Study on significance of pillar dimensions on stability of caverns in rocks with high overburden

The present study aims to examine the influence of pillar widths on the stability of caverns. Case study considering two caverns viz. Powerhouse Cavern (PHC) and Transformerhall Cavern (TC), in a major hydro-electric project in the eastern Himalayas is considered. 2D and 3D numerical analysis was carried out for w/B ratios 1.5 and 3.0 respectively. Primary aspects like major principal stress and development of plastic zone were investigated for the two pillar widths. An optimum pillar width was observed that resulted in reduced stress acting along the cavern periphery, a better stress distribution, and no overlap of plastic zones between the caverns. Further, the optimum pillar width resulted in a better stress-redistribution with progress of excavation and the in-situ stress became constant at an earlier stage of excavation. Observations from comparative analysis revealed that a pillar width nearly equal to the largest dimension or twice the width of the larger of the caverns in the group resulted in a better stability and hence can be considered as the optimum width. Furthermore, the analysis suggests that along the pillar width, maximum stress was observed at mid-height, and it is more in the vicinity of the face of the caverns.