Evaluation of safety and deformation characteristics of the secondary stope sandwiched between backfills in underground iron mines

In a large underground iron mine (over 2000, 000 t/a) in China, a large volume of mined void could not be filled in time, which posed a threat to the working face. Physical and numerical models were used to investigate the stability of the mined void sandwiched between the backfills. Digital image correlation (DIC) method was employed in the physical model to calculate the deformation field of the roof and backfills. Stress meters and multi-point borehole extensometers were installed in the mine to observe the change of vertical stress and horizontal displacement in real backfills. Principal component analysis (PCA) and cluster analysis were applied to classify the deformation state around mined void. The deformation field was exhibited by the contour and curves of displacement, which was consistent between the physical model and numerical simulation. The contour indicated that highly deformed zones were ellipse-shaped and wedge-shaped in the backfills and roof respectively. The deformation process was classified into three states (stable state, transition state, and unstable state) by using the density-based clustering. The data obtained from the in situ monitoring showed that the vertical stress increased by 60–80 kPa and the horizontal deformation was small in the backfills close to mined void. Classification of deformation states indicated that the mined void (temporary opening) remained stable during production plan.