Install Geotextile Fabric

What happens here

A layer of black geotextile fabric is installed. This fabric is first laid along the bottom of the trench, extending underneath the entire base of the retaining wall and then brought up along the back side of the trench. The narrator explicitly states its purpose: to separate the base soil (native soil) from the drain stone. He lifts a section of the fabric to reveal the distinct separation between the two materials, emphasizing that this prevents the soil from mixing with and contaminating the drain stone.

Goal: Separate native soil from drain stone to prevent contamination and maintain drainage