Test Study on True-Triaxial Loading and Unloading for Marble with Unloaded Single Face

To accurately evaluate rock stability around the excavation face of roadway（tunnel）, a true-triaxial unloading disturbance testing system is applied in the loading and unloading tests of marble samples, where single-face was unloaded in the direction of the minimum principal stress. Through the analysis on instability mechanism of rock around roadway excavation face with high geostress, two kinds of failure modes, including energy accumulation failure and stress concentration failure, were simulated in different loading and unloading paths. Then, stress-strain curves, and failure and strength characteristics were further analyzed. The results indicate that there is dilatancy when the stress on unloading surface decreases; the main failure surface is near the free face; moreover, with the increase of the axial load, the splitting failure area extends and the critical unloading value increases. With the increase of the confining pressure, the stress peaks and yields increase, and the curve slope between the yield and peak is reduced. The composite failure mode with local tension-splitting-shear failures develops into the overall splitting failure. Under the same confining pressure, the unloading failure strength is 80% of the loading failure strength, and the marble sample in unloading is more likely to fail than in loading. The Hock-Brown criterion is modified using the calculated fracture strength value.