Stability Analysis and Rock Support Optimisation of Headrace tunnel at Mai Hydropower Project, Nepal

Abstract

There is a huge potential of underground works in the himalayan region specially in Hydropower. Hydropower is one of the most economic source of Nepal due to its enormous potentiality in Neplese water resources. Construction of underground structures is not a easy task rather incomplex geology of Nepal. Proper understandings and engineering assessment of Neplese geology is utmost requirement to have optimised, cost effective and safe design of underground structures. This is even more important for underground structure proposed in Mai Hydropower Project.

Mai Hydropower Project (MHP), based on run of the river sheme has intalled capacity of 15.6MW with 16m3/s of design discharge and Gross head of 121.6m. MHP lies in Ilam district of eastern Nepal and dwells in sedimentary units of Siwalik zone according to geological formation of Nepal. The underground structure proposed in Mai Hydropower Project is headrace tunnel of2147m in length. The major rocktypes existing in the project area are mudstone, sandstone and siltstone and some minor shear zone has occupied some part of tunnel.

To carryout the engineering assessment of MHP headrace tunnel, the entire tunnel has been divided into eight different sections in different rock types with highest and lowest overburden of rockmass. Stability analysis and support optimisation of Headrace tunnel in MHP has been carried out during this thesis work. Preliminary, a semi-analytical, Sakurai's approach has been used to analyse the tunnel stability based on tunnel strain in all the sections selected. Analysis of possible wedge failure has also been carried out using UNWEDGE software. This analysis however, shows that all the wedges that are in sandstone and mudstone sections are stable. And lateron 2-D numerical modelling program Phase2 has been used to analyse elasto-plastic behavior of all the sections in detail. Furthermore, hydraulic fracture assessment has been carried out for all the sections applying water head inside the periphery of tunnel. This assessment has been accomplished using phase2 program and result shows that there is no effect on hydraulic fracturing. As this project is in construction phase and different support system has been finalised by the consultant for different sections of MHP tunnel, the support system defined by the consultant (Sanima Hydro& Engineering (P) Ltd.) has been used in Phase2 for the evaluation. And finally, support system has been optimised for those sections where the proposed support is insufficient as per the Phase2 analysis.

Preliminary assessment of MHP tunnel shows that two sections, one in Mudstone and other inshear zone have severe squeezing problem and afterwards Phase2 analysis has also verified that there is maximum displacement occured at these sections. Other six sections have few squeezing problem where the support system defined by the consultant seems sufficient to have minimum support problems.

Finally, the overall discussions on stability analysis of MHP headrace tunnel have been presented and some reccomendations have also been proposed for further execution of work.