Cavern Design Philosophy under High Stress Rock Condition a Case Study in Sri Lanka

For stabilizing of rock cavern stress condition and orientation, rock properties and cavern geometry have determinant effects. Numerical and analytical study and knowledge about self-support arching can assist to designer in appropriate understanding of cavern and ground interaction. The comparative studies by E. Hoek, showed that the mushroom shaped cavern was not an acceptable design because of high value stress concentration. The elliptical cavern provide best stress distribution. In this research 5 basic concept in a case study cavern design under high stress condition has been studied. Orientation of cavern and cross section shape have a dominant influence in stress redistribution and forming self-support arch, respectively. Very small tangential stress may create radial crack in periphery of the opening. If the rock mass is hard and brittle, rock burst problem may arises in the area of high compressive stress when the rock mass strength is lesser than the imposed compressive stress. If the strength of rock is low compare to the stresses on it. Instability problems may arise after few years of excavation. Uma Oya underground hydro power project has chosen as a case study. High value of horizontal stress (k>1) might cause buckling effect in walls. Result indicates that high value of horizontal stress might cause local tensile spalling. Plastic zones extend to the pillar between to caverns more than 30 to 1.5 m in the roof. Even after rock bolting, there is considerable tensile zone. About 5 m deep tensile zones are calculated in the walls and a tensile zone of 2 m deep in the roof. The thickness of arching are calculated 4.8 to 5.5 for power house and 1.6 to 1.7 m for transformer cavern, via FEM analysis.