If a site slope is 30%, is benching of the underlying soil recommended?

• A. Yes
• B. No
• C. Only if the soil is highly permeable
• D. Only if the bed is elevated

Answer: (B)

On slopes, excavating to create flat benches can destabilize the soil. A 30% slope is fairly steep, and removing material to form benches can undermine the toe of the slope and create new failure surfaces. When soils are saturated or experience seepage (common near septic systems), shear strength drops and the risk of a slide or erosion increases. Because of these stability concerns, benching the underlying soil on a 30% slope is not recommended. In such cases, choose designs that avoid benching—like locating the system on level ground or using an alternative (raised/mound or terraced) approach with professional guidance.