Role of geology and special rock mechanics tests in TBM tunnelling – A perspective

A growing need to switch over to the clean sources of energy, such as hydropower, is being felt in India after burning much of our coal reserves to an alarming stage. Incidentally, India has been gifted with numerous perennial rivers originating from the great Himalayas favoring hydropower generation. However, these young folded mountains with an extremely variant geology created several problematic situations delaying many tunnelling projects.

The major construction work of any hydro power project is the construction of head race tunnel (HRT) which had made use tunnel boring machine popularly known as TBM. There are number of ongoing projects of NHPC Ltd and NTPC Ltd such as Parbati Stage II and Tapovan-Vishnugad projct, etc. deploying TBMs for tunnelling. As huge amount of money is involved in deploying TBM, it is important that it should successfully complete the project. Thus, suitability of TBM needs to be established with the help of various geological and geophysical investigations coupled with insitu and laboratory tests. The role of geology, uncertainities involved and consequent hazards have an immense influence on the operational success of TBM and hence need attention.

TBM performance prediction models, namely, NTH, CSM, RMi-NTH & QTBM, are frequently used owing to their increased popularity over the years. RMi- NTH model is an empirical performance prediction method based on historical field performance of machine in certain rock types. Empirical graphs and equations obtained from regression analysis between rock properties, ground condition, machine parameters and rate of penetration are used in this. The CSM model on the other hand involves starting from the individual cutter forces and determines the overall thrust, torque and power requirement of the entire cutter head.

This paper discusses the geological problems encountered while driving tunnels with TBM, specially, with respect to Indian conditions. Different types of specialized rock mechanics tests that are carried out and available at Indian School of Mines, Dhanbad for enabling the selection of TBMs are also presented alongwith some results. Typical performance achievable in a given rock type is estimated using the RMi-NTH model. A case study of TBM drivage pertaining to Tapovan-Vishnugad Hydel project, NTPC Ltd. is considered for the analysis. The suggested approach with field corroboration could be useful for predicting the possible advance rate in a given formation.