What is the friction angle (φ) value of clay?

• A. 30 degrees
• B. 0 degrees
• C. 15 degrees
• D. 45 degrees

Answer: (B)

The friction angle (φ) of clay is typically considered to be 0 degrees. This value reflects the behavior of cohesive soils, such as clay, which rely primarily on cohesion for their shear strength rather than friction. In geotechnical engineering, the shear strength of clay is represented using the Mohr-Coulomb failure criterion, where the cohesion component plays a significant role, particularly when the moisture content is high or when the clay is saturated.

Clay's low friction angle indicates that it does not develop significant internal friction between particles when sheared, in contrast to granular soils like sand or gravel, which exhibit higher friction angles due to their interlocking nature and the role friction plays in mobilizing shear strength.

While values such as 15 degrees or other degrees mentioned may apply under specific conditions or certain types of clay at different moisture contents or densities, the general value accepted for the friction angle of clay, particularly undisturbed clay, is effectively zero. This characteristic is critical for understanding soil mechanics and designing foundations, retaining structures, and earthwork operations involving clay materials.