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.

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.

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

Strike and dip directionsManeria 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˚

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

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)

Uttarkashi Outlet

Maneri Intake

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

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

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

Solutionsinitially = 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

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