Sunday, 1 May 2016

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.

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