Determination of the minimum thickness of crown pillar for safe exploitation of a subsea gold mine based on numerical modelling

Sanshandao gold mine, located at the east coastline of Bohai Sea in the Shandong Province, is the first subsea metal mine in China. Since the mining activities are carried out under sea, it is of vital importance to maintain the stability of the crown pillar and to keep the sea water out from the excavations. In this paper, the minimum required thickness of crown pillar is determined based on 3D numerical modelling and analysis. A realistic geometric subsea gold mine is modelled by integrating the usage of SURPAC and FLAC3D. The numerical analysis is carried out by FLAC3D, in which the influences of sea water pressure as well as mining sequences have been considered. The distributions of the principal stresses, displacements, plastic zones and pore pressures in the crown pillar are obtained by simulating the cut-and-fill stoping method at different excavation levels (above level −115 m). The field displacement observation shows that the vertical deformation rate of crown pillar is smaller than 0.023%. It reveals that the reserved safety factor is about 1.43 when using cut-and-fill stoping method from level −165 m to −115 m in the subsea gold mine. The mining activities may extend to level −95 m according to the numerical analysis results. A four-year-field practice shows that the numerical analysis is helpful to determine the minimum crown pillar thickness in the challenging subsea gold mine.

► The first subsea metal mine in China is reported under safe exploitation.
► The subsea gold mine is geometrically modelled by integrating SURPAC and FLAC3D.
► The influences of sea water and mechanical properties of wet rocks are discussed.
► The minimum thickness of the crown pillar is numerically determined.
► The mining activities extend from level −165 m to −115 m and may rise to level −95 m.