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:
- a 62.5 metre (~205 foot) high concrete gravity dam at Nathpa, on the Satluj River;
- concrete intake works and tunnels;
- four 500 metre long underground desilting chambers;
- 27 kilometres of head race tunnel (HRT), running from Nathpa to Jhakri;
- 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.
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.
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.
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.
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.
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.
|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.
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.
|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.|
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.|
|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.|
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.
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.