Creep And Fatigue Properties Of Salt Rock Under Long-Term Cyclic Loading And Its Engineering Application

Due to the advantages of low permeability,stable mechanical property,excellent damage self-healing ability and solution mining,salt rock is deemed as the ideal medium for the construction of compressed air energy storage.As one of the most important mechanical properties of salt rock,creep characteristic is an important factor affecting the long-term stability of compressed air energy storage.During the long-term operation of compressed air energy storage,due to the characteristic of periodic gas injection and production,the surrounding rock of the storage will subject to the effect of cyclic loading.However,the duration of gas pressure variation in the storage is relatively long,the cyclic loading and unloading frequency acted on surrounding rock of the storage is low.Therefore,the surrounding rock deformation process of the storage is a creep process under low-frequency cyclic loading.In this process,on the one hand,the gas pressure change in the storage will alter the stress state of the surrounding rock and affect its creep property.On the other hand,cyclic loading and unloading effect will also cause fatigue damage to the surrounding rock.It is surely predictable,due to the creep and fatigue coupling effects,this deformation process will become extremely complicated.Based on this,the creep and fatigue properties of salt rock under long-term cyclic loading and its engineering application were systematically studied in this paper.The main research contents and results were described below:（1）Uniaxial compression test under different stress loading rates and triaxial compression test under different confining pressures were carried out on salt rock,the influence of stress loading rate on peak stress,strain at peak point,elastic modulus,Poisson ratio,macroscopic failure mode,energy evolution characteristic of salt rock were studied,the cohesive force and interior friction angle of the salt rock used in the test were obtained.（2）Creep and unloading rebound test under different axial stresses were conducted on salt rock,the influence of axial stress on the mechanical characteristic index of salt rock,such as creep strain and creep rate,were analyzed,the long term strength of the salt rock used in the test was determined,the creep damage evolution process of salt was discussed.On this basis,The total strain of salt rock was divided into instantaneous elastic strain,instantaneous plastic strain,viscoelastic creep strain and viscoplastic creep strain,the proportional relationship between these four part strains was analyzed,and the influence laws of axial strain on these four part strains was revealed.（3）High frequency cyclic loading fatigue test under different maximum cyclic stresses were performed on salt rock;meanwhile,the micro structural variations in salt rock before and after the test were monitored using scanning electron microscope（SEM）and nuclear magnetic resonance（NMR）system.The influences of maximum cycling stress and cycle of numbers on the macro characteristic index of salt rock,such as stress-strain curve,elastic modulus,cumulative irreversible deformation and irreversible deformation rate,and micro structural parameters,such as pore size,porosity,permeability,tortuosity,were analyzed,the damage evolution was characterized by pore growth,and then the fatigue damage evolution equation of salt rock based on meso structure was established.（4）Low frequency cyclic loading creep test under different maximum cyclic stresses were carried out on salt rock,the change laws of macro characteristic index of salt rock,such as stress-strain curve in a single cycle,elastic modulus,creep strain,creep rate,cumulative irreversible deformation and irreversible deformation rate,with maximum cycling stress,cycle of numbers and time were studied,the results from low frequency cyclic loading creep test,constant loading creep test and high frequency cyclic loading fatigue test were compared,the similarities and differences of mechanical properties of salt rock under these 3 different test conditions were also discussed.（5）Based on the theory of component combination creep model,a new 7-element nonlinear creep model describing the whole creep process of salt rock were proposed,then the nonlinear creep model was transformed to a nonlinear fatigue model reflecting the fatigue deformation behavior of salt rock according to the“homology”of fatigue characteristic and creep characteristic of rock.On this basis,a nonlinear constitutive model of salt rock considering creep fatigue coupling effect for salt rock under low frequency cyclic loading was established,and the secondary development of the model was realized based on the on the secondary development platform of FLAC^3Dsoftware,and the computer application program of the model was obtained.（6）The change laws of surrounding rock deformation and volume loss of compressed air energy storage with operation time were analyzed using the FLAC^3Dsoftware,the minimum storage pressure,maximum storage pressure and pillar width of the compressed air energy storage under the simulated formation conditions were determined.