The paper describes the results of stress perturbation in a large hydroelectric project and its impact on the design of the powerhouse of the project. The direction of the principal horizontal stresses (sh) at the desalting Complex is measured as N30°E which is quite consistent with the World Stress Map direction in the North Eastern quadrant. This direction is more or less normal to the Main Central Thrust which is the dominant geological structure and is within 2 Km from the proposed location of the desalting chamber. However the power house data indicates that sh has a direction of N40°W which is around 10 km downstream of the desalting chamber. This 70° rotation of the regional stress has been attributed to the nearby fault which might have perturbed the stress. We tried to understand the phenomena by the application of numerical modelling. Based on 2-D distinct-element modelling, we reconstructed the local stress field inside and around the fault in actual geological context. The resulting stress distribution reveals that major directional stress changes occur around the surrounding and stress deviations can reach values as large as 66 °. We have established simple relationships controlling stress changes around a preexisting fault zone as a function of
the remote stress ratio σ1 and σ3
the friction coefficient on the discontinuity, and
the strike of the discontinuity relative to the far-field stress.
The above studies confirm that a major geological structure can considerably influence the stress setup around it. Any major structures designed within this perturbed stress due to the geological structure may have to be designed accordingly as the case with the powerhouse of the Pipalkoti project
The geometry, shape, dimensions, excavation sequence and orientation of underground structures are dictated by the in-situ stress tensors around the proposed structures. Over a considerable stretch within a particular tectonic regime the stress tensor is found to be uniform until and unless perturbed by any geological structures. Though considerable advances have been made in the measurements of in-situ stress over the past 25 years the interpretation of these measurements with respect to geological structures have received significant attention only over the past 10 years. From different hydro and mining sites all over the world a large number of observations implicating the rotation of stress orientation as much as by 90° near geological structures are reported. A number of investigators have also postulated various theories to explain the phenomena. Jamison [1] observed rotation of stress by 25° in the central Sierra Nevada Mountains and by 45° at the Nevada test site compared to stress direction along the San Andreas fault based on hydro fracturing tests and focal mechanism. Aback [2] presumed that a strength contrast between a low frictional resistances of fault inside a strong crust is responsible for rotation of stress in the San Andreas Fault in Central California. Chandler and Martin [3] found stress rotation from 40° to 90° in Underground Research laboratory, Canadian Shield by biaxial overbore tests.
