Excavation of large diameter and deep vertical shafts in poor and extremely poor geological conditions: A case from Tehri Dam Project, India

摘要

The prestigious multipurpose mega-hydroelectric project i.e. Tehri Dam Project envisages generation of 2000 MW of hydroelectricity, in two stages, by constructing a 246.2m high earth and rock fill dam (highest in Southeast Asia and fourth highest in the world), at Tehri, across river Bhagirathi in Uttarakhand state of India. The first stage of this project (IC 1000 MW) has already been commissioned in 2006 and on completion, of both the stages, the Tehri hydropower complex will be able to provide 6200 million units of annual energy (Peaking). An intricate network of underground openings (Tunnels, Adits and Shafts) at the project site aggregates to 30 kilometers of tunneling. The paper, however, focuses on the geotechnical problems encountered during the excavation of the two vertical shafts located on the right bank, connected to the two diversion tunnels and two control gate shafts connected to the head race tunnels on the left bank, which are located in poor to extremely poor geological regime. The emphasis is however on the concept of influence of geometry (particularly reverse curve) on the stability of the structures. Two large diameter (each of 14m excavated diameter) and 230m deep vertical shafts, connected with the right bank diversion tunnels T-3 and T-4 respectively were constructed so as to function as shaft spillways. These vertical shafts join eccentrically (by 6m) with the tunnels at lower level, through a swirling device, which imparts a swirling motion to the flow in the tunnels for energy dissipation. These shafts were provided with the 9m diameter de-aeration ducts and 26m long, llm deep separation chambers to release the air separated from the air-water mix. The poor geological conditions (with rock mass of Q value as low as 1.33, encountered in T-3 shaft) and the intricate design of the structure posed a stiff challenge during the full widening of these shafts. Because of the reverse curve geometry, the junction of the main shafts with the de-aeration ducts, was the area of high stress concentration and failures were more frequent in this zone throughout the length of the shafts. The lowest por- tion of the shafts i.e. the swirling device area where the slant profile in the rock created overhang areas was another critical area. Similarly the junctions of Intermediate Level Outlet (ILO), for T-3 shaft and that of construction adit, for T-4 shaft were also vulnerable. sequence of excavation and the support system was revised as per the changing geological conditions and the work was completed without facing major hurdles. The construction of four control gate shafts viz., CGS- 1, CGS-2, CGS-3 and CGS-4 was taken up from the platform (at E1840m) on the left bank. These 110m deep shafts of excavated diameter of 13m between El±840m and El±830m and 11m below El±830m are connected to the four head race tunnel (HRT) at El±729m. During the course of geological investigations a 35m wide and extensive deformed/tectonised zone was delineated, which occupied a huge structural wedge confined within two major shears one longitudinal (L-11)shear and one diagonal (D-3) shear between El±910m-El± 835m on the cut slope and on the platform at El 840m (CGS area). In view of the extremely adverse geological conditions few innovative steps like multiple drifting through the deformed zone in the shaft and five tiers of concrete pile shafts in the area around the control gate shafts were resorted to.