Between the gravel and the underlying soil trench, what material should be placed?

• A. Concrete
• B. Gravel
• C. Filter fabric
• D. Plastic sheeting

Answer: (C)

Using a separator fabric between the gravel and the native soil trench is essential to keep fines from clogging the gravel layer and the drainage voids. This geotextile acts as a filter and barrier, letting water pass through while preventing soil particles from migrating into the gravel. That helps maintain the trench’s infiltration capacity and long-term performance. Concrete would block drainage, plastic sheeting would hinder infiltration and trap moisture or cause other issues, and replacing it with just more gravel wouldn’t stop soil from mixing with the fill. So the fabric is the proper choice to preserve flow and prevent clogging.