In construction engineering, what material is primarily used to improve drainage and ensure stability under foundations?

Gravel is primarily used to improve drainage and ensure stability under foundations due to its physical properties. It consists of coarse particles that create voids allowing water to flow through, thereby preventing the buildup of hydrostatic pressure which can lead to foundation issues. Additionally, the angular shape of gravel particles helps interlock, providing a firm base that distributes loads effectively, reducing settlement risk. This combination of drainage capabilities and structural support makes gravel the preferred material in foundation settings, particularly in areas where groundwater levels are a concern. In contrast, other materials like clay and silt have high plasticity and low permeability, which can retain water and create expansive conditions detrimental to foundation stability. Sand can provide some drainage benefits but lacks the load-bearing capacity and cohesiveness of gravel when it comes to foundation support.