Protective water barrier pillars (PWBPs) are inter-mine barrier pillars. An adequate PWBP can protect active mine workings from the danger of inundation from adjoining inundated workings. This paper discusses a hydro-mechanical coupled numerical modeling approach for the design of PWBPs, considering different flow regimes. The coupled model considers the effect of seepage through the roof, pillar, and floor on the mechanical strength of the rock mass and vice-versa. A statistical model based on the extent of positive volumetric strain zones (ZoPVS,

Providing a suitable protective water barrier pillar (PWBP) is common to reduce inundation hazards in underground coal mines. The imperative factors influencing its performance include the water head acting on the pillar, cover depth, pillar width, strength properties, and permeability characteristics. The mechanical failure of such pillar is a stress-controlled phenomenon, whereas the hydraulic failure is a strain-based phenomenon. A finite-difference numerical modeling approach was developed to study the hydro-mechanical coupled behavior of protective water barrier pillars. The mechanical stability was evaluated in terms of the percentage of failed (ZoF) and intact zones. The influence of the strain-controlled weakening on the permeability of the flow medium was studied through the coupling of the mechanical and hydraulic effects. The coupled steady-state model was used to estimate the outflow rate and its hydraulic stability. The adequacy of the protective pillar was also investigated by assessing mechanical stability and capability to resist hydraulic pressure against the maximum expected water head. A seepage rate-based classification system has also been proposed to evaluate the seepage potential and assess the hydraulic stability. The model has been validated for two case studies at the cover depth of 136–189.5 m and the existing pillar width of 16–42 m against the water head of 25–141 m.

A reliable design of protective water barrier pillars is critically essential for safe operation in underground mine workings. Different approaches are applied for the design of such barrier pillars, but none of them have ever evolved as a standard. This paper presents the outcome of a literature review covering various aspects of protective water barrier pillar design, including its hydro-mechanical modeling. Width/height (w/h) ratio, cover depth, water head, permeability, and discontinuities have been identified as the critical parameters influencing the performance of such protective pillars. For a given water head and the depth of cover, a lower w/h ratio of the pillar can result in an increased seepage from the pillars and inundation hazard in the worst condition. The review shows that in-depth research is required for an improved understanding of the mechanism of water flow and the failure of such pillars. This will not only help in assessing the adequacy of existing pillars but also in forming guidelines for the design of new protective pillars based on an updated knowledge base and improved understanding of the subject.

Most of the underground coal mines in the Indian Geo-mining conditions are developed and depillared by Bord and Pillar method. Caving behaviour and strata control are the major concerns to be considered while extraction of coal seams in such workings. This paper describes the results of a scientific strata control monitoring study during depillaring in a contiguous seam working. It includes instrumentation for determining the induced stresses in the pillars and roof-floor convergence in the galleries and junctions with distance from the goaf edges in the working. The mining induced stresses on the pillars were observed to be more in the top seam than that in the bottom seam working. Convergence monitoring was done with Telescopic Convergence Indicators (TCI) and Remote convergence recorders. In general, higher convergence was observed in the junctions as compared to the galleries. The junctions located at distant location suffered more convergence than the galleries is nearer to the goaf edge. Based on the field observation, convergence classification has also been done in terms of average distance of the goaf edge from the point of observation to assess the state of 'No-convergence', 'Initial convergence' and 'Maximum convergence'. The convergence observed in the bottom seam is more than that of the top seam.