In-situ Stress Measurement for the Design of Underground Desilting Chamber: A Case Study from Eastern Himalayas

The importance of in-situ stress measurements for underground structures has been well established in the recent years. The in-situ stress field is one of the primary parameters influencing the performance of underground structures particularly in the Himalayas where there is strong influence of topography and geology on insitu stresses. It is always advisable to measure the in-situ stresses, in whatever best way possible. Hydraulic fracturing method is one of the popular techniques used for determination of in-situ stresses in rock mass. In-situ hydraulic fracturing tests were carried out in vertical drillholes in the proposed cavern of desilting chamber of Tawang Hydroelectric Project, Arunachal Pradesh, to evaluate magnitude and direction of the maximum (max.) & minimum (min.) horizontal stresses. In-situ tests were carried out in horizontal drillholes to assess the vertical stress. The orientation of max. horizontal stress was found to be N630E whereas magnitude of max. and min. horizontal stresses were determined as 11.62 MPa and 7.42 MPa, respectively. The angle of difference between the average max. horizontal stress direction (N630E) evaluated and the orientation of the desilting chamber suggested by the designers based on the joint set pattern and functional requirement (N750E), is comparable with the recommendations of Nilsen and Thidemann (1993) for stable orientation of underground structure. The vertical stresses were also estimated using the height of the rock cover at test locations and it is found that estimated vertical stresses based on rock cover was higher than average measured in-situ vertical stress.