Many challenges have been encountered during construction of Subansiri Lower Dam, which will impound water for a 2,000 MW hydro project. Finding ways to overcome these challenges – including delays in completing infrastructure – have provided valuable lessons learned with regard to dam design and construction.
By Biswajit Das
Design and construction of Subansiri Lower Dam, on the Subansiri River in Arunachal Pradesh, India, involves many challenges. These include land not being available when construction was scheduled to commence, a limited period available for construction because of monsoons (from mid-April to mid-October), the need to handle high flood flows and poor rock conditions.
To meet these challenges, the design of the dam has undergone drastic and repeated revisions that also affected the schedule and planning for the construction work. As of November 2011, the dam had attained a level of elevation 138 meters, just below the spillway at elevation 145 meters. It is estimated that concreting work to reach the final elevation of 210 meters will be completed by February 2014.
Building Subansiri Dam
The 2,000 MW Subansiri Lower project, owned by NHPC, consists of a 133 meter-high concrete gravity dam with nine spillways equipped with radial gates. Energy dissipation is provided via a ski jump and preformed plunge pool that can handle a design flood of 37,000 m3/s. Figure 1 shows a layout plan of the project.
Upstream and downstream cofferdams are used to dewater the construction site. The upstream cofferdam is 31 meters high and 143 meters from the dam axis; the downstream cofferdam is 21 meters high and 312 meters from the dam axis. Five tunnels – each 9.5 meters in diameter and with an average length of 100 meters – were built to divert the river during construction. Each tunnel has inlet and outlet structures and diverts 4,550 m3/sec during the non-monsoon period, which lasts about seven months. The cofferdams were designed to wash out each year during the monsoon period in April and be rebuilt after the monsoon in October, depending on rain and flood variation.
The contract for building the dam and appurtenant works was awarded in December 2003 to a consortium of Boguchandgesstroy, Soyuzgidrosptsstry and Soma Enterprise Ltd. The contractors were asked to mobilize immediately and to finish the work in 72 months. However, the land on which the dam was to be constructed was not available because clearance was needed from the Forest and Environment Authority. After 13 months, the land covering only the dam and tunnels area was handed over. But land needed for the supporting infrastructure was not handed over until 41 months after the contract was awarded because the authority raised many objections, which had to be mitigated through prolonged discussions and examinations by all departments.
Further hindrances were encountered during construction of the diversion tunnels, due to geological variation in the rock characteristics in and around the diversion tunnels and inlet and outlet structures. This situation resulted in a series of landslides and lower production by the rock cutting machines than anticipated. Despite this and other hindrances, the contractors continued the work quickly, and NHPC directed them to deploy more personnel and equipment to mitigate the time loss.
The river was diverted in December 2007, and by April 2008 the foundation excavation was completed. The exposed rock was repeatedly scraped using rock cutting machines and cleaned using an air-water jet. In addition, NHPC performed geological exploration to establish the suitability of this rock for the foundation of the dam. One problem: The foundation rock was encountered 10 meters above the anticipated level, requiring revision of the construction drawings. Contractors were asked not to excavate, lower the foundation level or perform block concreting during that season. NHPC decided instead that concrete would be laid 500 mm thick over the foundation rock to protect it during the monsoon season.
Revised construction drawings were completed in October 2008, and the contractor immediately began reconstructing the cofferdams. Once the foundation was exposed, it was cleaned and prepared for concrete. However, the plant and machinery for producing the concrete was not ready because the land was not made available until April 2008, immediately before the rainy season started. Thus, installation of the concreting plants started in October 2008.
Normally, such plants require at least 18 months to install and three months for testing and commissioning. However, with an accelerated effort, two plants with a capacity of 240 m3/hr each were commissioned in March 2009. This enabled contractors to install 60,000 m3 of concrete on the dam foundation before work was suspended in May 2009 because of the onset of the monsoon season.
|With a capacity of 1,200 tons per hour, the aggregate processing plant for 2,000 MW Subansiri Lower is the largest such plant in India for dam construction. The plant is 3 km downstream from the dam on an island that was created using a shoal and riverbed material.|
Work began again in November 2009, and the two conveyors for the concrete placement system were commissioned in October 2009 and January 2011. As of early December 2011, about 575,000 m3 of dam concreting had been achieved, with the dam at elevation 138 meters, which is 54 meters high from the deepest foundation level. It is expected that dam construction will be completed by February 2014.
Installing the concrete plants
The concreting plant and machinery deployed to build Subansiri Lower Dam has a much greater capacity than originally envisaged to make up for lost time. The original capacity in the contract was 660 m3/hr, while the actual deployed capacity is 1,056 m3/hr. Below are the major elements of the concreting system.
Aggregate processing plant
With a capacity of 1,200 tons/hr, the aggregate processing plant, supplied by Metso Minerals, is the largest in India for dam construction. It is 3 km downstream from the dam on a river island that was created using a shoal and riverbed material. During installation of the plant, there was no access via land, so transportation of men, materials and equipment was accomplished by boat and a 70 ton self-powered barge barge.
The plant consists of a primary jaw crusher, secondary cone crusher, tertiary vertical shaft impact crusher and sand classifier to produce the required quantity and quality of graded aggregate needed – from 150 mm size to sand. A series of conveyors, reclamation tunnels and wet screens are used to clean and wash the aggregate after processing and prepare it for transportation by the main conveyor to the batching and mixing plant.
The aggregate processing plant was commissioned in 2009, but the conveyor bridge and structure were not ready at that time.
Conveyor bridge to transport aggregate
In 2008, NHPC was given clearance to construct a bridge to convey aggregate to the concrete plant, with the restriction imposed by the Forest and Environment Authority that river flow cannot be obstructed or disturbed by construction of a pier. This forced the contractor to design a suspension bridge with a single span of 300 meters, the longest single span suspension bridge in India. The bridge was was completed in two working seasons. From October 2008 to April 2009, only the pile foundation for the pylon with pile caps on both banks and the anchor blocks could be constructed. The protection work was washed by a flood, but the piles were not damaged. During the next working season, from October 2009 to April 2010, the reprotection work – consisting of tetrahedrons around the pylons – as well as cables and deck erection were completed, along with the conveyor belt erection over the bridge.
The aggregate conveyor system, which is 1.6 km long and has a capacity of 1,200 tons/hr, was installed and commissioned in March 2010. The system feeds aggregate via the conveyor and a tripper conveyor to a designated stockpile area over two reclamation tunnels. One tunnel is used to convey aggregate and the other sand. The aggregate sizes are 150 mm, 40 mm, 20 mm and 10 mm. Pneumatically operated gates on the crown of the tunnels are operated from a control room to discharge a particular size of aggregate onto the conveyor belts inside the reclamation tunnels for delivery to the mixing plant.
|The concrete batching and mixing plant consists of four 220 cumec-capacity plants, a twin shaft mixing plant, and a chilling and ice plant from KTI Germany. This is the single largest batching and mixing plant for dam construction in India.|
Batching and mixing plant
This plant consists of four 220 m3 capacity plants supplied, erected and commissioned by Schwing Stetter. It also contains a BHS twin shaft mixing plant from Germany and a chilling and ice plant from KTI Germany. This is the single largest batching and mixing plant for dam construction in India. It covers an area of 600 meters by 50 meters.
From the reclamation tunnels, aggregate is delivered to a series of inundation bins through shuttle conveyors. There are four bins for each size of aggregate (including sand), totaling 20 bins each with a capacity of 200 tons. These bins are continuously fed by running chilled water at 2 degrees Celsius until the aggregates are cooled to desired level. To ensure continuous production of cooled aggregate, the bins remain under different stages of operation.
Cooled aggregate is then delivered to inline bins fitted with weigh batchers and fed to the twin-shaft 6 m3 mixing plant through a conveyor, where ice is also fed to the desired quantity. The cement is fed to the silos from the cement storage house by conveyor and pneumatic feeder, and cement from the silo is fed to the weigh batcher and then to the mixer via screw conveyors. The admixer feeding system, near the mixing plant, feeds admixtures to the mixer through the weigh batcher. The mixing plants have a cycle time of 90 seconds, and each mixer has a peak capacity of 240 m3/hr.
Concrete conveying and placing system
The chilled concrete – produced at 8 C to 10 C, depending on ambient temperature – is carried by a 30 ton-capacity concrete carrier covering a distance of 1.2 km. Travel time is kept to 10 minutes or less to limit the temperature rise of the concrete to 1 C. The carrier discharges into two concrete hoppers of 12 m3 capacity each, which in turn discharge the concrete to two independent line conveyors feeding the two tower belts, each with a rated capacity of 400 m3/hr.
The tower belts, with a 100 meter radius jib length, have a lifting capacity of 25 tons at maximum jib length. The independent placement conveyor belts are held by the tower crane boom, with a system of wire ropes and a hydraulic winch, and can be moved in any position and angle during concreting to discharge through an elephant trunk chute. Mobile concreting equipment includes hydraulic vibrators fitted to excavators, dozers to spread concrete, mobile cranes for shuttering fixing, hydro blasters for cutting and cleaning, and drilling and grouting equipment. This equipment can be shifted from one dam block to the other by tower belts that cover 90% of the dam area in the initial and final stages of concreting. The remaining 10% is covered by a Creter crane with a capacity of 200 m3/hr, which is designed to be fed by five 15 m3 capacity hi-boy concrete carriers.
Concrete compaction and other equipment
Concrete compaction is accomplished using three sets of hydraulic vibratory cylinders, each set consisting of eight 150 mm cylinders, with a total capacity of 240 m3/hr. Each set is fitted to a CAT 320 excavator. Two sets are used, and one is kept as a standby. Concrete is laid in 500 mm layers and compacted in a continuous operation. Sufficient numbers of hand-held vibrators, both pneumatic and electric, are kept for compaction in congested areas, as well as to meet an emergency situation during an equipment breakdown.
Two hydro blasters with a capacity of 500 bar are mounted on truck chassis and deployed for cutting and cleaning of concrete. Each concrete lift is 1.5 meters thick and block width is 19.5 meters. The shutter panels of 1.5 meter high and 3 meters wide are cantilever climbing type and are handled by small cranes. There are six spillway blocks under construction using six tire-mounted mobile cranes.
Effects of design changes and contract restrictions
Because of the design changes to deal with the many challenges faced, the overall plan for construction of the dam had to be redrawn. Even though contractors have mobilized sufficient capacity to achieve sustained production of 120,000 to 140,000 m3 a month and 4,000 to 6,000 m3 per day, such production cannot be achieved consistently. In February 2011, the progress achieved was 79,300 m3.
Overall utilization of the concreting plant as per the original plan was 75%, and with the revised plan this is reduced to 40%. The major restriction imposed on the dam seat area is reduced from 22,600 m2 to 9,800 m2; the ratio of mass concrete to heavily reinforced concrete is reduced from 13:1 to 6:1. Below the spillway level to the foundation level, mass concrete volume is reduced by 700,000 m3.
The contract states the difference in levels between the highest and lowest blocks of the dam must not exceed 3 meters. At the same time, it is stipulated that work continue during the monsoon starting with the third season, when the dam level should be above the upstream cofferdam level of elevation 124 meters. With the diversion tunnels channeling total flow of 4,550 m3, the remainder of the annual flood of 12,000 m3 will flow over the dam. This was mitigated during the past monsoon season, when four blocks in the central area were kept at elevation 119 meters and other blocks in both banks were raised to elevation 138 meters. When the flood water passed over the central blocks, the upstream water level rose to elevation 132 meters, creating an afflux of 13 meters. With the 3 meter restriction, work during the monsoon is not feasible.
However, the India Standard allows a difference of 9 meters between adjacent blocks. In that case, the river water can be diverted through the central blocks and the wing blocks can be raised in larger steps during the monsoon. The designer must approve this option or completion of the dam will be delayed by another year.
Another restriction is that concreting above elevation 125 meters can begin only after a cutoff wall at elevation 94 meters is completed and the galleries for the wall are backfilled with concrete. This condition was imposed perhaps on the apprehension that stress around the galleries will exceed permissible limits. However, the contractor conducted a three-dimensional stress analysis that shows the stress concentration around the galleries will remain well within the permissible limit even when the concrete attains elevation 170 meters and the galleries are not backfilled. The dam designer has not yet reconciled with this analysis.
Once these issues are resolved, the scheduled completion date of February 2014 is feasible.
Biswajit Das is executive director of Soma Enterprise Ltd., one of the companies involved in construction of Subansiri Lower Dam.