ABSTRACT
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 spacing and types of the fracture weaknesses. Hardness is not directly related to strength; normally only relevant to ease of drilling. Rock failure is normally in shear; uniaxial compres sion in laboratory tests produces oblique failure shears. Compressive strength of most rocks is greater than applied engineering stresses; exceptions are weak clay, and any heavily weathered or densely fractured rock. (UCS concrete = 40 N/mm2 = 40 MPa)
Tensile Strength (To, St) Rarely measured or applied directly. Generally about UCS/20 to UCS/8 for rocks. Flexural strength relates to tensile strength on the outer surface, and is not easily measured or defined. Elastic mica plates give slate high flexural strength.
