Analysis of ground movements in the post-mining area of the lignite mine “Babina” (W Poland)

Post-mining areas require constant monitoring due to the risk of secondary ground movements of a continuous or discontinuous nature. This study focuses on the analysis of ground movements in the post-mining area of the “Babina” mine in Western Poland. The complex nature of the “Muskau Arch” glaciotectonic area, subjected to both underground and open pit mining, further complicates the understanding and monitoring of these movements.

The study site was subjected to precise levelling monitoring between 2020 and 2023, and the results of five measurement campaigns form the basis of the analysis, with the first measurement being the reference for the subsequent ones. For this purpose, a geodetic monitoring network was established to cover key locations in the “Babina” mine area. The network was made up of 99 control points set-up as concrete pillars with metal control bolts and was connected to 4 reference benchmarks of the national levelling network situated beyond the post-mining area. In addition, 7 local research networks have been established in specific study sites. The levelling measurements were carried out using geodetic precision levelling. The total length of the levelling lines was approximately 36 km. The ground movements recorded between the first (initial), carried out in May 2020, and the last measurement campaign, in September 2023, range from -5.58 mm to +4.97 mm. Ground movements in the specific dense networks range from -39.46 mm to +115.39 mm.

Three interpolation methods were used to obtain continuous maps of the ground movements from discrete levelling observations: Inverse Distance Weighted, Radial Basis Function and Ordinary Kriging. The latter technique produced the smallest root-mean-square errors assessed with the cross-validation technique. In the result, we obtained 24 maps representing elevation changes between the first and the last measurement, as well as maps representing movements between consecutive measurements.

The maps of ground movements were used to identify areas of statistically significant displacements and to analyse the evolution of these displacements over time and in relation to the extent of past underground and open-pit mining activities and current land use. The results indicate present-day activity of the ground in the post-mining area that varies in the magnitude and direction of movements in different parts of this complex area. The observed seasonal fluctuations may indicate relationship of benchmark movements with periodic change of ground water level and the effect of climate change.

Our findings confirm the presence of ground movements in post-mining area five decades after the end of mining activity and of varied nature, as well as substantiate the need for further investigation of this activity in the study area. These should include detailed analysis of the relation between groundwater level and benchmark heights and correlation of movements with the extent of shallow underground mining.

The research has been financed from the OPUS National Science Centre projects grant no. 2019/33/B/ST10/02975 and grant no 2021/43/B/ST10/02157.