Earth Dams
1. Introduction
Definition:
Earth dams are embankments of earth or rock materials built to store water, control floods, or serve other water management purposes. They are among the oldest types of dams, relying on compacted soil to create a barrier that resists the flow of water.
Uses:
- Water storage for irrigation, drinking water, and industrial use
- Flood control
- Recreation (e.g., creating reservoirs for boating and fishing)
- Sediment control
Examples of Earth Dams:
- Tarbela Dam, Pakistan (largest earth-filled dam in the world)
- Aswan High Dam, Egypt
- Fort Peck Dam, USA
2. Types of Earth Dams
Earth dams can be classified based on the materials used and their structure:
Homogeneous Earth Dams:
- Description: Made entirely of one type of material, usually impermeable clay or compacted earth.
- Application: Suitable for sites where materials are uniformly available.
- Example: Small agricultural dams or ponds.
Zoned Earth Dams:
- Description: Consist of different zones of materials; a central impermeable core is surrounded by more permeable materials.
- Application: Common for large dams, where different materials can be used to optimize cost and structural integrity.
- Example: Central clay core with shells of sand and gravel.
Earth-Rockfill Dams:
- Description: Combination of earth materials and rock; rockfill provides stability, while earth material acts as an impermeable core.
- Application: Sites with a combination of rock and earth materials available.
- Example: Fort Peck Dam, USA.
Rockfill Dams:
- Description: Mainly composed of rock, with an impervious membrane (like asphalt or clay) on the upstream face to prevent seepage.
- Application: Where large quantities of rock are available near the site.
- Example: Oroville Dam, USA.
3. Design of Earth Dams
The design of earth dams involves selecting appropriate materials, ensuring stability under various loading conditions, and preventing seepage and erosion.
A. Key Components
Upstream Slope:
- Usually gentle (1:2.5 to 1:3) to prevent wave action from damaging the slope.
- Protected with riprap (large stones) or other erosion-resistant materials.
Downstream Slope:
- Slightly steeper than the upstream slope (1:2 to 1:2.5).
- Needs adequate drainage to prevent saturation and instability.
Core:
- The central impervious zone that prevents water from passing through the dam.
- Made of clay or other low-permeability materials.
Shells:
- The outer zones of the dam, consisting of more permeable materials (sand, gravel).
- Provide structural support and stability.
Drainage System:
- A system of drains and filters to collect and remove seepage water, reducing uplift pressures and preventing piping.
- Often includes a drainage blanket and vertical drains.
B. Stability Analysis
Slope Stability:
- Essential to prevent landslides or collapse.
- Stability is analyzed for both upstream and downstream slopes under different conditions (dry, steady seepage, rapid drawdown).
Seepage Control:
- Proper design to minimize seepage through the dam, using a core and filters.
- Avoid piping (internal erosion) by ensuring that seepage velocities are low and filters prevent the movement of soil particles.
Foundation Treatment:
- Preparing the foundation to prevent seepage under the dam.
- Methods include grouting, cutoff walls, and excavation of unsuitable materials.
C. Construction Considerations
Material Selection:
- Use of local materials to minimize cost.
- Selecting materials with suitable properties (permeability, strength, compaction).
Compaction:
- Layers of soil are compacted to achieve desired density and strength.
- Proper moisture content is essential for effective compaction.
Quality Control:
- Regular testing of materials and compaction during construction.
- Monitoring of seepage and slope stability post-construction.
4. Case Study: Tarbela Dam, Pakistan
Background:
- Location: Indus River, Pakistan
- Type: Earth-Rockfill Dam
- Height: 143 m
- Length: 2,743 m
- Volume: 106 million cubic meters
- Purpose: Water storage for irrigation, hydroelectric power generation, flood control.
Design Features:
- Core: Central clay core for impermeability.
- Shells: Outer shells made of compacted gravel and rockfill to provide stability.
- Upstream and Downstream Slopes: Protected with riprap to prevent erosion.
- Drainage System: Well-designed drainage system to control seepage and maintain slope stability.
Performance:
Tarbela Dam has been operational since 1976 and has effectively served its purposes, demonstrating the reliability of well-designed earth-rockfill dams.
Arch Dams
1. Introduction
Definition:
An arch dam is a curved structure that relies on its arch shape to transfer the load of the water to the abutments (side walls of the canyon). These dams are typically made of concrete and are constructed in narrow, steep-walled valleys.
Uses:
- Water storage
- Hydroelectric power generation
- Flood control
Examples of Arch Dams:
- Hoover Dam, USA
- Glen Canyon Dam, USA
- Idukki Dam, India
2. Types of Arch Dams
Arch dams can be classified based on the extent of curvature and the method of construction:
Constant Radius Arch Dams:
- Description: The radius of curvature is constant throughout the height of the dam.
- Application: Suitable for sites with uniform abutments.
- Example: Glen Canyon Dam, USA.
Variable Radius Arch Dams (Double-curvature):
- Description: The radius of curvature varies, usually decreasing towards the base, providing a double-curved surface.
- Application: Common for larger dams, where varying curvature helps distribute stresses more evenly.
- Example: Hoover Dam, USA.
Cylindrical Arch Dams:
- Description: An arch dam with a horizontal cylindrical shape.
- Application: Suitable for long spans across narrow gorges.
- Example: Koyna Dam, India.
3. Design of Arch Dams
The design of arch dams focuses on utilizing the arch shape to transfer loads efficiently to the abutments, ensuring stability and safety.
A. Key Components
Crest:
- The top edge of the dam.
- Houses spillways and other structures for water discharge.
Base:
- The bottom part of the dam that rests on the foundation.
- Must be well-anchored to resist sliding and uplift.
Abutments:
- The side supports that anchor the dam to the valley walls.
- Critical for the stability of the arch dam.
Spillways:
- Structures that allow excess water to pass safely over or around the dam.
- Must be designed to handle maximum flood flows.
B. Load Analysis
Water Pressure:
- Acts perpendicular to the surface of the dam.
- Greatest at the base due to the depth of the water.
Self-weight:
- Acts vertically downward due to the weight of the concrete.
- Provides stability against overturning and sliding.
Temperature Stresses:
- Changes in temperature cause expansion and contraction of the concrete.
- Need to be considered in design to prevent cracking.
Earthquake Forces:
- Horizontal and vertical forces due to seismic activity.
- Important for dams located in earthquake-prone areas.
Silt Pressure:
- Pressure exerted by silt and sediments deposited against the upstream face.
- Adds to the hydrostatic pressure.
C. Stability Analysis
Arch Action:
- The curvature of the dam allows it to transfer the horizontal water pressure into compressive forces along the arch, which are then transferred to the abutments.
- Ensures that the dam is primarily under compression, which concrete can handle well.
Foundation Stability:
- The foundation must be strong and stable to support the loads.
- Requires thorough geological surveys and possibly foundation treatment (e.g., grouting) to improve stability.
Uplift Pressure Control:
- Measures to prevent water from seeping under the dam and reducing its stability.
- Drainage galleries and grouting are common methods used.
D. Construction Considerations
Materials:
- High-quality concrete is essential for strength and durability.
- Reinforcement may be used to handle tensile stresses.
Cofferdams:
- Temporary structures to divert water during construction.
- Allow for the construction of the dam in dry conditions.
Construction Joints:
- Necessary to accommodate shrinkage and temperature changes.
- Must be watertight to prevent seepage.
Temperature Control:
- Measures to control the temperature of the concrete during curing to prevent cracking.
- Use of cooling pipes and delayed curing.
4. Case Study: Hoover Dam, USA
Background:
- Location: Colorado River, USA
- Type: Concrete Arch-Gravity Dam
- Height: 221 m
- Length: 379 m
- Volume: 2.48 million cubic meters of concrete
- Purpose: Hydroelectric power generation, water storage, flood control.
Design Features:
- Double-curvature Arch: Varies in thickness from 200 feet at the base to 45 feet at the top.
- Spillways: Two large spillways on either side of the dam to handle flood flows.
- Foundation Treatment: Extensive grouting to improve the stability of the foundation rock.
Performance:
Hoover Dam has been operational since 1936, providing reliable power, water storage, and flood control. It stands as a testament to the effectiveness of arch dams in harnessing the natural landscape to manage water resources.
Conclusion
Earth Dams:
- Simple to construct, utilizing locally available materials.
- Require thorough stability and seepage control measures.
- Suitable for a wide range of applications, including irrigation and flood control.
Arch Dams:
- Utilize the strength of the arch shape to efficiently transfer loads.
- Suitable for narrow, steep-walled valleys.
- Require high-quality materials and precise construction techniques.
Both types of dams play critical roles in water management and resource utilization, each with unique advantages and challenges that make them suitable for specific site conditions and project requirements.