Visco-elastop-lastic constitutive relationship of rock and modified Burgers creep model

The classic Burgers creep model could only describe the viscoelastic behaviour of rock material before the third creep-phase,but weak rock usually was visco-elasto-plastic.So according to this shortage of Burgers model,a new plastic cell was developed based on Mohr-Coulomb criterion,which was assumed to be in absolute accordance with the plastic flow law of Mohr-Coulomb when rock failed.And then the plastic cell acted in series with the classic Burgers model,and a modified Burgers creep model was built and the corresponding visco-elasto-plastic constitutive relationships were deduced.The modified model could simulate visco-elasto-plastic deviatoric behavior and elasto-plastic volumetric behavior.In addition,some methods to solve model parameters were given and some corresponding programs were developed to deal with the test data.And the model parameters of an engineering example were fitted and the values were obtained through weighted mean ones.It was shown that the creep testing curves were coincident well with the theoretic curves,validating that the modified Burgers creep model was felicitous to characterize the creep behaviour law of rock.