ANALYSIS OF UNDERGROUND CAVERNS

Abstract

In the present study, it has been proposed three dimensional analyses of underground powerhouse caverns. The underground caverns are Machine hall and Transformer hall and are located in a close proximity to each other. The caverns are analyzed under in situ stress conditions. Elastic- continuum approach is chosen for the analysis of caverns. The in situ stress ratio is 0.25 (6h: 6„). The out of plane stress ratio (6Z: 6v) is also 0.25. High stress concentration is observed at the corners of both the cavities. Due to low deformation modulus of Slates underlying Dolomites and low in situ stress ratio, there are large vertical displacements in the region. Due to low in situ stress ratio the horizontal displacements are less. The excavation are made in one go and stepwise excavation is beyond the scope of the study. Due to low deformation modulus and low stress ratio, high compressibility of the powerhouse and surrounding is observed. A fmite element program called "ABAQUS" is used to analyze the given problem. After excavations it has been observed that the volume of both the cavities reduces. The walls of both the cavities come closer to each other and crown comes down and the invert moves up. The cavities are highly unstable under the combination of such in situ stresses. The cavities also experienced high shear stresses, which are greater than the shear strength of the material. The shear failure is observed mostly around the walls of the cavities. After analysis it has been observed that factor of safety for the compressive strength and shear strength, both comes out to be less than one. Thus heavy support system is supposed to be given around the cavities based on rock mass characteristics. The other rock mass classifications are derived from GSI and other rock mass characteristics. Further Elasto-Plastic analysis of the cavities is done in Dolomite region and then the results are compared with that of Elastic analysis.