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