Saturday, 19 March 2016

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. 

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