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NEW DELHI: Learning from recent incidents of tunnel collapse, including at Silkyara in Uttarakhand, the road transport ministry has issued a standard operating procedure (SOP) specifying that highway owning agencies must evaluate at least three alternative alignments with clear differentiators — shortest path, geologically/ geotechnically favourable, least social impact, and cost-optimised route – before settling on a choice.The SOP issued on Tuesday says the alignments must avoid critical constraints such as fault lines, eco-sensitive zones (ESZs), heritage sites, steep slopes and existing urban settlements. As per the newly introduced SOP, all alignments should respect terrain geometry and portal (entry) locations should be optimised for access, gradient, drainage, and minimal land acquisition. It also says that portal selection must be done keeping in mind stable slopes, drainage channels and land cover.There have been several instances of portal collapse during construction of tunnels.A probe into the Silkyara tunnel collapse had found that the project report was prepared on the basis of limited geological and geotechnical investigations and that the alignment could have been modified to eliminate weak or shear zones. A shear zone refers to highly deformed, weak and thinned rocks resulting from high level of regional stress; such zones require special attention, greater monitoring of ground behaviour and more support to avoid any mishap. “Without rigorous, standardised studies and data-backed alignment evaluations, tunnel projects may face delays, cost escalations, and stakeholder conflicts,” the SOP says.It has mandated developing a Geotechnical Investigation Interpretative Report (GIR) for projects that synthesises and interprets the geological, geotechnical, and hydrogeological data collected during field investigations. This must include corridor-wise geological synthesis, including lithological and structural mapping, assessment of rock mass quality and classification and groundwater inflow.Sources said that since fixing the right alignment is key to safe and timely construction of tunnels, the ministry has now decided that all alignment proposals will undergo due diligence by the ministry. The agencies concerned will have to submit details of traffic volume count survey, peak-hour analysis, and origin-destination surveys.
