Time-delayed deformation in rock is a common phenomenon which complicates interpretation of laboratory tests and creates uncertainty when results from laboratory tests are applied in the field. It is therefore desirable to include such effects in constitutive models used to describe rock behavior. One such model is the FORMEL model, which was developed to describe relationships between static and dynamic moduli of sandstones. The model has primarily been applied for prediction of stiffness and strength based on log data. In this study, we have introduced creep in the FORMEL model by recognizing that local failure events which induce non-elastic deformation may trigger new events of local failure and subsequently more non-elastic deformation, and assuming that each cycle in this chain of events has a fixed and finite duration. The resulting model possesses all basic features of delayed deformation – transient creep and possibly steady state and accelerating creep – depending only on the stress level. This extension of the model also introduces strain rate effects.

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