Surface Deformation Assessment in the Bulqiza Chrome Mine Using Empirical and Numerical Modelling

INTRODUCTION

Underground mining frequently leads to surface instability such as subsidence, sinkholes, and landslides. In the Bulqiza chrome mine in Albania, decades of extraction and the transition from cut-and-fill to sublevel stoping have increased rock-mass deformation, resulting in fissures, caving, and surface failures. This study focuses on Profile XIV, where both continuous subsidence and a sinkhole are present, in order to evaluate the accuracy of predictive methods used to assess mining-induced deformation.

AIM

This study aims to assess the surface impacts of underground mining in the Bulqiza district by applying both empirical subsidence modelling and numerical simulations using Finite Element Methods. The study compares predicted results with observed deformation, evaluates the influence of caved zones (goaf) and tectonic structures, and verifies the suitability of using a combined empirical and numerical approach for deformation assessment.

METHODS

Geological and mechanical properties were defined through field investigations and archived mine data. An empirical model with a subsidence coefficient of K = 0.9 was used to calculate the critical collapse depth (Hcal) and compare it with the effective mining depth (Hfac). Numerical simulations were then performed with the Rocscience FEM software for two surface-deformation profiles: one exhibiting continuous subsidence and the other featuring a surface sinkhole. Each profile was modelled under different conditions, including the presence or absence of goaf and the inclusion or exclusion of tectonic influence. Surface displacement was used as the main indicator for assessing deformation.

RESULTS

The empirical model indicated a low likelihood of funnel formation in the subsidence profile, where Hcal was smaller than Hfac, while in the sinkhole profile, Hcal exceeded Hfac, confirming a high probability of collapse consistent with field observations. Numerical modelling supported these findings. In the subsidence profile, vertical displacement remained small around 14 mm regardless of whether the goaf was included, and no funnel formation was predicted. In the sinkhole profile, displacement increased to 24.3 mm when the goaf was considered without tectonics. When tectonic effects were included, displacement increased substantially to values between 40.4 and 61 mm, closely reproducing the actual sinkhole conditions. These results show that tectonics strongly amplifies surface deformation.

CONCLUSIONS

This study demonstrates that both empirical and numerical methods effectively reproduce the types and magnitudes of surface deformation observed in the Bulqiza mine. Numerical modelling closely matched actual conditions, particularly when tectonic effects were incorporated. While goaf conditions had little effect in the subsidence zone, they significantly increased deformation in the sinkhole area. The findings confirm that tectonic structures are a major factor controlling surface collapse and that a combined empirical and numerical approach provides a reliable method for assessing mining-induced surface impacts in Bulqiza and comparable underground mining environments.