Tunnelling and Underground excavations in the Himalayan mountains is a formidable task, and the challenges were further amplified when the project was initially started by some other executing agency and after various technical & financial issues came to complete halt. After some years, these partially constructed projects are ageing and handed over to other agencies to complete it.
In case of Teesta VI HEP, the partially constructed project was handed to NHPC in 2019-20 i.e. after about 10 years to complete the balance works. After a decade of hiatus and the influence of worldwide epidemic situations like COVID 19 has made the work of completing the project much more challenging.
In this paper we will discuss in detail about the various challenges faced by engineering geologists from re-investigation to execution. Re-examination of information from a variety of sources like available progressive excavation data, additional topographical survey, geological / geophysical investigation, site specific vulnerable tunnel spans, surface mapping etc have been done to re-assess the geological conditions of surface and underground works. Accordingly, innovative underground investigation techniques, excavation methodologies, strengthening techniques of subsurface strata, support measures, tunnel re-alignment, cut slope design etc have been adopted that best suited the geological conditions. Along with this we will also discuss various challenges during re-sarting and re-stablishing the collapse faces and during crossing various perineal Nala and its feeder channels with low cover zone and adverse geology. We will also discuss various adopted solutions, remedial measures and mitigation plan to tackle these adverse geological problems during progressive tunnelling.