Unconfined Compression Tests on Basalt Rocks from Hawaii

Abstract

Unconfined compression tests were performed on 66 basalt samples to obtain index properties and elastic constants. The samples were obtained from a boring drilled at the University of Hawaii at Manoa. These values were compared to results obtained from other studies to determine correlations and observe vertical trends at the site. Properties obtained from nondestructive tests on the basalt rocks included unit weight (oven-dried, saturated-surface-dry, and apparent), unit weight through the use of a CoreLok machine, absorption, RQD, and percent recovered. Properties obtained from compression tests included unconfined compressive strength, Young’s Modulus, axial strain at failure or at 50% of ultimate load, failure type, and failure plane angle to horizontal. The results indicate that there appears to be two different layers of rock at the location of the boring. There is an upper layer of rock characterized by lower unit weights, higher absorption and lower strength and stiffness, compared to the lower layer of rock. This points to significantly different rock types, probably from different lava flows with somewhat different original magma composition and viscosity. It is worth noting that RQD, percent recovery and axial strain do not show discernible distinctions between these two rock units and thus appear to be less useful as indicators of distinct rock units, at least as encountered at the site. Strong correlations were observed between absorption and unit weight, as expected. Strong correlations were also noted between the various unit weights and the results of the unconfined compression tests. In particular, there are reasonably strong correlations between index properties in terms of unit weight and absorption, and test results in terms of unconfined compression strength and stiffness. No discernible correspondence was observed between field parameters in terms of RQD, percent recovery or axial strain versus index parameters and strength test results.