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To obtain time-dependent responses of rock in excavation engineering projects, a series of creep tests of dacite are conducted through stepwise unloading of confining pressure. Based on the test results, time-dependent behaviour and unloading failure mechanism of dacite are studied in terms of creep rate, failure mode, long-term strength, and time-dependent damage. According to the creep rate, creep deformation can be divided into decelerating creep stage, steady creep stage, and accelerating creep stage during the last unloading failure process. Time-dependent deformation exhibits an anisotropic characteristic. Local shear bands are found in failed samples, which is markedly different from tensile failure in a short-term unloading test. Time-dependent damage of dacite under unloading condition is the result of combined effects of cracks. Creep failure of dacite under unloading condition caused by concentrating in a local zone and coalescing to form one or more macroscopic shear bands. This study gives an insight to understand the mechanism of unloading creep failure of rock and provides an experimental foundation for developing a time-dependent constitutive model under unloading condition.

The time-dependent behaviour and failure mechanism of dacite under unloading condition