What is the maximum allowable elevation variation across the subgrade beneath a sand filter bed?

• A. 15 cm
• B. 30 cm
• C. 45 cm
• D. 60 cm

Answer: (B)

Maintaining a flat subgrade beneath the sand filter bed is essential for even support and uniform infiltration. When the subgrade elevation varies too much, the bed can tilt, causing uneven contact between the sand layer and the underlying soil. This changes how effluent moves through the sand and into the soil, leading to low spots where water pools and high spots that stay dry. That uneven loading can cause stress on the bed and its distribution system, potentially shortening the drainfield’s life and causing clogging or failure. The maximum allowable variation is thirty centimeters, which provides a practical tolerance for field conditions while protecting performance. Exceeding this limit increases the risk of differential settlement and poor treatment, while smaller variations stay within the intended tolerance.