Evaluation of In-Situ Stresses for the Design of Underground Desilting Chamber: A Case Study

Evaluation of in-situ stresses is essentially required for the design of underground structures inside rock. The size, shape and orientation of large underground caverns depend on the magnitude and direction of in-situ stresses. 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. Insitu 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 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. The results of in-situ stress measurements were compared with available correlations and geological interpretations also. Further, an attempt has been made to estimate the tensile strength of rock from crack initiation and reopening pressure. All the above aspects have been discussed in detail in this paper.