Low permeability in backfill soil is essential for managing rainwater effectively. When backfill soil has low permeability, it restricts the movement of water through the soil, allowing for better control of surface water runoff and minimizing erosion. This characteristic helps prevent saturation and the associated risks of soil instability, flooding, and structural damage.
On the other hand, high permeability would allow water to pass through quickly, which can lead to water pooling and increased runoff, potentially overwhelming drainage systems. Organic content can affect soil structure and drainage but is not as directly related to rainwater management as permeability. Grain size can influence drainage but doesn't provide a comprehensive understanding of how well the soil can retain or release moisture. Low permeability effectively supports the necessary balance for rainwater management, making it the most critical characteristic to consider in this context.