Experiments on mechanical properties of salt rocks under cyclic loading

The primary purpose of underground gas storages is to provide gas for seasonal consumptions or strategic reserve.The periodical operations of gas injection and extraction lead to cyclic loading on the walls and surrounding rocks of gas storages.To investigate the mechanical behaviors of different host rocks in bedded salt deposit,laboratory experiments were conducted on the samples of rock salt,thenardite,glauberite and gypsum.The mechanical properties of rock samples under monotonic and cyclic loadings were studied.Testing results show that,under monotonic loading,the uniaxial compressive stress(UCS) of glauberite is the largest(17.3 MPa),while that of rock salt is the smallest(14.0 MPa).The UCSs of thenardite and gypsum are 16.3 and 14.6 MPa,respectively.The maximum strain at the peak strength of rock salt(halite) is much greater than those of the other three rocks.The elastic moduli of halite,thenardite,glauberite and gypsum are 3.0,4.2,5.1 and 6.8 GPa,respectively.Under cyclic loading,the peak strengths of the rock specimens are deteriorated except for rock salt.The peak strengths of thenardite,glauberite and gypsum decrease by 33.7%,19.1% and 35.5%,respectively;and the strains of the three rocks at the peak strengths are almost the same.However,the strain of rock salt at the peak strength increases by 1.98%,twice more than that under monotonic loading.Under monotonic loading,deformation of the tested rock salt,thenardite and glauberite shows in an elastoplastic style.However,it changes to a ductile style under cyclic loading.Brittle deformation and failure are only observed for gypsum.The results should be helpful for engineering design and operation of gas storage in bedded salt deposit.