A method for calculating generalized relaxation modulus for different rock types

To assess the long-term stability of underground structures, it is crucial to understand the time-dependent behaviors of different rock types. Creep and stress relaxation are typical behaviors of generalized relaxation, which can explain temporal changes in rock stress and strain. In this study, generalized relaxation tests for Class I and Class II rocks were conducted to analyze their time-dependency behavior. A method was proposed for calculating the generalized relaxation modulus (GRM), which may be regarded as an extension of the stress relaxation modulus in linear rheological theory. Experimental results showed that GRM does not depend on α, and its value under uniaxial compression was larger than that under triaxial compression, indicating an obvious confining pressure effect. GRM is inversely proportional to stress level but strongly positively correlated with tangential Young’s modulus. The generalized relaxation compliance (GRC) also was considered, and the GRC/GRM ratio was taken as the local stiffness coefficient of rocks. When the stress reached the yield strength, the local stiffness of the rock decreased before failure. This ratio can predict the long-term strength of rock masses and stability of underground engineering.