Assessment of in-situ initial rock stress in underground powerhouse cavern of Karun3 dam

Karun3 is a 205 m high concrete dam constructed at 25 km to the south of Izeh in Iran. Large amount of rocks was extracted to construct necessary underground structures for establishing a 2000 MW power generator. A number of tests have been conducted to evaluate in-situ stress at the mean depth of 230 m below the surface by means of Borehole Slotter. 25 tests were performed at total of 9 holes. Stresses were measured at 6.45 m to maximum of 29.20 m from borehole collar. Analysis of the results was made using Kirsch and Hooke Laws in which the modulus of deformation had to be defined. Dilatometer tests in the same boreholes were performed in order to find the foregoing modulus. The results show that the magnitude of the maximum principal stress has been evaluated to be about 5.5 MPa. The direction of maximum principal stress is about 253 degrees. These results have been successfully used for the cavern orientation selection and the section configuration designing. The longitudinal axis of the cavern has been designed in the direction to be coincident with the maximum principal stress in order to minimize the stress concentration around the cavern.