The upper layers of backfill should contain what type of soil to reduce rainwater infiltration?

To effectively reduce rainwater infiltration in backfill areas, it is ideal to use moderately low-permeability soil for the upper layers. This type of soil allows for some drainage while still holding back excess water, thus reducing the potential for water accumulation and minimizing erosion or the risk of saturated ground.

Moderately low-permeability soil strikes a balance between allowing for enough water passage to prevent flooding and retaining moisture to support vegetation and soil structure. It can mitigate the adverse effects of too much water entering the subsurface, which may lead to issues such as compromised foundations or increased hydrostatic pressure.

In contrast, high-permeability soil may allow too much water to pass through too quickly, which could exacerbate drainage problems rather than alleviate them. Low-permeability soil, on the other hand, could restrict drainage too much, leading to waterlogged conditions. A paved surface could also effectively prevent infiltration, but it doesn’t provide the benefits associated with maintaining some soil and vegetation. Thus, the use of moderately low-permeability soil in the upper layers is the most effective choice to manage rainwater infiltration appropriately.