Dhauliganga Dam – First concrete face rockfill dam in India
The planning, design and construction of Dam founded on deep alluvium
Dhauliganga Dam is the first concrete face rockfill dam constructed in India. Special design and construction techniques were necessary to overcome the difficulties caused by the 70m thick alluvium overburden in the river bed with a large number of pebbles and boulders. This article discusses the dam, the design features and the construction procedures adopted.
Foundation geology
The alluvial foundation material at the dam site consists of boulders and cobbles in a sandy matrix. The alluvium fill extends to a depth of 50-60m below the bed level of the river. It consists of an upper layer of sand and gravel with pebbles and boulders, a central layer of the sand lens up to 20m thick, and an underlying zone of sand and gravel with pebbles and boulders. Boulders of the size of 2 to 3 m in diameter embedded in the sand gravel layer exist in the river alluvium. Standard penetration tests were conducted to determine the liquefaction potential of the sand layer. The N – values obtained were above 25. However, a few values less than 25 were also recorded. It was concluded that considering the load imposed by the dam, there was no possibility of liquefaction potential in the foundation. A ground failure that could endanger the dam’s overall stability was ruled out.
Selection of dam
The dam proposed in the feasibility report was a central core rockfill dam with a deep core trench down to the bedrock. The total dam height of this proposed solution was 111m above the bedrock at the bottom of the cut-off core trench. Two dam types were considered, i.e. an earth core rockfill dam with a vertical core and a concrete face rockfill dam. A concrete face rockfill dam with a plastic concrete cut-off wall at its upstream end was finally adopted as a solution for the Dhauliganga Dam site.
Layout
The general layout of the dam complex consisted of a 56m high concrete face rockfill dam with a spillway on the right bank. A spillway consisting of two low-level sluices (plus an auxiliary tunnel spillway ) was designed to pass a PMF of 3210 cumecs. The spillway was designed as a low-level type for the combined function of flood release and flushing during monsoons of sediment accumulated in the reservoir during monsoons.
Dam section
The upstream slope of the dam was provided as 1.5H: 1V. The overall downstream slope ( including berms ) was provided as 1.6H: 1V, although the slope provided between berms was 1.5H: 1V. The flatter slopes were provided as a conservative provision due to alluvial foundation and seismic consideration ( PGA for MCE 0.36 ).
The Face Slab
The thickness of the face slab was kept as 0.3m + 0.003H. A Uniform thickness of 30cm all along the height was considered, but finally, the variable thickness was adopted as a conservative provision.
The plinth
The plinth on alluvium has been provided uniformly at El 1305m all along the length. The natural ground level on the left terrace was ± 1310m while the river bed was El + 1295.0m
F. Grouting Through Cut-Off Wall in the Ends
The cut-off wall was of shallow depth at the ends, about 5 to 10m. Providing additional grouting in the rock through the cut-off wall was considered desirable. The grouting in rock was carried up to 20m from the top of the cut-off wall. In the left abutment where the plinth beam could be rested upon a rock, grouting was done to act as a curtain.
Construction
A. RIVER DIVERSION
A river diversion scheme was planned for the lin 30 monsoon flood for the year. The lin 30 years frequency flood was estimated as 1350 cumecs while the maximum observed flood at the site (observed in 1995) was assessed as 1075 cumecs. Based upon this consideration, a concrete-lined, 10m dia, 750m long diversion tunnel was constructed, and upstream and downstream cofferdams of 27m and 10m height, respectively, were constructed. After river diversion, during the last four years (2001 to 2004), the maximum flood observed at the dam site was 600 cumecs only. The work of the diversion tunnel was taken up in 1998 before the award of the main civil works packages as an advance activity and was completed in March 2001.
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DMR is a leading engineering consulting firm in India that leverages Experience and Core Expertise in Design and Engineering to offer Concept to Commissioning solutions throughout the lifecycle of projects in infrastructure sectors.
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© Copyright 2023. All Rights Reserved