Determining the minimum thickness of isolation layers for subsea metal mining: An analytical study

Mining submarine metal deposits poses a significant threat of seawater inrush due to the complex hydrogeological environments of submarine deposits. Designing rational safety isolation layers to serve as a barrier against the intrusion of overlying seawater is critical. This work develops an integrated analytical framework to determine the minimum thickness of isolation layer, accounting for the dual effects of subsea mining-induced disturbances and wave loading. The beam theory was applied to derive an analytical solution for the height of the water-conducting fracture zone (WCFZ) in seabed mining, and the quantitative relationship among the height of the WCFZ, rock mass strength, and mining span was revealed. Based on the theoretical model of seabed dynamic response, this study analyzed differences in seabed liquefaction ranges under various liquefaction criteria, as well as the effects of different wave conditions and seabed parameters on the dynamic response of the seabed were analyzed. The liquefaction criterion integrating mean effective stress and excess pore water pressure is preferable for conservative isolation layer design. Low-permeability sandy sediments demonstrate higher sensitivity to wave loading and are the types of seabed that require special assessment in the design of submarine isolation layers. Additionally, a design method was proposed for determining the minimum thickness of isolation layers by integrating the height of WCFZ, surface sediment layer and the protective layer. A case study at the Sanshandao submarine gold mine reveals that under extreme wave conditions, the minimum safety isolation layer thickness is 125 m, corresponding to mineable ore bodies below −135 m elevation. Compared with the current actual mining design, the results indicate that the implemented isolation layer thickness incorporates a safety margin of approximately 30 m relative to the theoretical minimum.