequate treatment.
- Ignoring the reserve area requirement.
- Confusing seasonal high water table with a perched water table.
Rule of thumb
Design to the worst credible condition found in the test pit, and always identify a reserve area of equivalent suitability before committing the primary field.
Worked example
A test pit shows 40 cm of sandy loam over mottled clay, with bedrock at 1.4 m and a trench planned at 0.6 m depth. The limiting layer is the clay contact at 0.4 m, so separation from trench bottom is only about 0.2 m in the upper zone. This is inadequate, and the designer must raise the system, use a raised bed, or relocate rather than rely on the favourable percolation rate.