What material is typically used as backfill for retaining walls?

Using loose gravel as backfill for retaining walls is primarily beneficial due to its ability to enhance drainage and prevent water buildup behind the wall. Retaining walls are designed to hold back soil and other materials but can be susceptible to hydrostatic pressure if water accumulates. The inclusion of loose gravel allows water to flow freely through the material, reducing the risk of pressure buildup that could lead to wall failure or movement.

Moreover, the porous nature of gravel facilitates efficient drainage, allowing water to escape rather than pool behind the wall. This characteristic is crucial for maintaining the structural integrity of the retaining wall since excess moisture can weaken the soil and compromise the wall's support.

While other materials like dirt and clay mixtures or sand might have their uses in certain applications, they do not provide the same drainage properties as loose gravel. Clay, for instance, can retain water and exacerbate the issues related to hydrostatic pressure, while sand may not offer the best stability in terms of filling voids and supporting the wall.

In summary, the use of loose gravel is paramount for effective backfilling behind retaining walls, making it the best choice among the options provided.