From Subsidence to Uplift in the Kłodawa Salt Mine: A Zero-Vertical-Velocity Zone Linked to Deep Convergence

Rock-salt mines deform through chamber closure and vertical translation of the surrounding rock mass, yet these responses are commonly evaluated separately. This study integrates underground levelling, convergence monitoring, and mine geometry to identify the depth at which vertical motion changes from subsidence to uplift in the Kłodawa Salt Mine, Poland. The database comprises levelling campaigns conducted between 1962 and 2021 and 337 convergence series containing 7671 observations. Annual vertical velocities were calculated between actual survey epochs. The zero-vertical-velocity zone (ZVVZ) was estimated between adjacent measured levels with opposite median velocities; bootstrap resampling and mutual-nearest benchmark pairing quantified sampling uncertainty and local consistency. No transition was bracketed within the monitored 450–630 m interval in 1987–1992. In 1992–1994, shallow levels continued to subside while deeper levels uplifted, yielding crossing depths of 570.4 m in Field 1, 514.4 m in Field 2, and 476.8 m in Field 3. Field 1 retained the reversal in the 1994–2021 average, with a crossing near 585.3 m, although that long interval cannot resolve intermediate changes. Independent, mostly later convergence records show a marked increase in closure below 690 m and increasing horizontal dominance with depth. The ZVVZ is therefore a spatially variable kinematic boundary within a mining-modified deformation field, not a strain-free horizon: roof descent and floor uplift may generate rapid closure while mean vertical translation remains near zero. The method provides a reproducible monitoring state for deep salt mines. Future work should re-adjust the historical levelling networks, acquire synchronous levelling and convergence data at 690–780 m, and test excavation, geology, and backfilling controls using a calibrated three-dimensional viscoplastic model.