Abstract:

The unconventional failure phenomenon of rocks，such as slabbing and rock burst，occur frequently in deep engineering，which bring huge financial loss and casualties.The occurring mechanism of the un-conventional failure is still unclear，and the stress state and levels of rocks is the main factors inducing unconventional failure.The surrounding rocks of deep engineering is under a one-dimensional，two-dimensional or three-dimensional stress conditions，corresponding to the stress state of the rock specimen in the uniaxial，biaxial or true triaxial compression tests，respectively.In this study，the uniaxial，biaxial or true triaxial compression tests were carried out to reveal the occurring terms of the unconventional failure of rocks.The isolated ore pillars left after room-pillar mining are usually under one-dimensional compressive stress.Through uniaxial compression tests of rock specimens with different height-width ratios and width-thickness ratios，it is found that the strength of rock specimen exhibits obvious characteristics of non-linear changes as the height and width of rock specimen increase.As the thickness increases，the uniaxial compressive strength increases.As the height increases，the uniaxial compressive strength tends to increase first and then decrease.This is mainly related to the slabbing failure of the dwarf and thin rock specimens.Combining the uniaxial compression test of cuboid rock specimens and the reduction of rock mass mechanical parameters，the safety evaluation of the isolated pillars can be conducted.Based on the fact that the surrounding rocks of underground engineering are often under a two-dimensional stress state，and the fact that rocks far away from the boundaries of underground engineering are often under a three-dimensional stress state，the biaxial compression and true triaxial compression tests of cubic rock specimens have been carried out.The rock strength in biaxial or true triaxial test shows obvious intermediate principal stress effect.Under the same minimum principal stress，the biaxial compressive strength and true-triaxial compressive strength of rocks increases first and then decreases with the increase of the intermediate principal stress，and the change rate of biaxial compressive strength decreases firstly and then shows a little increase.An exponential rock strength criterion was constructed by defining the strength increment parameter ν and the intermediate principal stress position parameter λ.It is considered that low confining pressure limitation，asymmetric confining pressure limitation and short crack propagation path are the main terms for unconventional failures of rocks under multi-dimensional stress conditions.

Key words: hard rock, stress state, stress level, strength characteristic, fracture mode, intermediate principal stress effect