Working in India: Anatomy of a Hydro Project - Desilting Intakes (Part 8)

My previous posting covered the underground desilting chambers and tunnels, but including the intakes themselves was going to be a bit much for a single post, so I'm breaking them out into a separate post. I should also note that during my time on the project, the intakes were not completed, so while I have plenty of "before" images, I unfortunately have no "after", only "during construction" photos (and early construction at that). 

When I arrived in February 1999, the intakes themselves had not been excavated, and the eventual tunnel mouths had not yet been exposed.

(Note: I added the image below some weeks after initially completing this post, having forgotten to include it the first time around.)

A plan view of the intakes showing the trash rack and intake bellmouths for each of the intake tunnels. The triangles mid-way down the image in each tunnel represent the transition between square and horseshoe profiles that is mentioned below.

I should note that some of the photos below, especially the ones showing concreting work, are not in chronological order. 

A close-up of the intakes area, taken before the intake tunnels were daylighted to the surface. The rock in this area of the Himalayas is very soft and prone to rock falls - as the Himalayas are relatively young in geological terms, mother nature hasn't had as much time to beat the mountains down (compared to, say, the Rocky Mountains in North America) and the rock has a high content of mica, which is very very soft. It was not uncommon to be able to walk up to a boulder on the side of the road and break off pieces with your hand. As a result of this, the project required a considerable amount of rock stabilization work, as is evident in this photo with row upon row of cable anchors in the rock above the intakes. Horizontal holes are drilled deep into the rock, and cables are inserted into the holes, anchored, and then stressed and tied off to anchor plates that are cast into the concrete on the surface. The dark spots on the concrete are the heads of the cable anchors. The intakes were out of the arc covered by the cable crane, and therefore all work in this area had to be supposed by the crawler cranes. The three tunnel openings to the right of the image are the dam access galleries, and the front face of the dam itself would have been to the left of these tunnels, between them and the zig-zag stairway. The pile of fill has been placed there to allow cable anchoring operations to continue down the rock face. The top of the zig-zag ladder represents the level of the intakes access road, and the top of the dam. Two crane platforms were eventually brought up to this level to allow concreting work on the intakes to proceed. The cable crane used on the dam, and shown in a previous post, did not reach far enough to cover the intake area.

Viewed from a higher elevation, and further upriver, this view from Spring 1999 shows the terracing of rock anchors better, along with the access road and the cable crane location. The road at the top left of the image is National Highway #22.

During my time on the project, as the cable anchoring of the rock face was completed, the fill pile in front of the intakes was slowly removed. The intake tunnels connecting the four underground desilting chambers to the intakes were daylighted, and concrete work began.

A close-up of the intakes area, with a similar viewing angle to the first image in this post, taken after the intake tunnels were daylighted to the surface but before concrete work on the intake structure began in earnest. In this photo, only the concrete foundations for the intakes has been started. An NCK Rapier crawler crane is at work. The intakes were out of the arc covered by the cable crane, and therefore all work in this area had to be supposed by the crawler cranes. Later in the project, when concrete work began in earnest, an NCK was mounted on each of the two concrete platforms (about the same level as, and on either side of, the top of the crane boom in this photo). If you look closely on the left side of the photo, you can make out the many flights of stairs that were required to descend from the road level to the base of the intakes. This was fine on the way down, but the climb back up required a bit more effort (and maybe a break or two to catch your breath)! 

March 14, 2000: formwork is up and concreting of the intakes is underway. The crawler crane has not yet been moved to one of the platforms over the intakes. 

Taken on May 6, 2000, the placement of concrete for the invert of the intakes bellmouth is now complete. The concrete work in the foreground, at the bottom of the image, is the concrete of the dam coming up the right bank. The upstream cofferdam has been breached for the summer months, and the river is flowing through the dam site. 

May 12, 2000: Reinforcing steel (rebar) being installed at the base of the intakes. The beginning of the bellmouth rebar (the vertical bars to the right) is taking shape.

June 5, 2000: These are two ski-jump concrete forms for the crown (top) of the intake bellmouths. I would have rendered these in AutoCAD, so that the Steel Fabrication Shop could build them (which is where this photo was taken). If I remember correctly, the bellmouths were parabolic in shape. I think both forms were required to pour a single bellmouth, but after 15 years I may be mistaken. Due to a flood in August 2000, these forms never saw use while I was on the project. In my time on the project, I spent some time at the Steel Fab Shop making sure work was fabricated per the drawings I prepared.

June 5, 2000: The partially completed intake crown transition formwork, looking from the downstream (inside) end to the upstream (outside) end. Steel was bent to the shape required, as provided on drawings I prepared. Every single rib in this structure was a different shape, and had to be drawn and calculated separately. The far end was the trickiest both to draw and to build. The forms were then skinned with planks and plywood, as can be seen here. The entire transition form could be transported in four parts to reduce the weight. The whole assembly would be propped up by short support towers. The concrete work formed by this formwork would be just downstream of the concrete formed by the ski-jump formwork in the previous photo.

July 8, 2000: The transition of Intake #2 takes shape. Located just behind the yet to be poured bellmouth, the transition section changes from the rectangular shape of the bellmouth to the inverted horseshoe shape of the intake tunnel over a length of perhaps 10 to 12 metres. The designers provided probably 5 or 6 intermediate shapes for the transition, and then I got the job of transferring the designer approved shapes plus the intervening shapes into AutoCAD, along with all the support structural members. In the photo, the concrete walls are poured up to about half height, and the transition overt form (from the previous photo) can be seen in the background.

A worker sprays water on the recently poured concrete that forms the bottom of the intake structure.

There was some concern that flooding on the river might top the concrete shown in the above photo, and end up flooding out the underground desilting works. As such, steel bulkheads were constructed for each of the four intake tunnels, to be installed just downstream of the intake works. Each bulkhead had to be shaped to match the rock profile of each tunnel: we had an advanced (for the time) laser tunnel profile instrument that would provide an exact profile of the tunnel at the location of each bulkhead, which I then imported into AutoCad and turned into a fabrication drawing. The bulkheads were anchored into the rock of each tunnel, and were designed by the Chief Design Engineer to withstand the potential hydraulic head of a large flood.

March 13, 2000: The contractor was worried about the risk of flooding of the intake tunnels and the desilting works behind, and so steel bulkheads were constructed at the inlet to each tunnel. In this photo, the bottom half of a bulkhead is being installed in Intake #2. The bulkhead was cut more or less to the shape of the rock, and then filled in. You can see the shape of the tunnel invert (floor) here, already poured in concrete.

March 13, 2000: The upper half of the Intake #2 bulkhead is swung into place by a small (yellow) Escorts scissor crane that is hidden in behind. The top half is later welded to the bottom half.

March 14, 2000: The partially completed Intake #2 bulkhead, with the top half installed. The lower port in the bulkhead is for man access, while the upper is a fan port. During the August 2000 flood, all the bulkheads held back the weight of water and silt that were thrown against them. Although there was some initial water ingress through the open fan ports (and the fans themselves), the silt quickly built up in front of the bulkheads and sealed the holes shut. The bulkheads therefore prevented the damage to the desilting works from being much more serious than it was.

March 16, 2000: The interior view of the Intake #2 bulkhead. You can see a bit of daylight coming in around the outside of the bulkhead, and this would later be filled in with a combination of steel plating and concrete (if I remember correctly).

As I've alluded to, the construction of these bulkheads was somewhat prophetic considering the flood event in August 2000, but I will cover that flood in a separate post. My next post on this project, though, will cover the 27 kilometre long Head Race Tunnel.