Tunnelling Through Unconsolidated Fluvio-Glacial Material in Hurla Approach Tunnel, Parbati Power Station (Stage-II), India

The Hurla Approach Tunnel of the Parbati Hydroelectric Project (Stage-II) was excavated through challenging ground conditions comprising unconsolidated fluvio-glacial deposits. The tunnel alignment encountered saturated slope-wash material, significant groundwater inflows, and frequent collapses. This paper documents the geological conditions, excavation difficulties, and the engineering measures adopted to achieve successful tunnelling. Conventional methods such as forepoling, grouting, and rib supports proved inadequate in highly unstable and water-charged ground. Ultimately, innovative use of progressive pipe roofing combined with Tube-a-Manchette (TAM) grouting, drainage, and staged excavation led to safe completion of the tunnel. The adopted methodology provides useful insights for tunnelling in similar geotechnical settings.