LIMITS ON STRENGTH AND DEFORMATION PROPERTIES OF JOINTED BASALTIC ROCK MASSES

A study of strength and deformation measurements for basaltic rocks, along with consideration of the influence of fracturing using a rock mass classification system, documents the range of brittle response for basaltic rock masses. Although basalts vary widely in composition and other physical factors, many of the properties of a basaltic rock mass appear to vary within a factor of about 10. Typical values of strength parameters for intact basalt at ambient temperature (20-degrees-C) and negligible confining pressure are Young's modulus, 78 +/- 19 GPa; Poisson's ratio, 0.25 +/- 0.05; tensile strength, -14.5 +/- 3.3 MPa; unconfined compressive strength, 266 +/- 98 MPa; and cohesion, 66 MPa. Corresponding values for a basaltic rock mass that incorporate the weakening effects of scale are deformation modulus, 10-40 GPa; Poisson's ratio, 0.3; tensile strength, -0.1 to -2.5 MPa; uniaxial compressive strength, 10-90 MPa; and cohesion, 0.6-6 MPa. A measured deformation modulus for ambient pressure in the vertical direction, 20 GPa, is 1.5-3 times larger than that in the horizontal directions, 13.5 and 6.5 GPa, reflecting strength anisotropy due to column or block geometry for one particular basalt. Values of tensile and cohesive strength for the basaltic rock mass are generally one to two orders of magnitude lower than corresponding values for intact basalt. The shear strength of joints appears to vary considerably from flow to flow.