Working in India: Anatomy of a Hydro Project - Desilting Intakes (Part 8)

My previous posting covered the underground desilting chambers and tunnels, but including the intakes themselves was going to be a bit much for a single post, so I'm breaking them out into a separate post. I should also note that during my time on the project, the intakes were not completed, so while I have plenty of "before" images, I unfortunately have no "after", only "during construction" photos (and early construction at that). 

When I arrived in February 1999, the intakes themselves had not been excavated, and the eventual tunnel mouths had not yet been exposed.

(Note: I added the image below some weeks after initially completing this post, having forgotten to include it the first time around.)

A plan view of the intakes showing the trash rack and intake bellmouths for each of the intake tunnels. The triangles mid-way down the image in each tunnel represent the transition between square and horseshoe profiles that is mentioned below.

I should note that some of the photos below, especially the ones showing concreting work, are not in chronological order. 

A close-up of the intakes area, taken before the intake tunnels were daylighted to the surface. The rock in this area of the Himalayas is very soft and prone to rock falls - as the Himalayas are relatively young in geological terms, mother nature hasn't had as much time to beat the mountains down (compared to, say, the Rocky Mountains in North America) and the rock has a high content of mica, which is very very soft. It was not uncommon to be able to walk up to a boulder on the side of the road and break off pieces with your hand. As a result of this, the project required a considerable amount of rock stabilization work, as is evident in this photo with row upon row of cable anchors in the rock above the intakes. Horizontal holes are drilled deep into the rock, and cables are inserted into the holes, anchored, and then stressed and tied off to anchor plates that are cast into the concrete on the surface. The dark spots on the concrete are the heads of the cable anchors. The intakes were out of the arc covered by the cable crane, and therefore all work in this area had to be supposed by the crawler cranes. The three tunnel openings to the right of the image are the dam access galleries, and the front face of the dam itself would have been to the left of these tunnels, between them and the zig-zag stairway. The pile of fill has been placed there to allow cable anchoring operations to continue down the rock face. The top of the zig-zag ladder represents the level of the intakes access road, and the top of the dam. Two crane platforms were eventually brought up to this level to allow concreting work on the intakes to proceed. The cable crane used on the dam, and shown in a previous post, did not reach far enough to cover the intake area.

Viewed from a higher elevation, and further upriver, this view from Spring 1999 shows the terracing of rock anchors better, along with the access road and the cable crane location. The road at the top left of the image is National Highway #22.

During my time on the project, as the cable anchoring of the rock face was completed, the fill pile in front of the intakes was slowly removed. The intake tunnels connecting the four underground desilting chambers to the intakes were daylighted, and concrete work began.

A close-up of the intakes area, with a similar viewing angle to the first image in this post, taken after the intake tunnels were daylighted to the surface but before concrete work on the intake structure began in earnest. In this photo, only the concrete foundations for the intakes has been started. An NCK Rapier crawler crane is at work. The intakes were out of the arc covered by the cable crane, and therefore all work in this area had to be supposed by the crawler cranes. Later in the project, when concrete work began in earnest, an NCK was mounted on each of the two concrete platforms (about the same level as, and on either side of, the top of the crane boom in this photo). If you look closely on the left side of the photo, you can make out the many flights of stairs that were required to descend from the road level to the base of the intakes. This was fine on the way down, but the climb back up required a bit more effort (and maybe a break or two to catch your breath)! 

March 14, 2000: formwork is up and concreting of the intakes is underway. The crawler crane has not yet been moved to one of the platforms over the intakes. 

Taken on May 6, 2000, the placement of concrete for the invert of the intakes bellmouth is now complete. The concrete work in the foreground, at the bottom of the image, is the concrete of the dam coming up the right bank. The upstream cofferdam has been breached for the summer months, and the river is flowing through the dam site. 

May 12, 2000: Reinforcing steel (rebar) being installed at the base of the intakes. The beginning of the bellmouth rebar (the vertical bars to the right) is taking shape.

June 5, 2000: These are two ski-jump concrete forms for the crown (top) of the intake bellmouths. I would have rendered these in AutoCAD, so that the Steel Fabrication Shop could build them (which is where this photo was taken). If I remember correctly, the bellmouths were parabolic in shape. I think both forms were required to pour a single bellmouth, but after 15 years I may be mistaken. Due to a flood in August 2000, these forms never saw use while I was on the project. In my time on the project, I spent some time at the Steel Fab Shop making sure work was fabricated per the drawings I prepared.

June 5, 2000: The partially completed intake crown transition formwork, looking from the downstream (inside) end to the upstream (outside) end. Steel was bent to the shape required, as provided on drawings I prepared. Every single rib in this structure was a different shape, and had to be drawn and calculated separately. The far end was the trickiest both to draw and to build. The forms were then skinned with planks and plywood, as can be seen here. The entire transition form could be transported in four parts to reduce the weight. The whole assembly would be propped up by short support towers. The concrete work formed by this formwork would be just downstream of the concrete formed by the ski-jump formwork in the previous photo.

July 8, 2000: The transition of Intake #2 takes shape. Located just behind the yet to be poured bellmouth, the transition section changes from the rectangular shape of the bellmouth to the inverted horseshoe shape of the intake tunnel over a length of perhaps 10 to 12 metres. The designers provided probably 5 or 6 intermediate shapes for the transition, and then I got the job of transferring the designer approved shapes plus the intervening shapes into AutoCAD, along with all the support structural members. In the photo, the concrete walls are poured up to about half height, and the transition overt form (from the previous photo) can be seen in the background.

A worker sprays water on the recently poured concrete that forms the bottom of the intake structure.

There was some concern that flooding on the river might top the concrete shown in the above photo, and end up flooding out the underground desilting works. As such, steel bulkheads were constructed for each of the four intake tunnels, to be installed just downstream of the intake works. Each bulkhead had to be shaped to match the rock profile of each tunnel: we had an advanced (for the time) laser tunnel profile instrument that would provide an exact profile of the tunnel at the location of each bulkhead, which I then imported into AutoCad and turned into a fabrication drawing. The bulkheads were anchored into the rock of each tunnel, and were designed by the Chief Design Engineer to withstand the potential hydraulic head of a large flood.

March 13, 2000: The contractor was worried about the risk of flooding of the intake tunnels and the desilting works behind, and so steel bulkheads were constructed at the inlet to each tunnel. In this photo, the bottom half of a bulkhead is being installed in Intake #2. The bulkhead was cut more or less to the shape of the rock, and then filled in. You can see the shape of the tunnel invert (floor) here, already poured in concrete.

March 13, 2000: The upper half of the Intake #2 bulkhead is swung into place by a small (yellow) Escorts scissor crane that is hidden in behind. The top half is later welded to the bottom half.

March 14, 2000: The partially completed Intake #2 bulkhead, with the top half installed. The lower port in the bulkhead is for man access, while the upper is a fan port. During the August 2000 flood, all the bulkheads held back the weight of water and silt that were thrown against them. Although there was some initial water ingress through the open fan ports (and the fans themselves), the silt quickly built up in front of the bulkheads and sealed the holes shut. The bulkheads therefore prevented the damage to the desilting works from being much more serious than it was.

March 16, 2000: The interior view of the Intake #2 bulkhead. You can see a bit of daylight coming in around the outside of the bulkhead, and this would later be filled in with a combination of steel plating and concrete (if I remember correctly).

As I've alluded to, the construction of these bulkheads was somewhat prophetic considering the flood event in August 2000, but I will cover that flood in a separate post. My next post on this project, though, will cover the 27 kilometre long Head Race Tunnel.

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.