What does the angle of internal friction (φ) specifically signify about a soil's resistance to shear failure?

The angle of internal friction (φ) specifically signifies the frictional resistance that a soil offers against shear failure. Shear failure occurs when the applied shear stress, which is a force acting parallel to a surface, exceeds the soil's internal resistance, causing it to deform excessively or collapse. The angle φ quantifies the contribution of friction and interlocking between soil particles to this resistance. Essentially, it represents how much internal friction the soil can mobilize when subjected to a normal stress, which is a force acting perpendicular to a surface. A higher normal stress on the soil particles leads to greater contact forces, thereby increasing the frictional resistance, and the angle φ dictates how effectively this normal stress is converted into shear resistance. For granular soils, like sand or gravel, where there is little to no cohesion (the attractive forces between particles), the angle of internal friction is the primary component of shear strength. A larger angle of internal friction indicates that the soil particles are more angular, rougher, or more densely packed, allowing them to resist sliding and interlock more effectively. Consequently, a soil with a higher φ can withstand greater shear stresses before failing, particularly under the same confining normal stress, making it more stable. For example, a pile of dry sand with a high angle of internal friction can stand at a much steeper slope without collapsing compared to a sand with a lower φ, directly demonstrating its superior resistance to shear failure.