Influence of Overburden Rock Characteristics on Stress Arching and Surface Subsidence Over Salt Caverns

The urgency for energy transition from non-renewable to renewable sources has highlighted the importance of salt caverns for storing gases and disposing of nuclear wastes. While salt caverns are valuable for storage due to their low permeability and self-healing features, they present challenges, notably creep deformations in cavern walls leading to surface subsidence. Research in this field has predominantly focused on precisely determining the creep properties of rock salt through advanced laboratory tests and developing sophisticated constitutive models to simulate salt rock behavior numerically. However, the properties of the overburden rocks are often overlooked, with their numerical modeling typically relying on simpler constitutive models, such as the isotropic linear-elastic model, or being represented merely as a uniform surcharge equivalent to the weight of the overburden rock at the top of the rock salt deposit. Nonetheless, the arching of stresses above salt cavern fields is heavily influenced by the properties of the overburden rocks, indicating that these properties are probably influential on the geomechanical behavior of salt cavern fields. To date, research on the influence of overburden rock properties on the geomechanical behavior of salt cavern fields remains unexplored. This technical note aims to shed light on this issue by examining the impact of overburden rock properties on stress redistribution and surface subsidence above a hypothetical cavern field. A 3D Finite Element Method model simulating the creep process for a field of 36 caverns within a rock salt deposit was developed. The findings reveal that the stiffness of the rock mass significantly influences the arching of stresses above this hypothetical cavern field. The more competent the overburden rock mass, the greater the stress relief in the mining area, and the wider the unmined area receiving the redistributed stresses. Thus, the differential stresses within the rock salt deposit in the mined region are reduced as the overburden rock mass becomes stiffer and stronger. Since creep deformations heavily depend on differential stresses, the properties of the overburden rock mass significantly influenced both the rate of cavern convergence and the rate of surface subsidence of the hypothetical cavern field over time. The results presented in this technical note highlight the potentially crucial role of overburden rock properties in the geomechanical behavior of cavern fields and suggest that future research should carefully consider the behavior of overburden rocks in numerical models of cavern fields.