Friction angle of rock

Answer 1

Formal answer: The angle of internal friction is measure of the ability of a material (could be rock or soil or whatever) to withstand a shear stress. It is the angle (φ), measured between the normal force (N) and resultant force (R), that is attained when failure just occurs in response to a shearing stress (S). Its tangent (S/N) is the coefficient of sliding friction. Its exact value is determined experimentally. Technical answer: The angle of internal friction is a critical parameter of the Mohr-Coulomb failure criterion: qu = 2*c*tan(45+φ/2), where qu is the unconfined compressive strength of the material (in this case rock), c is the cohesion and φ is the angle of internal friction. Practical answer: Because there are a lot of different types of rocks out there - each with a different geomechanical behavior - it should come as no surprise that there are a lot of different angles of internal friction of rock. The equation that defines the angle at which a material breaks is the (45+φ/2) part of Mohr-Coulomb. It has been observed that many hard materials tend to break in shear at an angle of about 60 degrees during uniaxial compression. So if that tends to be the case, then a good guesstimate for the angle of internal friction of many rocks is 30°. However, bear in mind that the value can vary greatly, less than 10° for some very soft rocks and more than 50° for some very hard rocks.