What does the term "friction angle" refer to in soil mechanics?

The term "friction angle" in soil mechanics specifically refers to the angle of internal friction that plays a crucial role in determining the shear strength of soil. This value is a measure of the resistance of soil to sliding forces, which is critical when analyzing stability in various geotechnical applications such as slopes, foundations, and retaining structures.

The friction angle is derived from the Mohr-Coulomb failure criterion, which relates shear strength to normal stress and provides insights into the soil's behavior under loading conditions. A higher friction angle indicates better shear resistance, which is especially important for assessing the safety and stability of structures built on or in the soil.

Understanding this angle is essential for engineers when calculating bearing capacities, designing slopes, and predicting the behavior of soils under various loading scenarios. The other options, while related to soil mechanics, do not accurately define the term "friction angle." For instance, the angle at which soil begins to erode pertains to erosion mechanics and not directly to internal friction. Similarly, the angle of repose applies to how granular materials stack and rest against gravity, and the angle of compaction involves different principles related to the density of soils and does not directly relate to shear strength.