Experimental Study On Creep Properties Of Deep Siltstone Under Cyclic Wetting And Drying And Its Constitutive Models

The change of moisture content usually occurs in deep rock during excavation and retaining of underground engineering,which results in unfavorable effect on the long-term stability of deep rock.Siltstone from-906 m of Zhujidong coalmine in Huainan is taken as mainly research object.Laboratory tests choose cyclic wetting and drying to simulate change of moisture content,and carry out creep tests on siltstone under cyclic wetting and drying to investigate the influence of moisture content on the long-term stability of deep rock.It provides scientific evidence for the excavation and supporting of underground engineering.The results are as follows.A series of uniaxial compressive test of rock are carried out.The trend of strain-stress curve is shifted up hollowly when rock is in compaction phase.Then,the elastic stage occurs and the strain approximately follows a linear relationship with the stress.Soon,there occurs plastic yielding stage which has a shorter period of time.The sudden brittle failure of specimen occurs after it reaches peak strength.From observation of failure mode,it can be found that the main failure surface parallels with axial of specimen.What's more,there are a small of local shear failure.The uniaxial compressive creep tests of siltstone under cyclic wetting and drying on the siltstone were carried out.The test results show that axial instantaneous deformation occurs when the specimen is applied with instantaneous stress.And then the slope of curve increases with the increase of stress.It shows transient creep and steady-state creep at this stage.With increasing of axial stress,the creep property of specimen is obvious gradually.When the loading reaches the last stair,axial deformation increases gradually with the time going.And the creep curve shows three stage of creep properties obviously,which are transient creep stage,steady-state creep stage and accelerated creep stage.Finally,specimen is failure at accelerated creep stage.With the increase of cyclic wetting and drying,all of instantaneous strain and creep strain of siltstone increase,and the axial steady creep rate has a nonlinear increase.the whole time for creep failure of siltstone decrease gradually,and the proportion of transient creep stage and accelerated creep stage in the whole time increase gradually.The instantaneous deformation modulus of siltstone decreases.The failure characteristics of siltstone present a transformation from tension failure to shearfailure.The triaxial creep test of siltstone under six wetting and drying cycles is carried out with the confined pressure.It can be found that with the increase of confined pressure,the instantaneous strain,creep strain and its rate of siltstone gradually decrease under the same stress,the instantaneous deformation modulus increase gradually.It shows that confined pressure has a restraining effect on the deformation of siltstone.According to characteristics of the uniaxial compressive creep curve of siltstone under cyclic wetting and drying cycle,the generalized kelvin chain model is selected to describe creep properties.The creep parameters under different stress levels are obtained by using the Origin software,and by substituting parameters into the creep equation of generalized kelvin chain model,it is found that the experimental data and model curve can fit well,which shows the model can describe the creep properties of deep siltstone after cyclic wetting and drying.A nonlinear viscous element is proposed in light of the creep mechanism of rock.A nonlinear viscoelastic body which can describe the acceleration creep property is presented by a nonlinear viscous element is in parallel with the existing SAN venan body.And,combining with the generalized kelvin chain model,a nonlinear viscoelastic plastic model is built which can fully reflect the three stages of rock creep.By using the creep test data to identify the model,the results show that the nonlinear viscoelastic creep model can describe the creep properties of siltstone under cyclic wetting and drying.