## 1 Introduction  
Rock salt possesses characteristics of low porosity, low permeability, reasonable short-term mechanical strength and stiffness and a propensity to creep stably under deviatoric stresses. Rock salt is soluble in water (\(\sim\)1:7 volume ratio for salt and saturated brine), allowing caverns to be dissolved, and salt caverns possess good safety characteristics (environmental and physical security). Studies in the last seven decades have led to commissioning of dissolved salt caverns for the storage of liquids, gases and even solid wastes [1; 2; 3; 4; 5]. In several countries (USA, Germany, etc. ), salt mines are used to store radioactive wastes. In addition to high safety, dissolved caverns with adequate borehole connections may have large volumetric capacity, fast injection and withdrawal speeds and low operating costs [7]. Underground energy storage (oil, gas, compressed air) has been implemented in very thick salt or salt domes [8]. The USA, Germany, France, Canada and other countries have established underground oil or gas storage, used for commercial short-term or seasonal storage, or as national strategic energy reserves (as in the USA Strategic Petroleum Reserve). Because of the demand for commercial and strategic energy storage, the implementation of large-volume salt cavern underground storage in China has begun. The key to salt cavern storage security is to ensure the extremely low permeability of the rock salt so as to effectively block the leakage of oil and gas. This is straightforward in thick, clean, deep deposits. However, the rock salt in China we are working with has several characteristics, such as shallow depth, low salt thickness, high impurity content (hard gypsum mudstone, gray mudstone, salt mudstone, sandy mudstone), and so on [9; 10]. The physical and mechanical characteristics of these deposits are complex and challenging to measure. Therefore, it is an important design aspect to carefully study the layered rock salt, especially the permeability characteristics, and to develop an understanding using field data and laboratory tests to develop a model. The permeability characteristics of rock salt have been extensively studied. Field tests show that rock salt permeability is generally less than \(10^{-17}\) m\({}^{2}\)[11; 12]. [PERSON] and [PERSON] [13] created a conceptual model for far-field Salado hydrology involved permeability in anhydrite layers and at least some impure halite layers. They note that the pure and most impure salt have negligible permeability because of low porosity and a lack of porosity inter-connection. Their research indicated that in the near-field of an opening in salt rock (excavation damaged zone (EDZ)), dilation, creep and shear can increase the permeability (this should be most severe at layer interfaces between salt and non-salt rocks because of deformation incompatibility). They report typical average permeability values for anhydrite of approximately \(10^{-18}\)-\(10^{-20}\) m\({}^{2}\), and for pure halite less than \(10^{-20}\) m\({}^{2}\)[13]. [PERSON] [14] combined gas permeability and P and S wave velocity measurements under hydrostatic and triaxial loading conditions on rock salt specimens from the Gorleben salt dome and the Morsleben salt mine. Isotropic loading markedly decreased permeability, tending toward the in situ matrix permeability (<\(10^{-20}\) m\({}^{2}\)), with a concomitant wave velocity increase because of progressive closure of grain boundary cracks. The experiments show that permeability change is not only a function of dilatancy, but also of microcrack linkage [14]. [PERSON] and [PERSON] [15], using before-and-after CAT-scans on 100-mm cores, showed explicit evidence of damage as a thick external annulus of slightly higher porosity (microcracks along grain boundaries), explaining why permeability is so sensitive to isotropic stress in the laboratory.