Experimental study on hardening effect of creep in rockfill material

The excessive long-term deformation may lead to crack and damage problems in high earth-rockfill dams. The creep effect of rockfill is crucial for the long-term deformation of rockfill dams. The generally applied creep models are mainly based on the single-stage creep tests, without taking account of the coupling effect of creep and loading during multistage processes. This study improves the triaxial apparatus by developing a stress controller to maintain stable axial stress for long durations. The improved apparatus achieves the switch between strain-controlled mode for loading and stress-controlled mode for creep. Then, a multistage loading-creep coupled test method is proposed, providing a clear determination of the creep starting point. A series of multistage loading-creep coupled tests are specially designed and conducted for rockfill materials which are crushed weakly-weathered granite from the Nuozhadu Rockfill Dam. The test results indicate that the multistage loading-creep processes show apparent cyclic and piecewise characteristics. The creep strain is found to have a hardening effect similar to the loading-induced plastic strain, making the post creep loading behave as a reloading elastic process first and then an elastoplastic process. It is further shown that the hardening effect is an inherent property of creep of the rockfill and can be interpreted by the classical elastoplastic theory.