Comparison of Intact Rock Strength Criteria for Pragmatic Design

This paper compares several intact rock strength criteria against an extensive database comprising 128 data sets with over 7,000 laboratory test results. It includes over 1,000 new test data from several recent tunneling projects in Australia. The database includes sedimentary, igneous, and metamorphic rocks with unconfined compressive strength (UCS) values ranging from less than 10 to over 300 MPa. The criteria assessed are Modified Weibols-Cook, Lade, Hoek-Brown and two of its modifications, and Christensen. Several interpretations and conclusions are drawn. Any of the criteria can provide good fits to test data that do not show much scatter. However, curve-fitting becomes problematic when there is variability in the test data. The variation in test data swamps the differences between criteria. On balance, the Hoek-Brown criterion, with its parameters m(i) and a derived as recommended and extended to three dimensions in the literature where required, produces a better fit to the majority of rock types considered. (C) 2016 Society of Civil Engineers.