The Tehri hydropower scheme in Uttar Pradesh in the Himalayas in India should incorporate a set of underground caverns for the hydropower station and at a later stage analogous amount of openings for installation of machinery of the pumped storage station. This paper deals with experience of the 3D numerical modeling and monitoring of the set of the underground structures of the first stage of the project.
At the design stage of the project more then 10 years ago 2D static and dynamic FEM analyses were performed for two most important parallel caverns - those of machine hall (MH) and transformer hall (TH) (Fink et al, 1991). STATAS/DYNAS FEM package was used for numerical modeling on available mainframe computers (Yufin 1979). For obvious reasons the geological model taken for the analysis at that time could not be so detailed as at the recent period, when excavation of the underground openings is almost completed. The results showed high sensitivity of the set of two openings to the adopted sequence of excavation/anchoring of caverns. For the case of dynamic loading the primarily importance was given to the absence of the rigid vault structures in both caverns. Importance of the thorough monitoring program for the construction period was high lined. Accounting for different design of the future powerhouse cavern of the pumped storage station in a 3D case (Barton et al, 1994) until now has no practical consequences. Recent analyses has been performed for three principal cases: 3D for real complex geological structure of the site, 3D for average uniform geological structure and 2D for one of representative cross-sections of the real 3D case. Two last cases were analyzed as the reference basis both for the main case and previous analyses of these and independent authors. Results of extensometer measurements are compared with those of numerical simulations.
The geology of the region is described by Singh & Goel (1999a). The rock mass in the vicinity of all caverns is composed prevailingly of phyllites of different degree of jointing and fracturing. The set of caverns is situated at the depth of approximately 300 m.
An exact assessment of the initial state requires the simulation of the loading history. This was obtained by superposing gravity and the stress state considered as initial stress state. The result is a zero-deformation state associated with gravity induced stress state. Initial stress Independent of gravity is accounted for in a similar way. The analyses domain is assumed to have no horizontal and vertical movements in the lower boundary and no horizontal movements on the vertical surfaces. The obtained stress state of the rock mass served as input for the subsequent stages of analysis. For the sake of a more detailed study, of the rock mass stress-strain state two series (cases) of numerical analyses have been made in the 3-D formulation of the problem. (- case 1 considers the rock mass to be homogeneous continuum).