Working in India: Anatomy of a Hydro Project - Desilting Chambers and Tunnels (Part 7)

The waters of the Satluj River in the Himalayas of Northern India transition from green in the winter, when flows are low, to chocolate brown in summer when snow melt higher up causes flows to ramp up and the river carries increasing amount of sediment.  Not only is sediment bad for the turbines at the generating section, far down stream, but it also increases the abrasiveness of the water in the Head Race Tunnel (HRT), wearing away the rock and concrete lining of the tunnel. During the August 2000 flood, sediment laden flood waters sand blasted the exposed dam sluiceway piers right down to the reinforcing steel, and actually wore down the nubs on the steel itself. On top of this, the soft waters of the Satluj River are normally fairly hard on concrete to begin with, and don't need assistance in wearing it down. 

The Nathpa Jhakri Hydroelectric Project therefore includes a large desilting works, consisting of four 500 metre long underground desilting chambers and the related intake works and network of connection tunnels, plus a silt flushing tunnel that returns silt from the collection hopper of each desilting chamber back to the river.

The desilting chambers are labelled at the top centre of the diagram, and are shaded in blue, along with the intake tunnels and the start of the head race tunnel. The Intakes are coloured green, just above the dam.

A diagram showing the four desilting chambers in section, along with the upper access tunnel, stairwell shafts, and access galleries. 

When I arrived in February 1999, the excavation of the desilting chambers themselves was well underway, as well as most of the connection tunnels. The intake works, on the other hand, were still being excavated and concrete work had not yet begun.

Inside Desilting Chamber #1, looking downstream. Excavation of a lift, or bench, continues in the far background. A Caterpillar front-end loader and workers can be seen in the foreground. The stair tower behind the loader leads to one of the maintenance and inspection access galleries that would provide personnel access to the chambers once they were commissioned and in service. Health and Safety procedures and regulations in India are not what I am used to in Canada, and I recall that climbing to the top of this stair tower was one of the scarier things that I did while I was on this job - mind you, I am afraid of heights, but I recall there there were no railings on the top level. The bottom of the chamber will eventually narrow out to a trough running the length of the chamber. The idea behind the Desilting Chamber was that the size of the chamber would allow the speed of the water flow to slow down enough that the silt and sand particles would settle out into the trough at the bottom, and be returned to the Satluj River via the Silt Flushing Tunnel (SFT). The clean water, with silt removed, would be siphoned off the top of the chamber and directed to the Head Race Tunnel (HRT) which would transport the water to the turbines at the powerhouse at Jhakri, 27 kilometres away.

The port in the background of this image leads to the stair tower seen in the previous photo, and is one of the access gallery manholes indicated on the schematic above. The port itself is a metal collar with bolts, cast into the concrete, which would be used to secure and seal the watertight hatch that would eventually be installed there. When the chambers are full of water, this gallery would be underwater. The stair shaft that provides access to this gallery from above is behind and to the right of the photographer. The stairs were not complete when I left the project in 2001.

This is the downstream bellmouth in Desilting Chamber #3 while under construction in April 2000. The formwork on the invert and walls is still in place here, and concrete work is progressing up towards the overt. Chamber excavation has not proceeded very far here, and the chamber will get much deeper - this bellmouth is located at the very top of Chamber #3. This is where clean water, with silt removed, would leave the chamber to head for the HRT.

Taken the same day as the previous image, but in Desilting Chamber #4, this bellmouth is slightly more advanced in construction with the walls pretty much complete. Only the overt form installation and concrete pour remain. The lift lines can clearly be seen in the concrete - 6 lifts and part of a 7th can be counted from the top of the current lift to the top of the concrete work. The outlet tunnel can be seen in the background through the bellmouth opening.

A manifold of four tunnels join the bellmouths at the downstream end of each chamber with the Head Race Tunnel (HRT).

The inside of the outlet tunnel from Desilting Chamber #4, looking in the opposite direction from the previous photo. The support towers and the ski jump concrete form for the overt pour are sitting in the tunnel waiting for installation. The HRT is behind the photographer in this photo.

A worker looks at a profiling platform in Outlet Tunnel #2. Once the mass tunnel excavation was finished, a work platform such as this would be run through the tunnel on rails to determine where the rock line was intruding into the tunnel profile, and workers could have a go at removing the protuberances with pneumatic hammers. This platform was also fitted with a steel tunnel profile that marked the minimum excavation line, but for some reason that item is not visible here - perhaps this was taken just as the platform was being assembled.

This photo was taken at the junction of Nathpa Adit and the start of the Head Race Tunnel (HRT), with the outlet tunnels and Desilting Chambers behind the photographer. The masonry wall to the left of the spotlights is right at the end of Nathpa Adit. Concete curbs have been installed along this portion of tunnel, and the HRT overt form is just visible in the background to the left of the two spotlights. The Chief Surveyor stands in the foreground, and appears to be carrying the tripod that I was using that day (though apparently not for this photo).

Excavation of the desilting chambers didn't always go smoothly. I've previously mentioned the poor rock conditions on this project, and the chambers suffered from this as well. There were at least two rock falls during my time on the project, and possibly more. The company brought in a consultant geologist from Canada to advise on how to better support the rock during construction, and the end result was a large number of cable anchors running from chamber to chamber through the rock pillars between each chamber, in order to better hold the rock together.

Chamber excavation was not without its challenges and dangers. Even though the rock was stabilized and reinforced with rock bolts and lined with shotcrete, rock falls such as this one were not an uncommon occurrence. The rock bolts can be seen sticking up out of the rock debris - failures like this occured when the depth of the failure plane exceeded the depth of the bolt penetration into the rock.

A rockfall, this time in Chamber #3, in May 2000. The workers in the frame give a sense of scale to the amount of rock that came tumbling into the chamber, and rock bolts can be seen both in the debris and sticking out of the exposed rock face. After further study by geologists, it was decided to drill through the common walls between chambers and install cable tension anchors between chambers to support the rock in those walls. This work was done after I left, and was hindered by the fact that the chambers had mostly been excavated to near full depth by the time it became apparent they were necessary.

Also ongoing during my time on the project was the excavation of the intakes and intake tunnels. 

The inlet tunnels brought water from the Intakes through to the Desilting Chambers (you can see pictures of the Intakes in the Surface Construction Gallery). One of the jeeps is parked in Inlet Tunnel #3, which has had its invert lined with concrete. The screed form that shaped the concrete would have run on the rails on each side of the tunnel. These rails would later be removed and replaced with new rails mounted right on the concrete invert, to be used by the overt form. A steel bulkhead is visible in the background at the entrance to the tunnel.

This is a rebar installation platform in Inlet Tunnel #1. The tunnel invert concrete has been placed, and the rails have been relocated from the position in the previous photo to the top of the invert concrete. The platform is riding on those rails. As seen here, workers on the various levels of the platform work to install reinforcing steel (rebar) around the circumference of the tunnel overt (crown) prior to the lining of the tunnel with concrete. If you look closely, you can see that one of the workers is actually climbing on the rebar cage between the rebar and the rock wall. My supervisor, the Chief Design Engineer, designed this platform and I was responsible for producing the production drawings that were sent to the steel fabrication shop.

At the upstream end of the intake tunnels was the intake works. The intakes were large concrete structures intended to direct water into the tunnels and desilting chambers with as little turbulence as possible. Up front was a trash rack of vertical fins, intended to prevent large floating debris from entering the system, followed by bellmouths to each tunnel. Very little of this work was completed when I left the project.

Plan view of the intakes showing the trash rack and bellmouths to intake tunnels 1 through 4.

I will leave the intakes themselves for a separate post.