Saturday, 12 March 2016

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. 

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