What is the required slope of the surface of the underlying stone layer in an absorption trench?

What is the required slope of the surface of the underlying stone layer in an absorption trench?

• A. Between 30 mm and 50 mm vertical drop for every 10 m of horizontal length.
• B. Between 5 mm and 15 mm per 10 m of horizontal length.
• C. Between 60 mm and 70 mm per 10 m of horizontal length.
• D. Flat, no slope

Answer: (A)

In an absorption trench, the stone backfill must have a gentle downward gradient toward the outlet so liquids can move by gravity through the gravel and into the surrounding soil. This slight slope helps ensure all parts of the bed drain evenly and prevents water from ponding, which can clog the system and reduce performance. The required surface slope of the underlying stone layer is a vertical drop of 30 to 50 mm for every 10 meters of horizontal length, which provides enough grade for drainage without causing erosion or channeling. A smaller slope like 5–15 mm per 10 m would risk standing water and poor distribution, while a much steeper slope (60–70 mm per 10 m) could lead to uneven flow and potential damage. A flat surface would eliminate gravity-driven drainage altogether.

In an absorption trench, the stone backfill must have a gentle downward gradient toward the outlet so liquids can move by gravity through the gravel and into the surrounding soil. This slight slope helps ensure all parts of the bed drain evenly and prevents water from ponding, which can clog the system and reduce performance. The required surface slope of the underlying stone layer is a vertical drop of 30 to 50 mm for every 10 meters of horizontal length, which provides enough grade for drainage without causing erosion or channeling. A smaller slope like 5–15 mm per 10 m would risk standing water and poor distribution, while a much steeper slope (60–70 mm per 10 m) could lead to uneven flow and potential damage. A flat surface would eliminate gravity-driven drainage altogether.