Determination of support pressure for tunnels and caverns using block theory

• The algorithm used in the method can be easily adapted to the computer programme.
• The technique involves determination of geo-mechanical parameters for stability assessment.
• Alignment of the tunnel could easily be chosen to get the rock pressure to be minimum.

The estimation of support requirements to stabilize underground structures is of prime importance for rational design of these structures. The characterizing parameters of rock mass may vary with depth. Determination of these parameters by drilled cores and Ground-Probing-Radar (GPR) is difficult and expensive due to anisotropy of rock mass. Laboratory testing is also expensive. Also the in situ conditions are difficult to simulate in the laboratory. The designer is thus resorting to empirical methods and analytical methods to determine these parameters. Often, the analytical techniques may mesmerize the designer to feel the problem and its solution on the screen of the computer. In this paper, an attempt has been made to develop algorithm based on Block Theory with geological information & mechanical properties of rock for determining the rock pressure. Limitations of this technique are number of joint sets not less than three and width of the opening up to 25 m. The algorithm determines all the wedges formed at a time by 3, 4, 5, 6, …, n joint planes with excavation plane responsible for manifestation of rock pressure at roof/wall. All the permutations and combinations for wedge formation can be considered in this respect. Rock pressure for design is determined for reinforcement of the underground openings. Spacing of rock bolts is found out as an additional feature. The alignment of the opening for optimal reinforcement can also be determined. Case history of Tehri Power House, India is taken up for analysis. The empirical correlations developed by Goel (1994) are used for comparative study. It was found that no appreciable rock pressure was developed at walls. Roof pressure is determined to be 140 kPa, which is almost same as observed. It is thus established that block theory may be applicable for design criterion up to depth of 500 m.