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 - Dam (Part 5)

Having shown some of the scenery I photographed while in India, I should probably cover the reason that I was in India in the first place: the construction of the Nathpa Jhakri Hydroelectric Project. The project consisted of many parts, the major portions being as follows:

1. a 62.5 metre (~205 foot) high concrete gravity dam at Nathpa, on the Satluj River;
2. concrete intake works and tunnels;
3. four 500 metre long underground desilting chambers;
4. 27 kilometres of head race tunnel (HRT), running from Nathpa to Jhakri;
5. a 1500 MW (megawatt) powerhouse at Jhakri with six 250 MW turbines.

Of the above, 11km of #4 and all of #5 were on a different contract - the Continental Foundation Joint Venture (CFJV) I was working for was handling the remainder. On top of the list above, there was also a significant number of temporary works (bridges, shops, roads, etc) that were also the responsibility of the contractors, and which we were responsible for designing.

An overview of the dam site (looking west), with the upstream coffer dam just visible in the bottom half of the photo, just above the grassy foreground slope. Looking downstream, the bank on the left is the Left Bank and the bank on the right is the Right Bank. Simple. The diversion tunnel inlet is just out of site to the right of the upstream cofferdam. This photo was taken on April 27, 2000. The Satluj River can go anywhere from 50-80 cumecs (cubic metres per second) of flow in the winter months, all the way up to 2000 or more cumecs in the height of summer when snowmelt is at its greatest. During the summer, dam construction would halt, and the upstream cofferdam would be dismantled to allow the river to flow right through the dam site.

There are two main types of concrete dam: gravity and arch. The latter is kind of like a bridge arch on its side, with the top of the arch pointing upstream, and the bridge abutments braced against the side of the valley or gorge in which it is built (think Hoover Dam). We were building the former - a 62.5m high concrete gravity dam, intended to hold back the weight of the water through sheer weight of concrete and friction with the bedrock on which it is built. 

Another aerial view of the site at Nathpa. The concrete structures just left of centre are the intakes and the crane platforms built above them. To the right, barely visible, is the concrete structure of the dam slowly coming out of the ground. A bridge and several groups of workshops can be seen downstream, towards the top right of the image. The road running along the top left of the image is National Highway 22. The traveling end of the cable crane can be seen just below the highway, but up the slope from the intakes.

A schematic layout of the dam and intakes area. Top is upstream, bottom is downstream. The orange coloured "Adits" are access tunnels, some of which would have been filled in after construction. The Head Race Tunnel (HRT) leaves the sketch to the right, while the Flushing Tunnel returned silt to the river itself.

The dam creates a reservoir of water in the river, which enters the intakes and desilting complex, then makes its way through the headrace tunnel to the powerhouse 27km away. This creates 428 metres (1400 ft) of hydraulic head to turn the turbines, which produce electricity. Therefore, the dam needs to hold water back - it can't let water go through or around it, the water must pass through the intakes (unless deliberately allowed to spill over the dam through the spillways). 

Bedrock laid bare downstream of the dam - this area is called the apron, and was filled with concrete.

In order to ensure the dam is watertight, construction began by excavating away the riverbed until bedrock was reached. The bedrock in the Himalayas isn't the best quality, as it is fairly young rock, and it has a lot of cracks - those cracks were filled by injecting cementitious grout into the bedrock. Tunnels were excavated into the valley walls on either side of the dam so that a grout curtain could be made around and under the dam. Basically, any water that wants to push its way past the dam must do so by infiltrating cracks in the rock all the way around the outside influence of the grout curtain, and then back through cracks until it reaches the river valley again. The volume of water making this journey should be very small indeed. 

Workers in one of the grouting galleries on the side of the dam.

Once the bedrock had been uncovered and the cracks grouted, the dam concrete itself could start to be placed. The dam is made up of 11 blocks across its width, with only the centre blocks going the full depth of the reservoir - the wing blocks on the sides are a fair ways up the valley walls and are cast into a notch cut into the rock. I can't recall exactly, but I think each block was about 15m wide, and this was limited to allow expansion joints to be placed between each block to minimize cracking of the concrete. In addition, each block was poured in 1.5m deep lifts. The curing of concrete is a chemical reaction, and monolithic concrete pours create "heat of hydration" during the curing process. If you pour too big a block of concrete at one time, the heat created by the curing reaction will actually cause the surrounding bedrock and concrete to crack. Cracks in a dam are bad.

A concrete pour proceeds on a lower lift in a dam block, probably Block 4. The blocks proceeded in a staggered fashion, partly for scheduling purposes (to allow freshly poured concrete to cure) and the need to have some blocks above the summer flood levels, and partly to allow better dispersal of the heat generated by curing concrete (called heat of hydration). Each dam block was poured in Lifts of 1.5m in height, although some early lifts were poured in 0.75m half lifts due to concerns about cracking of the bedrock from heating. The concrete is dropped to the dam via a concrete bucket suspended by a cable crane that runs across the valley; the concrete buckets are supplied with concrete by trucks that deliver concrete to the loading dock high on the right bank. As shown here, the concrete lift is poured in three layers that advance from the downstream end to the upstream end of the dam. In Block 3 to the left, you can see the climbing forms that hold the concrete within the block during a pour. Once the pour is complete and the concrete has cured sufficiently, the forms are removed and are lifted up for the next pour. Block 4 only needs forms at the upstream and downstream ends because the rest of the poured concrete is confined by the neighbouring dam blocks.

A concrete pour proceeding, probably in Block 4. CFJV stands for Continental Foundation Joint Venture, made up of Continental Construction Limited (CCL) and the Foundation Company of Canada (FCC). The latter was my employer at the time, but by this time only operated in India under this name - back in Canada it was called BFC Civil, and has since been renamed again to AECON. 

Concrete was batched at a plant nearby, and trucked to a platform from which it was dumped into the concrete bucket in the photo above. The distance was so short, hopper trucks (instead of the mixer trucks one is accustomed to seeing) were used, and they could dump their concrete very quickly into the bucket. A cable crane spanning the river valley would pick up the bucket and dump it in the dam block being poured. 

To keep water from forcing its way between each dam block, a total of three different kinds of waterstop were placed at the front of each joint: one copper, one PVC, and one a bituminous substance that was poured into a diamond shaped groove formed between blocks. 

The three waterstops between dam blocks, probably Block 8 and Block 9, start at the base of the following block. In this case, Block 9 will start from this elevation and head upwards. In the previous pour, the two blocks were being poured monolithically. The copper waterstop is furthest upstream, the PVC (rubber) waterstop is furthest downstream, and there is a gap left in between the two into which liquid asphalt or similar substance is poured after the concrete pour. Three separate and different waterstops were used on each joint to provide redundancy.

The concrete pour continues at the upstream end of Block 4. The various waterstops (3 in total) can be seen cast into the front edge of Block 5. These waterstops prevent water from infiltrating along the construction joint between each dam block. The dam is poured in multiple blocks with construction joints between each block to reduce cracking of the concrete. The rebar over the gallery and stairs from Photo 19 can be seen to the bottom left of the photo. The exposed end of a mid-height gallery is visible in the side of Block 5 - I seem to recall that there were at least 3 levels, if not 4, of galleries in the dam. Each gallery is used to check for leakage in various parts of the dam, and to provide access to strain gauges that are incorporated into the dam to record stresses within the structure. The various galleries continue into the rock at each end of the dam on the left and right banks.

After a lift was poured in any given block, the concrete was allowed to cure. A scum would form on top of the curing concrete, which would be removed through high pressure water - a process called "green cutting". This would expose the aggregate (rock) on the top of the lift of concrete, which would allow the subsequent lift of concrete to bond better to the lift below. While a block was curing, the blocks on either side of it might be poured, to keep work going. The dam was poured in such a way as to keep one block several lifts ahead of the block behind it, partly so that the formwork of the leading block did not interfere with the lagging block. 

In this manner, the dam was slowly poured to its full 62.5m height. The five central blocks contain the spillways: Block 6 had the central spillway, with two smaller spillways on either side in Blocks 4 & 5 and Blocks 7 & 8. The spillways contained large steel gates that would normally be kept closed in winter, but could be lifted up in spring, summer, and fall to allow water to pass and regulate the level of the reservoir. 

Standing in the Block 7 sluiceway bucket area and looking up at the piers of Blocks 7 and 8. You can see the parabolic shape of the sluiceway in Block 8 taking shape here, between the two piers to the left. The inset portion of the side wall of the sluiceway is meant for the sluiceway liner, which was high-density concrete in some dam blocks and high strength steel in others. The Satluj River runs full of sediment in the summer months, due to the soft rock in the Himalayas, and the combination of fast flowing water and sediment is the equivalent of sandblasting to concrete. Add to this soft mountain water that eats concrete, and you need resistant liners for the sluiceways to prevent the concrete from being eaten away. The right bank loading dock can be seen at the top left of the image.

During my time on the project, none of the gates themselves were installed. Only two of the gate girders that would support the gates were installed, and one of those was washed away in a flood. 

A crawler crane places one of the 18-tonne gate girders in place on the right pier of Block 8. There are two of these girders for each of the sluiceway blocks. Two girders would be installed before a large flood occurred in August 2000, during which the left girder in Block 8 (not the one shown) would disappear and not be seen again for another year or so.

The two gate girders are now placed in Block 8 - the left girder would go missing in a flood in August 2000, ripped from its housing and buried in silt and sediment. I never saw it again, as I returned to Canada before it was found.

Behind the sluice buckets and dam structure is the concrete apron, constructed to protect the bedrock from the water flowing through the sluiceways from erosion that could undermine the downstream end of the dam. 

Excavation in the apron area. 

Excavation work on the dam didn't always go smoothly, especially if a pump failed.

Excavation in the apron area. One of the construction superintendents inspects the results of a pump that failed during the night shift. Workers were present when it happened, but for whatever reason were unable to find a replacement before the excavation flooded. The excavator from the previous photo can be seen sticking out of the water. Due to the steep sides of the excavation, the machine could not be removed without the use of a crane, which was apparently not available. The side of the excavation was turned into a ramp, and after the water was pumped out, an operator clawed the machine out of the hole after which it was overhauled by mechanics.

The apron area was excavated down to bedrock, and a pattern of reinforcing steel dowels were drilled into the rock, and a cage of reinforcing steel was constructed and anchored to the dowels. The whole area was later filled with concrete in blocks, similar to the dam itself.

Workers empty a concrete bucket in the apron area behind the main dam. They are pouring a leveling slab directly onto bare bedrock here. If you look closely, you can see the well worn surface of bedrock that has been subjected to the flow of water. The concrete to the right is the back wall of the sluice bucket section of the dam. The sluice bucket will cause water passing through the sluiceways to jump into the air at the back of the dam, and if it were to crash down on the rock behind the dam it would erode the rock away and eventually undermine the back side of the dam. Thus, a concrete apron is poured at this location to protect the rock. The 1435 painted on the dam, with the line next to it, represents an elevation above sea level (i.e. 1435 metres) as well as the top of the concrete apron at this location.

The concrete apron behind the dam follows the contours of the bedrock. The first task is to drill and grout in anchors of reinforcing steel, to prevent the apron from separating from the bedrock. This part of the apron is being infilled between two completed portions. I was responsible for laying out the anchor locations prior to them being drilled in the field.

Since the previous photo was taken, the anchor installation has been completed, formwork has been added to the downstream face of the pour area, and the rebar mat that will reinforce the exposed face of concrete has been installed (as detailed by yours truly).

Concrete is now being poured into this portion of the apron. The pour is staggered in lifts, and although the fresh concrete has not yet reached the upstream end, workers have already begun to finish the concrete surface at the downstream end.

The concrete apron follows the contours of the bedrock behind the dam, and here you can see the shape of the reinforcing steel echoing the shape of the rock. One of my jobs was to detail the steel in this area, and send the details to the steel fabrication yard for cutting and bending.

One of the more impressive pieces of temporary infrastructure built to construct the dam was the cable crane over the dam site. 

This view takes in both the tail tower (foreground) and the right bank anchor point (background). The next photo shows a closeup of this view.

With the cable reel on the (left bank) traveling unit in the upper right of the image, this photo looks all the way across the valley to the right bank anchor point (look in the upper middle of the photo). The traveler head can be seen on the cable near the centre of the image - this unit traveled back and forth across the valley, and the crane hook would rise and lower below the traveler.

Another view of the cable crane's traversing unit. The large wheel shown here is a reel for the power and control cables. The traverse speed along the track was quite slow, and so it was best if the traversing unit could be lined up with both the area of the dam being worked on and a portion of the loading dock on the right bank where concrete hopper trucks could dump concrete into the concrete bucket. Sometimes the cable would not intersect the loading dock, and the speed of a concrete pour would be severely curtailed as the traversing unit would have to go back and forth each time. When this happened, it was preferred to have a crawler crane carry out the pour.

The Cable Crane tail tower sits on the rails near the trestle. The trestle was constructed later than the rest of the track, once construction on the dam required more coverage area from the crane.

This photo was taken while riding on a platform hung from the Cable Crane, looking straight up. Talk about vertigo! I seem to recall that I wasn't very happy taking this photo, as I am not terribly fond of heights, and looking straight up while having a drop of several hundred feet below just doesn't do anything for me.

While the overall project ran from 1993 to 2004, I was only assigned to the project for two years from 1999 to 2001. As such, I wasn't able to photograph the final stages of dam construction. I will end this post with a photo taken in the concrete apron area, showing some of the people I worked with on this project. 

A number of CFJV employees stand on the bedrock in Block 11 prior to concrete being poured. To the right of the photo you can see the painted line that represented the divider between Blocks 10 and 11. Top left employee is a senior superintendent from British Columbia, while the man immediately right of him is the Chief Design Engineer (my boss at the time).

So, how do you build a dam in the middle of a flowing river? You don't. My next post on this project will cover the diversion of the river around the dam site.