Working in India: Anatomy of a Hydro Project - Head Race Tunnel (Part 9)

Some hydro-electric dams are tall enough themselves to provide the hydraulic head (water pressure caused by elevation difference, or depth) to drive the turbines, and the turbines can be placed immediately downstream of the dam itself. One of the more famous examples of this is probably the Hoover Dam (an arch dam over 200 metres high) on the Colorado River. The Nathpa Dam on the Nathpa Jhakri Hydroelectric Project, on the other hand, is a gravity dam of only 62.5 metres height, and the dam could not be built any higher due to other developments further upstream that would be flooded out. The slope of the river at the dam location is low enough that in order to achieve the hydraulic head required to drive the turbines at the generating station, it was necessary to drill and blast a 27 kilometre long Head Race Tunnel (HRT) through the rock of the Himalayas between the dam at Nathpa and the powerhouse at Jhakri. In my previous Parts 7 (Desilting Chambers) and 8 (Desilting Intakes) I covered the head works for the 16 kilometres of HRT that were in the Continental-Foundation Joint Venture's contract. 

This image shows the layout of the intakes, desilting chambers, with the HRT departing to the right of the image. Tunnels shown in red are the construction access tunnels.

The HRT construction was accessed by construction access tunnels, or adits, spaced along its length, beginning with the Desilting access tunnel (also known as Nathpa Adit). [From Wikipedia, an adit is a horizontal entrance tunnel to an underground mine works for the purpose of access, drainage, and ventilation - which accurately describes what our adits were intended to do.] During the summer of 2000, just before I was supposed to go on vacation, one of the key highway bridges collapsed and fell into the gorge. This cut off our only road access to outside civilization. As a result, my trip out started with what was probably a 6 kilometre trip through the Head Race Tunnel, starting at Sholding Adit and ending at Nugulsari Adit, before returning to National Highway 22 for the rest of the trip.

Desilting Works access tunnel, otherwise known as Nathpa Adit.

The HRT itself begins immediately downstream of the manifold of tunnels that combines the flow exiting the four desilting chambers.

This photo was taken at the junction of Nathpa Adit and the start of the Head Race Tunnel (HRT), with the outlet tunnels and Desilting Chambers behind the photographer. The masonry wall to the left of the spotlights is right at the end of Nathpa Adit. Concete curbs have been installed along this portion of tunnel, and the HRT overt form is just visible in the background to the left of the two spotlights. The Chief Surveyor stands in the foreground, and appears to be carrying the tripod that I was using that day (though apparently not for this photo).

These days, projects of this magnitude often use Tunnel Boring Machines (or TBMs) to achieve the long distance tunneling required to join Point A (in this case Nathpa) with Point B (Jhakri), which for us was a straight-line distance of 27 kilometres. This project, however, used the drill and blast method for the entire 27km. I suspect it was logistically impossible to support the operation of a TBM in our remote location, or even to ship the components there in the first place. The drill and blast method allowed tunneling to progress in multiple different locations along its length - once the adit was excavated into the proper location, the tunnel would hammerhead out in both directions towards the tunnel being excavated from the next adit either up or downstream. As one might imagine, this requires pin-point accuracy in order to have the two tunnels meet in the middle between the two adits. As far as I remember, the surveying crews were very successful in this regard, and I don't think there were any major mis-alignments of the tunnels. 

The tunnel itself was excavated in various benches, with the top bench being excavated first, and then the following benches were removed down to the bottom (or invert) of the tunnel. Drilling machines (mostly Tamrock Jumbos, with one or two Atlas Copco Boomers thrown in) would drill the rock face at the forward end of the tunnel, then the face would be loaded with explosives and blasted. Special low-profile tunnel loaders would load the rock into trucks, which would haul it out of the tunnels and to a spoil pile.

The initial benching work could be considered a bit claustrophobic.

The first bench. The tunnel overt (roof) is supported here by steel I-beams, curved to the overt profile, with the spaces between the beams filled with lagging - pre-cast concrete bricks that fit into the beams. In the background a steel tower is supporting what is presumably a particularly unstable part of the overt. CFJV Photo.

CFJV Photo.

One of the Tamrock Jumbo drilling machines. CFJV Photo.

Tamrock Jumbo. CFJV Photo.

Some early tunneling operations. CFJV Photo.

A loader rigged with a platform to allow workers to access the rock face. CFJV Photo.

A tight working area at the tunneling face. CFJV Photo.

The underground environment was very dusty and humid, and add in vehicle exhaust fumes and the air could get pretty nasty (and hard to see through). All of the adits had large ventilation fans and ducting installed to try and minimize the air quality problems, but this was not always successful. 

I was fortunate to arrive on the project after most of the early tunneling was complete, and in any case, I spent very little time underground as it wasn't my area of expertise (to be fair, I was a junior engineer, and didn't really have any area of expertise at the time to begin with).

Eventually, the tunneling was completed, and other supporting works began. Some portions of the tunnel were finished just in bare rock, but others needed reinforcing with either rock bolts or steel supports and lagging.

As with the Desilting Chambers, the HRT was excavated in lifts using drill and blast methodology - the face of the next lift is visible in the background with a worker sitting on top of it. Modern tunnel excavation uses Tunnel Boring Machines (TBMs), but the logistics of supporting such a beast in the wilds of Northern India ruled out that type of construction here. Also similar to the desilting chambers work, the HRT had its share of rock falls - this one occured in November 1999. A variety of forms of support were used for the rock in the HRT - everything from rock bolts, a combination of rock bolts and shotcrete, steel ribs and lagging (seen here), and full reinforced concrete lining were used. The steel ribs and lagging shown here obviously didn't prevent the rock fall, and this support work would have required replacement (and was probably beefed up) before excavation could continue.

This view is to the right and slightly up from the previous photo - the curved rib in the bottom left can be located in the previous image also. This photo clearly shows the framework of supporting steel ribs with concrete lagging in between. Concrete lagging was pre-cast outside of the tunnel, and each lagging (measuring say 6 inches in width and 2-3 feet in length) would have been installed individually between the channels in each side of the ribs.

This photo shows the tunnel overt, or crown, at the location of the rockfall shown in the previous two photos. Some of the concrete lagging can be seen about to fall out from between the steel ribs.

The rockfall shown in this image took place in December 1999 - I seem to recall that I had to rush out to site late in the afternoon of Christmas Eve to take these photos for insurance purposes. The other thing that I remember is that I had to wait probably 30-45 minutes before I could even take these photos - a cold metal-bodied camera and lens brought in from the outside to the warm and humid atmosphere underground would take a considerable time to defog. Both of my cameras at the time (film and digital) were subjected to many foggings, and it was a wonder that the film wasn't adversely affected by the condensation. The film camera died within two years of returning to Canada, and I always wondered if the rough handling in India - the dust and humidity - didn't have something to do with it. Mind you, it was 16 years old at the time as well. You can see the orange Tata-Hitachi excavator on top of the rock debris in the background, with a crushed Ashok-Leyland rock truck under the rock debris below it. Both pieces of equipment were damaged, with the truck a total write-off - luckily, I seem to recall that no one was killed. A plethora of rock bolts can be seen piercing the shotcreted face of the tunnel interior, and some can be seen in the area of the rock fall, indicating that further stabilization was required in this area.

Workers inspect damage to a Tata-Hitachi excavator, caused by a rockfall in May 2000. Equipment on the project were constantly being damaged by rock falls, especially the equipment that worked underground. The repair shops were always kept very busy. You will note that the workers here are not wearing hard hats, which always struck me as a dangerous practice indeed. Mind you, a hard hat probably wasn't going to protect a worker against the sort of rock that caused the damage to this excavator.

Two days later, I was photographing the removal of a large boulder that fell from the roof of a tunnel connected to the HRT. The boulder appears slightly larger than the excavator assigned to remove it.

For particularly unstable portions of the HRT, more robust methods were required to support the rock. Some sections of tunnel were lined completely in concrete.

For those sections of the HRT where it was decided that reinforced concrete lining was required to prevent a cave-in, the work proceeded in several stages - 1) reinforced concrete curbs were installed (seen here), 2) steel reinforcement was placed all around the walls and overt of the tunnel (also seen here) using the platform in the background, 3) the walls and overt were lined with concrete, and 4) the invert between the two curbs was lined with concrete.

Prior to placing the concrete walls and overt of the tunnel, a work platform mounted on rails is used to install the steel reinforcing around the tunnel's circumference. This photo was taken at the very start of the HRT, immediately downstream of the Desilting Works. Workers in the foreground lend scale to the size of the tunnel cross section.

A close-up of the concrete curb and rebar mat prior to the arrival of the HRT overt form. The inserts in the curb will eventually be used to support rails for the invert screed form. The shotcreted face of the tunnel wall can be seen behind the rebar mat.

This is the HRT overt form, looking back from an unlined section at the form. The overt form traveled on rails and would take the form of the finished tunnel shape. Bulkheads would be constructed at the unfinished end of the form, and then concrete was pumped between the form and the rock. Vibrators were hooked up to the form itself to vibrate the concrete and prevent honeycombing. The form would then be pulled in on itself using hydraulics, moved to the next unfinished section, and the work would continue. This was probably the largest and most complicated mechanism used during the construction of this project. Concrete pours on this part of the work were long and drawn out affairs, as there was not enough room for two concrete hopper trucks to pass each other between the concrete curbs let alone for a truck to turn around. A concrete truck would therefore have to drive all the way from the concrete plant to the marshaling area inside the nearest Adit, then back up the length of the tunnel to the location of the form before dumping its load of concrete into the concrete pump. The truck would then return to the concrete plant, with the subsequent truck backing down the length of the tunnel once the preceding truck was clear.

A diagram I prepared for a technical paper submitted regarding the project, showing the concrete placement process. Trucks would dump into the surge hopper (complete with a screw conveyor), which would feed the concrete pump. The concrete pump would supply concrete to feel points on the HRT form. 

The bottom articulated section of the HRT overt form, seen here almost in position but pulled in slightly from the curb. The form would push out right to the curb to prevent leakage during a concrete pour. As you can see from the size of the ribs, the form was designed to withstand significant pressure during a concrete pour.

Another view showing the sheer scale of the HRT traveling overt form.

Another image of the HRT travelling overt form, this time from the finished side of the form with the concrete overt lining in place. If you look closely towards the bottom right of the image, you can see my old Minolta Maxxum 7000 film camera mounted on a tripod, possibly while taking a long exposure but more likely while I waited for it to defog. The works lend scale to the side of the tunnel and machinery.

This is a film image taken of the HRT traveling overt form and concrete handling machinery. The equipment in the foreground is the concrete hopper and concrete pump - trucks would drop the concrete into the hopper, and the pump would lift the concrete to the top of the form and inject it between the form and the rock face of the tunnel interior. This is a partly completed section of tunnel, with the curbs and overt lining completed. Only the invert concrete work remains. Two workers on a ladder at the right of the frame patch up the surface of the concrete liner. This view appears in the very background of the previous image in this gallery.

Once the benches were placed, followed by the overt concrete, only the invert remained to be concreted.

The invert (floor) of the tunnel still needs to be concreted in this photo.

This is the HRT invert screed form that was used to complete the tunnel lining process. Concrete would be placed in front of the path of travel for this form, and the screed would move through the concrete to impart the circular tunnel shape. The unit mounted on the middle of the form is a vibrator, which would vibrate the entire form to provide a smooth finish to the concrete without honeycombing. The form rides on the rails seen to either side of the photo.

Another photo showing a completed section of concrete lined Head Race Tunnel. You can see that the process of screeding the invert concrete didn't produce a surface that was quite as polished as the formed surfaces of the curbs and overt. As I recall, only a portion of the 27 kilometres of HRT was lined with concrete like this section, although I do not remember the relative proportions. Only 16 kilometres of the total 27 kilometres was in the contract that my company had. As with all the underground works, water was a constant problem.

Based on a graphic I prepared for a presentation, there were 9 zones of Extraordinary Geological Occurrences (EGOs) within our 16 kilometre section of HRT. Some of these EGOs were fairly closely spaced, and it is quite likely that concrete was run for the entire section where these EGOs were to prevent cave-ins. Once the construction infrastructure was demobilized, it would have been very expensive to recover from a cave-in once the project was in service and producing power, so it really had to be done right the first time. 

There were also some underground works at Sholding to support the Head Race Tunnel, which I will probably cover in a future post.

Working in India: Anatomy of a Hydro Project - Desilting Chambers and Tunnels (Part 7)

The waters of the Satluj River in the Himalayas of Northern India transition from green in the winter, when flows are low, to chocolate brown in summer when snow melt higher up causes flows to ramp up and the river carries increasing amount of sediment.  Not only is sediment bad for the turbines at the generating section, far down stream, but it also increases the abrasiveness of the water in the Head Race Tunnel (HRT), wearing away the rock and concrete lining of the tunnel. During the August 2000 flood, sediment laden flood waters sand blasted the exposed dam sluiceway piers right down to the reinforcing steel, and actually wore down the nubs on the steel itself. On top of this, the soft waters of the Satluj River are normally fairly hard on concrete to begin with, and don't need assistance in wearing it down. 

The Nathpa Jhakri Hydroelectric Project therefore includes a large desilting works, consisting of four 500 metre long underground desilting chambers and the related intake works and network of connection tunnels, plus a silt flushing tunnel that returns silt from the collection hopper of each desilting chamber back to the river.

The desilting chambers are labelled at the top centre of the diagram, and are shaded in blue, along with the intake tunnels and the start of the head race tunnel. The Intakes are coloured green, just above the dam.

A diagram showing the four desilting chambers in section, along with the upper access tunnel, stairwell shafts, and access galleries. 

When I arrived in February 1999, the excavation of the desilting chambers themselves was well underway, as well as most of the connection tunnels. The intake works, on the other hand, were still being excavated and concrete work had not yet begun.

Inside Desilting Chamber #1, looking downstream. Excavation of a lift, or bench, continues in the far background. A Caterpillar front-end loader and workers can be seen in the foreground. The stair tower behind the loader leads to one of the maintenance and inspection access galleries that would provide personnel access to the chambers once they were commissioned and in service. Health and Safety procedures and regulations in India are not what I am used to in Canada, and I recall that climbing to the top of this stair tower was one of the scarier things that I did while I was on this job - mind you, I am afraid of heights, but I recall there there were no railings on the top level. The bottom of the chamber will eventually narrow out to a trough running the length of the chamber. The idea behind the Desilting Chamber was that the size of the chamber would allow the speed of the water flow to slow down enough that the silt and sand particles would settle out into the trough at the bottom, and be returned to the Satluj River via the Silt Flushing Tunnel (SFT). The clean water, with silt removed, would be siphoned off the top of the chamber and directed to the Head Race Tunnel (HRT) which would transport the water to the turbines at the powerhouse at Jhakri, 27 kilometres away.

The port in the background of this image leads to the stair tower seen in the previous photo, and is one of the access gallery manholes indicated on the schematic above. The port itself is a metal collar with bolts, cast into the concrete, which would be used to secure and seal the watertight hatch that would eventually be installed there. When the chambers are full of water, this gallery would be underwater. The stair shaft that provides access to this gallery from above is behind and to the right of the photographer. The stairs were not complete when I left the project in 2001.

This is the downstream bellmouth in Desilting Chamber #3 while under construction in April 2000. The formwork on the invert and walls is still in place here, and concrete work is progressing up towards the overt. Chamber excavation has not proceeded very far here, and the chamber will get much deeper - this bellmouth is located at the very top of Chamber #3. This is where clean water, with silt removed, would leave the chamber to head for the HRT.

Taken the same day as the previous image, but in Desilting Chamber #4, this bellmouth is slightly more advanced in construction with the walls pretty much complete. Only the overt form installation and concrete pour remain. The lift lines can clearly be seen in the concrete - 6 lifts and part of a 7th can be counted from the top of the current lift to the top of the concrete work. The outlet tunnel can be seen in the background through the bellmouth opening.

A manifold of four tunnels join the bellmouths at the downstream end of each chamber with the Head Race Tunnel (HRT).

The inside of the outlet tunnel from Desilting Chamber #4, looking in the opposite direction from the previous photo. The support towers and the ski jump concrete form for the overt pour are sitting in the tunnel waiting for installation. The HRT is behind the photographer in this photo.

A worker looks at a profiling platform in Outlet Tunnel #2. Once the mass tunnel excavation was finished, a work platform such as this would be run through the tunnel on rails to determine where the rock line was intruding into the tunnel profile, and workers could have a go at removing the protuberances with pneumatic hammers. This platform was also fitted with a steel tunnel profile that marked the minimum excavation line, but for some reason that item is not visible here - perhaps this was taken just as the platform was being assembled.

This photo was taken at the junction of Nathpa Adit and the start of the Head Race Tunnel (HRT), with the outlet tunnels and Desilting Chambers behind the photographer. The masonry wall to the left of the spotlights is right at the end of Nathpa Adit. Concete curbs have been installed along this portion of tunnel, and the HRT overt form is just visible in the background to the left of the two spotlights. The Chief Surveyor stands in the foreground, and appears to be carrying the tripod that I was using that day (though apparently not for this photo).

Excavation of the desilting chambers didn't always go smoothly. I've previously mentioned the poor rock conditions on this project, and the chambers suffered from this as well. There were at least two rock falls during my time on the project, and possibly more. The company brought in a consultant geologist from Canada to advise on how to better support the rock during construction, and the end result was a large number of cable anchors running from chamber to chamber through the rock pillars between each chamber, in order to better hold the rock together.

Chamber excavation was not without its challenges and dangers. Even though the rock was stabilized and reinforced with rock bolts and lined with shotcrete, rock falls such as this one were not an uncommon occurrence. The rock bolts can be seen sticking up out of the rock debris - failures like this occured when the depth of the failure plane exceeded the depth of the bolt penetration into the rock.

A rockfall, this time in Chamber #3, in May 2000. The workers in the frame give a sense of scale to the amount of rock that came tumbling into the chamber, and rock bolts can be seen both in the debris and sticking out of the exposed rock face. After further study by geologists, it was decided to drill through the common walls between chambers and install cable tension anchors between chambers to support the rock in those walls. This work was done after I left, and was hindered by the fact that the chambers had mostly been excavated to near full depth by the time it became apparent they were necessary.

Also ongoing during my time on the project was the excavation of the intakes and intake tunnels. 

The inlet tunnels brought water from the Intakes through to the Desilting Chambers (you can see pictures of the Intakes in the Surface Construction Gallery). One of the jeeps is parked in Inlet Tunnel #3, which has had its invert lined with concrete. The screed form that shaped the concrete would have run on the rails on each side of the tunnel. These rails would later be removed and replaced with new rails mounted right on the concrete invert, to be used by the overt form. A steel bulkhead is visible in the background at the entrance to the tunnel.

This is a rebar installation platform in Inlet Tunnel #1. The tunnel invert concrete has been placed, and the rails have been relocated from the position in the previous photo to the top of the invert concrete. The platform is riding on those rails. As seen here, workers on the various levels of the platform work to install reinforcing steel (rebar) around the circumference of the tunnel overt (crown) prior to the lining of the tunnel with concrete. If you look closely, you can see that one of the workers is actually climbing on the rebar cage between the rebar and the rock wall. My supervisor, the Chief Design Engineer, designed this platform and I was responsible for producing the production drawings that were sent to the steel fabrication shop.

At the upstream end of the intake tunnels was the intake works. The intakes were large concrete structures intended to direct water into the tunnels and desilting chambers with as little turbulence as possible. Up front was a trash rack of vertical fins, intended to prevent large floating debris from entering the system, followed by bellmouths to each tunnel. Very little of this work was completed when I left the project.

Plan view of the intakes showing the trash rack and bellmouths to intake tunnels 1 through 4.

I will leave the intakes themselves for a separate post.

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.