CP Systems

Tehri Dam

Eight jobs at India's tallest dam: dredging the Bhagirathi below the spillway, walling it, and repairing the tunnels and intakes that feed 1,000 MW.

Client
HCC / THDCIL
Location
Tehri, Uttarakhand
Dredged
~2.5 lakh cum
Baffle wall
47 m across the river
Scroll to the work

Working below the spillway

Tehri HPP (1,000 MW) and Tehri PSP only stop for short, scheduled shutdowns. Each CPS job had to fit inside one.

Below the dam the Bhagirathi rose and fell 8–10 m twice a day, leaving 3–4 hours of working time per shutdown. Inside the tunnels the only access was a crane down a 100 m surge shaft.

The dredged riverbed in the Bhagirathi gorge

River dredging, phase 1

Feb – Jul 2023Zero schedule of Tehri HPP

Dredging across 640 Rm of the Bhagirathi downstream of the spillway, done while the river level swung 8–10 m twice a day.

  • Dredging to THDCIL’s daily zero schedule
  • ~1 lakh cum dredged and hauled 14–15 km
  • 12 excavators (210) and 2 long-boom excavators (300)
  • 50–60 dumpers for muck transport
  • 12 dewatering pumps, 50 HP each, 7,000–8,000 LPM
  • Dedicated 30 cum batching plant and 6 transit mixers
  • All resources deployed 24 hours in two shifts
Excavators dredging the riverbed below Tehri Dam
Dumpers hauling dredged muck

Phase 1 dredging, photographed

  • River dredging operations
  • Water level at 604+ m
  • Excavation in variable water
  • Dredging, March 2023
  • Heavy machinery deployment
  • Water level 603.75 m

Baffle wall and micro piling

Feb – Jul 202347 m across the Bhagirathi

A baffle wall about 47 m long across the river between the spillway and the TRT of Tehri HPP, poured under water onto a micro-piled, saturated riverbed.

  • ~2,000 cum RCC in 4 months
  • 1,000 mm dia Hume pipe through the wall
  • 700 m of micro piling in saturated foundation, ~400 Rm in one month
  • Built in 5–10 m segments
  • Underwater concreting by mobile boom placer
  • Drilling and grouting of anchors on both banks
Boom placer pouring concrete at the baffle wall
Placing the 1000 mm Hume pipe for the baffle wall

Building the wall

  • Baffle wall construction
  • Micro piling operations
  • Concreting in progress
  • Wall segment formation
  • Foundation reinforcement
  • Completed phase 1 baffle wall

River dredging, phase 2

Jun – Jul 2024A 35-day shutdown

The second phase dredged 1,50,000 cum and built 70 m ramps either side of the baffle wall, all inside a 35-day shutdown and through monsoon rain.

  • 1,50,000 cum dredged from chainage 260 m to 660 m
  • ILO discharge never stopped; water never below EL 601.5 m
  • A 12-day approach road built through the river
  • A 12 m deep area found across the full river width
  • 20 more dumpers brought in mid-job
  • Emergency wall for an exposed THDCIL building foundation
Transit mixers on the ramp, June 2024

Phase 2 on the riverbed

  • Phase 2 dredging operations
  • Excavation in progress
  • Riverbed operations
  • Machinery at work
  • Dumper loading
  • Approach construction

Ramps and micro piling, phase 2

Jun – Jul 202414,400 cum concreted

Ramps 70 m long and 5 m wide along the Bhagirathi, piled and poured under water in the same shutdown.

  • 70 m ramps, 5 m wide, upstream and downstream
  • ~850 m of micro piling in the ramp foundation
  • Underwater concrete by boom placer
  • Built in 5–10 m segments because of seepage
  • Foundation held at EL 601 m against a target of EL 598–600 m
  • Shuttering fixed in standing water; buried formwork dug out
Drilling rig for micro piling on the bank

The ramps

  • Ramp construction
  • Concreting by boom placer
  • Ramp concreting in progress
  • Ramp foundation

Tunnel repair, 100 m down

HRT #3 and #4Tehri PSP, 8.5 m diameter

Repairing the 8.5 m head race tunnels meant lowering every worker and every bag through the surge shaft by crane, then moving the epoxy 700 m along the tunnel by hand.

HRT #3

Length repaired
23 m
Epoxy mortar
~375 MT
Done in
36 days
Target
45 days
  • 100–125 workers, round the clock
  • Material moved 700 m through the tunnel by hand

HRT #4

Length repaired
32 m
Epoxy mortar
~430 MT
Done in
37 days
Target
45 days
  • Custom electric mixers: a 110 kg batch in 5 minutes
  • 12 mm shear connectors at 300 mm c/c, curved shuttering to the tunnel profile
Epoxy mortar batching for the tunnel repair
Curved scaffolding matching the tunnel profile
Epoxy repair work inside the head race tunnel
Repair crew in a cage at the TRT outlet

Inside the tunnels

  • TRT/HRT repair work
  • Epoxy mortar in HRT #3
  • Material moved by trolley
  • HRT #4 plug breaking area
  • Foundation epoxy work
  • Tunnel repair operations
  • TRT preparations
  • TRT surface preparation
  • TRT epoxy application

Morning glory shaft epoxy treatment

Shafts T1 to T4Four months

Repair of the swirling-device bottom in all four surge shafts, with water falling continuously from 150–160 m above. A 25 m scaffold went up inside each 16 m shaft and the water was funnelled away from the work.

  • 25 m scaffold built inside a 16 m shaft
  • Water falling 150–160 m turned aside with custom GI sheets
  • Epoxy treatment, mortar seals and fibreglass wrap
Scaffolding inside the surge shaft

Epoxy treatment, intakes 1 and 2

31 May – 14 Jun 202516 days against 30 planned

Epoxy treatment at Intake Structures 1 and 2, EL 745 m, Tehri HPP. Planned for 30 days, handed over in 16.

  • Epoxy mortar at construction joints
  • Old cement concrete wearing coat removed
  • Epoxy paint on steel; steel work in the gates
  • Bitumen felt removed over joints
  • Four-course waterproofing with bitumen felt
  • Stainless steel plates fixed over the felt

Tehri in three numbers

2.5 lakh cum

dredged across phases 1 and 2 and hauled 14 km

805 MT

of epoxy mortar in HRT #3 and #4, both inside the 45-day target

16 days

for the intake epoxy treatment, planned for 30

More of the work

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