The certainty of uncertainties is the hallmark of Himalayan Geology. Complex geological signatures attribute to occurrence of multiple generation folds, faults, thrusts, shear zones (bedding/cross shears), highly fractured rockmass, perpendicularly intercepting perennial nalas/gullies as lineaments at almost every 100-500m intervals etc. along with diverse climes; being the primary traits.
Although requisite investigations are done as per codal provisions/guidelines; encountering geological uncertainties can't be ruled out whatsoever. Moreso, complex geology with mixed ground conditions and associated brittle-ductile deformation poses difficulties not only in surface excavations, rather infringes greater challenge for tunnelling in DBM and/or TBM; leading to geological uncertainties. Such instances; if substantial in degree & difficult to handle/mitigate are tagged as Extraordinary Geological Occurrences (EGOs), which in turn lead to time and cost over-run of the project.
Exemplifying EGOs as encountered in 120 MW Rangit-IV Project, envisaging construction of 6.5km long (7.3m dia.) HRT and 958m long SFT (5.1m dia.) comprised of heavy underground seepage of 7000LPM, low-cohesive Coal band (8m thick), >2D Cavity, >1D Chimney & long-persistent shear zones (>0.5D). Also, experienced high squeezing >5% radial tunnel closure, occurrence of Gassy conditions with tunnel fire and explosion, high tunnel temperature (46°C) etc. in Lower Gondwanas which affected the work-efficiency and in turn the progress of tunnel. Nevertheless, such EGOs were tackled conventionally, yet with certain on-site innovations.
In Indian context, based on past experience of various tunneling projects, of recent EGOs are being quantified for sufficing contractual framework towards avoiding disputes between developer and executing agency. However, denotation of EGOs vis-a-vis degree/dimension/extent are project-specific; yet limited.
Hence, this paper is an attempt towards advocating a moderate yet universal numerical assignment for EGOs negotiating uncertainties in complex geological set-up and putting up arguments with reasoning for such advocation, thereby urging the need for framing an appropriate code/guidelines for EGOs.