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.

Working in India: Anatomy of a Hydro Project - Diversion Tunnel (Part 6)

In a previous post, I presented photos taken during a period of construction on the 62.5m high gravity dam on the Nathpa Jhakri Hydroelectric Project on the Satluj River. At the end of the post, I asked the question: "How do you build a dam in the middle of a river in the mountains?" When you are a steep sided mountain valley, you can't just divert the water around the dam site - or can you? Well, you can - but it isn't easy. In our case, it required a diversion tunnel cut through the rock of the mountain side around the dam site on the right bank. The diversion tunnel was completed before I ever arrived on site, and was presumably plugged with concrete (or possibly just the steel gate at the upstream end) after I left.

After the diversion tunnel had been originally laid out, a rock slide at the intended inlet location meant that the tunnel had to be doubled in length. It had to be constructed on the right bank, so that it would not interfere with the construction of the desilting chambers and head race tunnel on the left bank. 

Layout of the dam area, with the diversion tunnel shown on the left side of the diagram.

When I arrived on the project in February 1999, the tunnel would already have been pressed into service for the winter months. The tunnel was only designed to handle up to a certain maximum flow (I don't remember what it was, but probably in the range of 200-500 cubic metres per second (cumecs), but the Satluj River varied between around 80 cumecs in winter to a normal maximum of around 2000 cumecs in the summer. In springtime, river flow rates from a nearby government monitoring station would be watched carefully, and as flows approached the tunnel maximum, flows would be removed from the tunnel and rerouted through the dam site. Dam construction would halt during the summer in areas below the water level. 

A close-up of the Diversion Tunnel (DT) inlet cofferdam (taken in September 1999), which forced the river to flow through the dam site. Summer flows were far too much for the DT to handle, and would cause significant damage if allowed to flow through the tunnel, so it would be blocked up for the summer so that maintenance could proceed. In August 2000, this cofferdam would be eroded away during a serious flood event, and the DT became filled with silt almost to the roof of the tunnel. The DT Bailey Bridge can be seen spanning the gap over the tunnel inlet. This bridge had a fairly limited load capacity, and this caused problems on several occasions. The cofferdam has clearly had traffic over it for some time, and was probably used to bypass the bridge for heavy loads, although I do not remember for sure at this point. A loader and two trucks have begun to dismantle the cofferdam.

A tight fit - one of the 17 tonne gate anchor girders arrives by truck across the DT Bailey Bridge. The bridge had to be realigned and beefed up especially to take this load, and even so, this photo shows the slight deformation of the bridge due to the weight. There was only inches to spare on either side as the girder crossed the bridge.

The next few photos show the sequence of removing the diversion tunnel cofferdam. The cofferdam would be removed in lifts (or layers) from top to bottom, until just one lift was left. The excavator then began removing the final lift from the upstream end, and moving to the downstream end, from where the cofferdam across the river itself would be started. 

In October 1999, the Diversion Tunnel inlet cofferdam is breached to allow the river to flow through the DT. Shown here, an excavator has removed the bulk of the cofferdam, and is about to breach the dam. Note the larger rip rap on the sides of the channel to prevent erosion.

Water is starting to make its way through the reduced cofferdam. A Hindustan 1025 off-road dumper is receiving material from the excavator.

The 1025 dumper is back to take more material. This material is stockpiled nearby, to provide a source of material to construct the upstream cofferdam that will prevent the river from flowing through the dam site.

The diverstion tunnel cofferdam is mostly removed, allowing flow through the tunnel for the 1999/2000 dam construction season.

Once the diversion tunnel cofferdam was removed, a new cofferdam was constructed across the river itself, to force the water to pass through the tunnel, and leave the dam site relatively dry.

An excavator starts to push off the upstream cofferdam that will block the river flows through the dam site.

Construction continues on the upstream cofferdam. A 1025 dumper and a dozer have joined the work.

In a somewhat precarious position, a dozer pushes material out to the end of the cofferdam, and is very close to closing the gap to the south bank.

The completed cofferdam, taken in March 2000. It has been in place since October 1999. Taken from upriver, the dam site is visible in the background.

With the cofferdam in place, the dam site would be excavated to clear sediment deposits from the summer season, and construction would resume. At some point in the spring, the river flows would increase once again, and the river would be allowed to flow through the dam once again, and the diversion tunnel would be blocked off for inspection and maintenance. The next series of photos show the tunnel interior in August 1999.

This photo was taken within the DT inlet during the August 1999, with the DT inlet cofferdam visible in the background. The river flows during the summer months were too much for the DT to handle, and would have caused damage to the tunnel lining, so the flows were removed from the tunnel and the summer months were used for maintenance purposes. You can see temporary stairs (made of sand bags) and a temporary ladder that were used to gain access to the tunnel floor.

Another view of the DT inlet, with the same access ladder from the previous image off to the right. To the left you can see the beginning of the concrete lining that was at the tunnel entrance. You can see that a considerable amount of water is leaking past the DT inlet cofferdam, and it was always advisable to have rubber boots (gumboots in the local parlance, and Wellingtons or Wellies in the parlance of my British-extracted supervisor) on hand.

This photo was taken with available light from the DT inlet, looking in the downstream direction. The Chief Design Engineer is walking further into the tunnel, right under the gate shaft for the DT inlet gate. The gate was lifted by mechanisms stored in a chamber accessible from the road above, although I never saw it used so I am not sure what the point of it was. Perhaps it was supposed to be used for the permanent closure of the tunnel after the dam was constructed, to avoid draining the reservoir, while still allowing the option of diverting the flows during the winter months for dam maintenance. The tunnel at this location was partly lined with concrete to prevent erosion from the turbulence at the tunnel entrance, but beyond the gate shaft the native rock was lined with shotcrete, a form of sprayable concrete.

A view further into the DT, about as far as you could go without a flashlight - the lighting here is from spotlights provided to aid the inspection and maintenance of the tunnel. The roof, sides, and floor of the tunnel are lined with shotcrete, and you can see a number of rock bolt heads sticking out of the tunnel roof. The rock in the Himalayas is fairly soft and of low quality, as rock goes, and has a high content of mica which is very soft indeed. This rock does not fare well when put in tension, and so long rock bolts were drilled and grouted into the rock face to provide additional strength and to try and prevent rock falls. You can see the channel worn in the tunnel floor from the water flow.

I have previously alluded to a large flood during August of 2000 that caused significant damage to the project works, and I will dedicate a future post to the post-flood damage assessment, but I will cover the affects to the diversion tunnel in this post. The flood consisted of an initial 12 metre high wall of water that swept down the river, and then a period of higher than normal river flows after the initial event. While miraculously not heavily damaged, the diversion tunnel was still affected - the inlet coffer dam was swept away, and a good portion of it (along with other river sediment and gravels) was deposited in the tunnel, filling it almost to the tunnel roof (or crown). As part of the recovery during the fall of 2000, the diversion tunnel had to be cleared out before it could be used again. This meant that a good portion of the 2000-2001 dam concreting season was lost.

Before I ever arrived on the project, the Diversion Tunnel had to be increased in length by a factor of 2, because a large rock slide occured at the location of the intended tunnel entrance. This slide remained unstable, and the flood in August 2000 reactivated a portion of the slide and wiped out a portion of the road. The Diversion Tunnel inlet can be seen near the top right of the photo, with the Bailey Bridge spanning it. The flood occurred in August 2000, while the DT Inlet cofferdam was in place to protect the tunnel from the summer flows. Unfortunately, the flood breached the cofferdam, washed it away, and a portion of the river flowed through the tunnel. The water velocity in the tunnel must have been much reduced, which served to allow the sand and silt carried by the water to settle out and fill the tunnel.

Work has begun in this photo on removing the debris from the Diversion Tunnel slide. Near the top left of the photo, you can see a Hindustan 1025 dumper and Tata Hitachi excavator working on clearing the top portion of the slide. This provides some idea of the scale involved here.

This is the outlet of the Diversion Tunnel, almost completely filled with silt, sand, and rocks after the flood breached the inlet cofferdam. This tunnel would have to be cleaned out before construction work could resume on the dam.

This, again, is the outlet to the Diversion Tunnel, but after the cleaning operation had begun. To the right are two workers, to give some perspective to the size of the tunnel. Lighting has been added to the tunnel in order to facilitate the cleaning operation. This lighting would not normally be present, and would be removed again before the tunnel could be used for its intended purpose.

This photo was likely taken just inside the DT outlet opening, showing the ribs intended to support the opening.

My boss walks ahead of me in the DT as we approach where the equipment is working to remove material from the tunnel. Wouldn't you like to be the worker assigned to make adjustments to the electrical panel on the metal stand situated in a large puddle of water?

After walking around the bend in the last photo, we approach the wheel loader as it works at the face of the silt deposit. The tunnel was fairly cramped for equipment, and this loader would have to turn around before it could deposit a bucket of silt into a truck. I'm assuming they either used a loader because it was the only one they had available, or because there wasn't enough room for an excavator to swing its boom. The tunnel would have originally been excavated using dedicated tunnel machinery, but a lot of that equipment was lost during the flood, with the remainder busy elsewhere. 

After the loader backs off to fill a truck, my boss climbs the silt face to see what lies ahead of the cleaning operation. This photo was taken on January 10, 2001, a few weeks before I left India for good, so I probably didn't see the tunnel fully cleaned out before I left.

The diversion tunnel was returned to service after I left the project in January 2001, although I am not sure if it was used at all until the fall of 2001.