What layer is built on top of subgrade to provide drainage and load distribution?

The correct answer pertains to the base course, which is a crucial layer in the construction of paved surfaces and other structures. The base course is built directly on top of the subgrade and serves multiple functions that are essential for effective drainage and load distribution.

One of its primary roles is to distribute the load from the surface pavement or structure above it. This helps to prevent excessive stresses on the subgrade, which might lead to settlement or failure. The base material typically consists of a well-graded aggregate that allows for adequate drainage, helping to channel water away from the surface and reducing the potential for moisture-related issues such as frost heave or erosion.

Additionally, the base course aids in maintaining the stability of the overall system by providing a solid foundation that can support traffic loads and prevent deformation under stress. Proper drainage is essential to achieve longevity and performance in any grading or paving project, which is why the base course is considered a key component of effective stormwater management.

In contrast, the other options focus on different aspects of drainage and stormwater management. Area drains are intended primarily to collect and redirect surface water but do not contribute to load distribution. Bioretention systems are designed for treating stormwater through natural processes but are not foundational layers for loads. The