2.  Location – Maneri to Uttarkashi, India  Length – 8.56km long, diameter 4.75m  Use – a power tunnel, part of the hydro-electric project on the River Bhagirathi. to harness the power potential of the rivers flowing from the Himalayas.

3.  Mountain ranges with rocks of different ages and mixed lithology  Tectonically disturbed – folded and faulted. Closely spaced jointing, brecciation and shearing  Affected by Main Central thrust and North Almora thrust  Quartzitic › Thin bands of slate, chlorite schists, phyllites, metabasics and basics belonging to the Garhwal group

4.  Strike and dip directions Maneria area N 10˚-80˚ 25˚-45˚, N 100˚-170˚ Heena area N 250˚-280˚ 25˚-35˚, N 160˚-190˚ Tiloth area N 290˚-350˚ 35˚-45˚, N 20˚-80˚

5.  Tunnel face collapse  Heavy water ingress  Cavity formation  Buckling of steel ribs  Squeezing ground conditions

6.  Problem › Tunnel face collapsed suddenly at ch.3549m › 300m³ of loose rock fell due to a heavy in-rush of water › 2 months later 200m³ of sliding muck  Cause › two 40cm wide cross shear zones intersected in the tunnel and water present in the quartzites  Solutions › Grouting with portland cement not possible as shear zone contains brecciated quartzite's, fineness module = 1.16 (attempted to retrieve face for 6 months without success) › Tunnel realignment through more metabasic rocks to the North (safer and easier for tunneling)

7. Uttarkashi Outlet Maneri Intake

8.  Problem › Major cavity (813m³) formed during excavation at ch. 5038 to 5050m. Continuous inflow of debris from above the muck pile  Cause › Shear zone at ch. 5050m with crushed quartzites heavily charged with water  Solutions › Face sealed with rolled steel joists › Drainage holes to drain seepage water › Cavity above forepoles filled with concrete by pipes › Muck below forepoles grouted with cement to consolidate the muck › Side drift excavated to attract seepage water away from tunnel face (not successful due to inflow of crushed material with water so was sealed. Drainage pipes used through bulk head instead. › Muckpile then grouted and tunnelling done by multidrift method from both ends

10.  Problem & Cause › At ch. 5550 to 5250m partially wet and thinly foliated metabasics had severe squeezing problems › 5 to 6 months after excavation, backfill concrete started cracking and ribs deforming  Solutions › Initially: ISMB 150 x 150mm ribs spaced 80cm supported the tunnel spaces dilled with concrete › Temporary (3 years): Supports stop buckling of ribs – invert struts fixed to each rib and ribs strengthened by laggings and Concrete poured into inner flanges of ribs › Final: Rock mass trimmed and re-supported by 150 x 150 spaced 75mm

11. At ch. 5550 – 5250m partially wet and thinly foliated metabasics were beset with severe squeezing problems 5 – 6 months after excavation, backfill concrete started cracking and ribs deforming Solutions initially = ISMB 150 x 150mm ribs spaced 80cm supported the tunnel spaces dilled with concrete Temporary (3 years) = supports stop buckling of ribs – invert struts fixed to each rib and ribs strengthened by laggings and Concrete poured into inner flanges of ribs Final = rock mass trimmed and resupported by 150 x 100 spaced 75mm

12.  Water-inrush, cavity formations and support failures due to squeezing was tackled  The problems were encountered because of the unknown and frequently changing geology along the tunnel alignment.  Inaccessible terrain covered with vegetation meant it was difficult to collect reliable geological information  Minimise surprises in the rocks by drilling advance boreholes at the tunnel face  Invert support is helpful in poor rock masses to prevent the buckling of the steel rib supports