Types of Dams Built in India
India builds three primary types of dams depending on geology, hydrology, and available construction materials:
Earthen Embankment Dams (70% of India's dams):
Built from compacted earth (clay core with random fill shells). Suitable for wide valleys with alluvial foundations. Heights typically 15–60 metres. Examples: Tawa Dam (MP), Ukai Dam (Gujarat), Hirakud Dam (Odisha).
Concrete Gravity Dams (20% of India's dams):
Massive concrete structures that resist water pressure by their own weight. Suitable for narrow valleys with hard rock foundations. Heights up to 260 metres. Examples: Bhakra Dam (HP), Sardar Sarovar Dam (Gujarat), Nagarjuna Sagar Dam (AP/Telangana).
Masonry Dams:
Older construction method using stone masonry with cement mortar — many dams built pre-1960 are masonry gravity dams. Still used for small check dams and barrages.
Composite Dams:
Combination of concrete and earthen sections — concrete in the river gorge for the spillway, and earthen flanks on either side where the valley widens. This is very common in Indian practice.
VRSIPL has constructed earthen dams, barrages, and canal head-works across Gujarat and Madhya Pradesh, with the Bilgaon Dam being a notable recent project.
River Diversion — The First Challenge
Before dam construction can begin, the river must be diverted away from the construction zone. This is arguably the most challenging and risky phase:
Cofferdam Method:
Temporary earthen or rockfill bunds (cofferdams) are built upstream and downstream of the dam site, diverting the river through: - Diversion tunnels (bored through the abutment rock) - Diversion channels (cut around one side of the valley) - Existing river bed slots (for stage construction)
Stage Construction:
For very wide rivers, the dam is built in stages — first one half of the river bed is closed with a cofferdam while the river flows through the other half, then vice versa.
Monsoon Protection:
All diversion works must be designed to safely pass the monsoon flood. Underestimating the diversion flood is the most common cause of cofferdam overtopping and work damage. Typically, the diversion system is designed for a 25-year return period flood during construction.
VRSIPL's approach:
On the Bilgaon Dam project, VRSIPL constructed upstream and downstream cofferdams with controlled overtopping design, allowing monsoon floods to pass safely while protecting the dam foundation zone.
Foundation Treatment for Dams
A dam is only as strong as its foundation. Foundation treatment ensures the dam base is watertight and can safely carry the structure's load:
For Earthen Dams on Alluvial Foundations:
- Core trench: Excavate a trench into impervious stratum and backfill with compacted clay - Sheet pile cut-off: Drive steel sheet piles through permeable overburden - Cement-bentonite slurry wall: For deep alluvial deposits (30m+) - Relief wells: Downstream pressure relief to prevent piping failure
For Concrete Dams on Rock Foundations:
- Stripping: Remove all weathered rock to expose fresh, competent rock surface - Dental treatment: Fill all cracks, joints, and cavities with concrete - Consolidation grouting: Inject cement grout at low pressure (3–5 kg/cm²) to fill near-surface rock joints - Curtain grouting: Deep grout curtain (depth = 30–50% of water head) to prevent under-seepage. Drilled at 3m spacing, injected at pressures up to dam height in metres × 0.5 kg/cm² - Drainage gallery: Internal gallery with drain holes drilled downstream of the grout curtain to relieve uplift pressure
Foundation treatment can represent 10–20% of total dam cost but is absolutely non-negotiable for safety.
Embankment Dam Construction
Earthen dam construction is an exercise in controlled compaction of millions of cubic metres of earth:
Zoning:
A typical zoned earthen dam has: - Impervious core (compacted clay, 15–20% of dam width) - Transition filters (graded sand/gravel) on both sides of the core - Random fill shells (compacted available soil/rock) - Upstream stone pitching or riprap for wave protection - Downstream toe drain for safe seepage collection
Construction Sequence:
1. Foundation preparation (stripping, core trench) 2. Build dam in horizontal layers (150–230mm loose thickness per lift) 3. Each lift: spread by bulldozer → moisture conditioning by sprinkler → compaction by sheepsfoot roller (core) or vibratory roller (shells) 4. Quality check: field density test every 500 cum (95–98% Standard Proctor Density required for core) 5. Continue lifting through dry season, protect partial dam from monsoon overtopping
Placement Rates:
A well-equipped contractor achieves 3,000–10,000 cum/day depending on borrow area distance, dam width at current level, and season.
VRSIPL deployed fleets of 20+ tippers, sheepsfoot rollers, and vibratory compactors on dam projects, achieving sustained placement rates of 5,000+ cum/day.
Spillway Construction
The spillway is the dam's safety valve — it safely passes flood flows over or around the dam without overtopping the embankment. Spillway types include:
Ogee Spillway:
The classic curved-crest concrete spillway designed for efficient high-velocity flow. Used on concrete gravity dams.
Chute Spillway:
A concrete-lined channel on the dam abutment, often with stilling basin energy dissipator at the downstream end.
Side Channel Spillway:
Flow enters a channel parallel to the dam crest, then turns 90° into a chute. Used where the dam crest is at a higher elevation than the spillway crest.
Shaft (Morning Glory) Spillway:
Circular overflow crest with vertical shaft and tunnel — used where space for a conventional spillway is unavailable.
Spillway civil construction involves:
- Massive concrete volume (often 50,000–200,000 cum for major dams) - Precise curved formwork for ogee crest - Reinforcement for cavitation-prone zones - Stilling basin with baffle blocks and end sill - Training walls and guide walls - Gates and hoisting arrangement foundations
Spillway concrete must be high-quality (M20–M30) with controlled temperature to prevent thermal cracking in massive pours.
