ABSTRACT
24 Rock Strength Strength of intact rock depends on component mineral strengths and the way they are bound together - by interlocking or cementation. Rock mass strength applies to a mass of fractured rock within the ground and largely relates to the fracture weaknesses. Hardness is not directly related to strength; normally only relevant to ease of drilling. Rock failure is normally in shear; unconfined compression in laboratory test produces oblique failure shears. Compress ive strength of most rocks > applied engineering stresses; exceptions are weak clay, and any heavily weathered or densely fractured rock. (UCS concrete = 40 N /mm2 = 40 MPa)
