In the Baglihar Hydroelectric Project in the Himalayas a powerhouse and transformer house were excavated for Stage I of the project. The Stage I excavations seemed to have influenced the initial stress field and deformability properties of the rocks encountered in Stage II, which has been proposed in close proximity to the Stage I excavations. During detailed investigations inside the exploratory drift to the proposed Stage II powerhouse the measured principal horizontal stress magnitudes were found to be very low as compared to the rock cover and deformability parameters; these values were also lower than the values found in Stage I investigations. The orientation of the maximum horizontal stress was however found to be same as measured in Stage I which was considered to determine the orientation of the long axis of the powerhouse. This paper examines the effect of Stage I construction on stress levels and deformability properties found in the exploratory drift for Stage II; the analysis used the 2D finite difference numerical analysis method backed by field data. The analysis has provided an understanding of the impact of Stage I excavations and redistribution of stresses around the exploratory drift in Stage II and explained the low values of the field data.
The Baglihar Hydroelectric Project is located on the river Chenab in Ramban District of Jammu & Kashmir. The completed underground power house of the Baglihar Hydro Electric Project Stage I is now generating 450 MW power. The powerhouse complex of Stage II is proposed to be located 180 m downstream of the Stage I powerhouse and is located inside a hillock rising to a maximum EL 1150 m to generate 450 MW of hydro power. The rock formations present in the hillock consist of hard quartzite with occasional slate partings, phyllitic quartzite and slates. [1, 2, 3, 4] For the design of the Stage II underground powerhouse a main access tunnel was excavated at El 708.3 m for carrying out different rock mechanics investigations including in-situ stress and deformability parameters. The stress measurements by hydrofracture method and deformability measurement by pressuremeter method were carried out between RD 105 and RD 120 which was intersected with very poor quality rocks. A low stress magnitude was evaluated even at high rock cover due to the fact that the fractured rock cannot hold high stresses. The deformability modulus was also found to be very low if we compare it with the deformability moduli determined at different components of the Stage I of the same project. The low values of both stress and deformability at Stage II were assessed by numerical modeling and also by developing a correlation coefficient from the data available from Stage I.
2. GEOLOGY OF THE STUDY AREA
A 121 m long and 6 m high arch shaped main access tunnel has been made along the centre line of the power house cavern at El. 708.3 m. The main access tunnel has been excavated largely in grey quartzite with occasional slate partings as well as slate bands.