In constructing a raised leaching bed, after removing the topsoil and digging a shallow depression, what is the next task?

• A. Scarify the subgrade to a depth of about 150 mm.
• B. Install the drainage layer.
• C. Backfill with topsoil.
• D. Compact the subgrade.

Answer: (A)

Scarifying the subgrade to a depth of about 150 mm prepares a loose, rough, and more permeable foundation for the raised bed. This breaks up any hard crust or compacted layer that would hinder water movement and uniform distribution of effluent. By creating a rough surface, you improve the mechanical keying and bonding for the drainage layer that sits on top, help prevent uneven settlement, and promote even infiltration into the soil below. After this step, the drainage layer can be installed on the prepared base, with topsoil placed above it to form the growing medium. Compacting the subgrade would reduce porosity and impede drainage, and backfilling with topsoil before laying the drainage layer would block the intended arrangement.