The influence of monotonic and cyclic long-term loading on dynamic elastic constants of rock salt

The influence of monotonic and cyclic long-term loading on dynamic elastic constants of rock salt

The dynamic and static elastic constants of rock salt are among important parameters in design and stability analysis underground excavations such as petroleum fluids storage, natural gas storage, compressed air energy storage, necleus waste disposal caverns and pillar and excavations underground mining in rock salt. All underground excavations above mentioned, in during lifetime of operation and at the conditions of their applications are affected by long-term monotonic and cyclic loads. So, two laboratory experiments creep under monotonic load, one is equal 5 MPa for a time period of 20 days and other is 14 MPa load for a time period of 15 days. Also, three laboratory experiments cyclic instantaneous loading with one cycle per two days carried out on cylindrical salt specimens for a time period of 25 days. The specimens tested here were obtained from the Khoshkalat mine, in distance 28 Km west Garmsar city. The cylinderical core specimens have been drilled with injection of compressed air by compressor apparatus under special conditions in laboratory. All the specimens are relatively pure halite (98% halite) with a slight amount (less than 2%) of anhydrite and clay minerals. So, the specimens color is white. The average crystal (grain) size is about 0.5-2.5 mm. Two specimens (L/D = 2.5) were prepared for uniaxial creep tests and 3 specimens (L/D = 2.5) for cyclic loading tests For determining the dynamic elastic constants of rock salt, ultrasonic tests carried out on specimens before and after creep and cyclic loading experiments. The results ultrasonic tests for long-term monotonic loads tests show that the dynamic elastic constants of rock salt before tests are greater than those after tests. Also, the results ultrasonic tests for cyclic loading experiments show that elastic coefficient and shear modulus of rock salt before tests are greater than those after tests and only the Poisson ratio before tests is less than that after tests