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Dam Good!

01 Aug 2015 Long Read
The Teesta IV dam, being built by HCC for the National Hydroelectric Power Corporation (NHPC), is the third of its kind in India, and the first RCC dam project for both companies.

Building a hydroelectric (hydel) power project is a complex process. In India, the majority of the run-of-the-river hydel projects involve significant underground work, including powerhouse, head race tunnel and associated component construction. However, dam construction has not seen significant technological enhancement over many years.

RCC for dam construction
Since its introduction in 1980 in Japan, over 500 roller compacted concrete (RCC) dams have been constructed world over. Now, India is catching on. The Ghatghar and middle Vaitarna dam have been constructed using this technology. Now, the Teesta IV dam, being built by HCC for NHPC, is the third of its kind in the country. This is the first time both NHPC and HCC have taken on an RCC dam project.

Salient features
The 160-mw Teesta IV Low Dam Project, built in the Brahmaputra basin on Teesta River around 25 km from Siliguri in West Bengal, falls in seismic zone No. 3 (moderate damage risk zone). The total length of this dam is 504 m with a height of 30 m above the riverbed level and 45 m above the deepest foundation level, with a stilling basin for energy dissipation. The dam can be broken up in three segments: The power dam and surface powerhouse on the left embankment, spillways in the middle and RCC dam on the right embankment. The RCC dam section is about 200 m long with a unique cross-sectional geometry having steps on both upstream and downstream sides.

Unique features of RCC
RCC has the same ingredients as conventional concrete but in different ratios, and increasingly with partial substitution of fly-ash for Portland cement. RCC is a mix of cement or fly-ash, water, sand, aggregate and common additives, but contains much less water. The produced mix is drier and essentially has no slump. It is placed in a manner similar to paving; the material is delivered by dump trucks or conveyors, spread by small bulldozers or specially modified asphalt pavers, and then compacted by vibratory rollers. This technique reduces construction time of dams by more than half. Also, compared to a conventional concrete gravity dam, depending on the size, construction cost is lower by nearly 5 to 30 per cent.

The construction
For the construction of Teesta IV Low Dam, the river was diverted into two stages. The first was from the left bank to the right so that the work on the spillway, power dam and powerhouse could be started. After completing the spillway and power dam works, the second stage of diversion was done through spillways for construction of the RCC dam.

RCC dam construction requires a huge setup for the continuous feed of concrete. HCC had set up a crusher plant to produce aggregates, silos for storing and procession aggregates, batching and mixing plants, chilling plants and ice flex manufacturing plants. All these were connected by a web of conveyor belts to produce and drop the RCC at the dam location. This was collected in dumpers standing on the dam body, relayed to the required location and poured.

Once sufficient quantity was gathered, it was spread using dozers into over 300-mm-thick layers. After spreading, 10 tonne rollers were used to compact the concrete. A special ´nuclear density gauge´ was used to test the compactness.

Once concrete passes the test, the layer is declared approved for further construction. This layer is then continuously cured until the next layer covers it. The challenge here is to keep the layer live before the next layer is laid. This was done with the use of chemicals called retarders, which delay the setting of concrete. Thus, the concrete is placed layer by layer in succession without any discontinuity, creating a monolithic dam.

Challenges in execution ¨Timelines were critical,¨ says Santosh Kumar, Project Manager for Teesta IV Low Dam, HCC. ¨As rains stop concrete work, we wanted to finish the job in one season. And the team rose to the occasion by finishing the 196 m dam in just 196 days without any holiday or break.¨ To this, Punnaswamy, consultant to HCC on RCC methodology, adds, ¨There were other construction and technical challenges. Here, the peculiarity is that you have steps on both sides. Every 0.9 m, you are closing in the dam by 1 m. It eats up the space quickly. So how do you manage your equipment within the space that you have? You should already have built the dam in your mind.¨

Here are some of the key challenges the HCC engineers had to overcome to maintain continuity of work:

  • Managing supply chain: Fly-ash was sourced from NTPC´s Kahalgaon Power Station in Bihar. It was transported in bulker trucks, which were emptied in the silos at project locations. The thermal power plant is around 320 km from the project site, which is a 20 to 32-hour drive for the trucks. About 115 tonne of fly-ash was required on a daily basis to meet the continuity of construction, translating into four bulker trucks daily. HCC had deployed 29 bulker trucks to source 210 tonne of fly-ash on a daily basis during peak construction time. Similarly, cement was sourced from Darjeeling-based Star Cement, around 50 km from the site. Around 75 tonne was required daily, translating into three bulker truck loads. In peak time, this requirement used to touch around 135 tonne. The aggregates were prepared at HCC´s crushing plant on site for which the boulders were handpicked from the riverbed. The site identified by NHPC to source boulders was around 25 km from the project location. Around 2,500 tonne of boulders were required daily, translating into around 100 dumper trips.
  • Synchronisation of equipment: HCC had to install a special four-stage crusher plant to produce aggregates. These were produced in four different size fractions: 50 to 25 mm, 25 to 12.5 mm, 12.5 to 5 mm, and 5 to 0 mm. Over 4,000 tonne of aggregate was required per day to meet the peak RCC demand. Hence, huge storage space was required. However, space was sparse and the team had to depend on just one months´ inventory. Three fully automated batching and mixing plants equipped with powerful twin shaft mixers for 24/7 operations were installed. Each plant had a capacity to produce over 120 cu m per hour of RCC. Of these three batching and mixing plants, two were operated simultaneously, whereas the third was kept as a backup for seamless operations. Alongside, an ice plant and chilling plant were installed to control the temperature of RCC during production. An inundation system with capacity of 3,600 tonne per hour was installed to bring down the temperature of aggregates before mixing. The whole system of aggregate conveyance was covered to reduce dust pollution. The production capacities of individual systems (aggregate production to concrete placement) were synergistically designed to meet the asking rate of RCC placement.
  • Excavation for foundation: Excavation of the dam foundation was to be done till EL 141. However, during excavation, the team encountered varied geology and had to go a little deeper. The riverbed rock level was uneven with several crest formations. These could not be levelled evenly as the machines could not reach these spots. Second, these could not be made flat by blasting as this could create cracks in the bedrock and weaken the foundation. Apart from this, several streams of coal seam were found trapped in the bedrock level. These coal seams are loose; when they come in contact with water, they turn mushy. As RCC could not be built on a loose foundation, these coal seams had to be removed carefully before work began. And, as they could not be excavated by machines, it was done manually, which took a lot of time.
  • Shuttering: The shuttering used for the RCC dam is made of heavy steel sheets. To lift and place these, shuttering hydras and tower cranes were used. As the RCC process is fast, managing the timecycle for placing the shuttering was critical and required precision in planning.
  • Maintenance of equipment: As RCC is a continuous process and a huge web of equipment is continuously in operation, maintaining it was critical. Even a small snag in one equipment could halt the entire chain.
  • Quality: The dam was built with the highest quality. This was realised through well-crafted and regimentally implemented quality assurance plans and commensurate quality control testing. The quality assurance involved aspects of the project: Raw materials, equipment, construction processes, shuttering and manpower resources.

And finally!
The HCC team prevailed over all these challenges to finish the RCC dam in a record of 196 days on May 12, 2015. The finishing works of the RCC dam are expected to be complete by June 30, 2015. In the powerhouse, major works are completed and installation of equipment is in progress. After installing the equipment, finishing works will be taken up and are expected to be completed by December 2015. The project will be commissioned by January 2016.

Project details
Client: National Hydroelectric Power Corporation.
Tel: 0129-258 8500. Fax: 0129-227 7941.
E-mail: webmaster@nhpc.nic.in
Website: www.nhpcindia.com
Contractor: HCC. Tel: 022-2575 1000.
Fax: (022) 2577 5732. E-mail: corpcomm@hccindia.com Website: ww.hccindia.com
RCC dam completion date: June 30, 2015
Project commissioning date: January 2016
Fly-ash: Sourced from NTPC´s Kahalgaon Power Station in Bihar
Cement: Darjeeling-based Star Cement
Aggregates: Prepared at HCC´s crushing plant on-site
Boulders: Sourced from 25 km of project location

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