Assessment of excavation damage and spalling potential at a mechanized longwall face: a numerical modeling study

Abstract

Abstract: Proper investigation of the face spalling mechanism with its associated rock mechanics indicators is crucial for effective ground control in deep longwall workings, particularly under massive strata conditions comprising high strength and large thickness. This paper deals with the three-dimensional elastoplastic modeling study to understand the damage and associated spalling characteristics in a retreating longwall working. The stress–strain redistribution was simulated with progressive face retreat till the occurrence of the first weighting. The quantification of the various rock mechanics indicators viz. major principal stress, axial strain, and lateral strain were presented in the three distinct face loading conditions, namely normal, peak-stress, and weighting. The excavation damage concept was implemented to quantify the extent of EDZ (excavation damaged zone) and HDZ (highly damaged zone) at the face. The role of the massive key roof in differential loading along the length of the face was also examined. Finally, a set of design criteria for the three-dimensional assessment of the extent of face spalling was proposed integrating the results of the numerical modeling and field observations. The study revealed that the central section of the face experienced a higher intensity of damage than the gate end section when the face was exposed to extreme stress conditions. The model results indicated that 52% of the face length extending over 130 m in the central section of the 250 m long face was affected by spalling. The spalling was initiated at the middle position of the 3.5 m high face and extended to its top position affecting the 0–3 m region ahead of the face. The study also showed that the extent of spalling during the first weighting varied between 24 to 40% of the zones within the affected region. The model-observed extent of face spalling was validated against the field observation. The modeling results also agreed well with the redistribution pattern of induced stresses, differential load transfer at the face, and the mechanism of damage in the overlying strata as well as the coal face generally observed in the field. The findings of this study present a unique perspective for the rational design of face length for effective roof control in deep longwall workings. Article Highlights: A numerical modeling approach was developed to quantify the extent of face damage and associated spalling in longwall workings.A design criterion was developed for the assessment of face damage based on the profile of stress-strain redistribution.The proposed criteria were used to quantify spalling and associated damage at a longwall face in three-dimension, and its results were compared with the field observation. © 2021, The Author(s), under exclusive licence to Springer Nature Switzerland AG.