What should surround the perforated pipe in a subsoil drain?

The correct choice of pea gravel or stone surrounding the perforated pipe in a subsoil drain is essential for effective drainage performance. Pea gravel or stone serves several critical functions. Firstly, these materials are permeable, which allows water to flow freely through them while preventing the entry of silt and clay that could clog the perforated pipe. This permeability helps maintain the intended flow of water away from the area being drained.

Furthermore, the angular nature of stone can create pathways for water movement, enhancing the overall hydraulic performance of the drainage system. The graded size of the gravel or stone provides both support and stability to the perforated pipe, ensuring it remains in place while allowing unrestricted water movement.

In contrast, dirt can compact and restrict water flow, leading to drainage issues, while sand, although more permeable than dirt, can still migrate into the perforated pipe and cause blockages. Clay has very low permeability and would significantly impede water movement, negating the effectiveness of the drainage system. Therefore, the decision to use pea gravel or stone aligns best with the principles of effective drainage design.