Application of Flattened Brazilian Test to Investigate Rocks Under Confined Extension

A failure envelope that ignores the intermediate principal stress is typically adequate for the design of rock structures. To obtain the complete envelope, the rock cylinders should be tested using confined compression as well as confined extension tests. While confined compression tests are common, confined extension tests are rarely carried out. Current techniques available to test rocks under confined extension conditions require shaping the cylindrical specimens to a dog-bone geometry. The limited data available in the literature indicates that the results from these dog-boned shaped tests produces strengths that are considerably greater than the confined triaxial tests carried out on traditional cylindrical specimens. Whether this increased strength is real or simply an artifact of the stress path for triaxial confined extension tests (intermediate principal stress equal to the maximum principal stress) versus confined compression tests (intermediate principal stress equal to the minimum principal stress) is unknown. We propose here a new methodology to test rocks under confined extension condition with zero intermediate principal stress. We used the flattened Brazilian test and calculated the major and minor principal stresses from the strain on the surface of the specimens. To test the rock under increased confined conditions, we tested specimens with increasing depth of flattening. We derived equations to calculate the major and minor principal stresses in a flattened Brazilian specimen considering the bi-modularity in the stress–strain equations. We tested Lac du Bonnet granite specimens for major principal stress range 15–37% of its UCS and observed that the minor principal stresses remains in the range of Brazilian strength of the material (11.6 MPa). The results suggest that the nonlinear Hoek–Brown failure criterion based on only compressive triaxial results provides a reasonable estimate of the tensile strength.