Lecture-07 

Embankment Dams: Types, Components, Design Principles, and Failure

Embankment Dam

Embankment dams are constructed using soil, rock fill, or a combination of both. They are categorized as non-rigid dams, relying primarily on the shear strength of their materials to resist external forces. The stability and strength of these dams are derived from the shear resistance of the materials used in their construction.

Types of Embankment Dams:

1.     Earth Dams:

o    Constructed primarily using compacted earth (soil).

o    Suitable for sites with abundant natural soil.

2.     Rockfill Dams:

o    Built using compacted rock fragments or boulders.

o    Often include an impermeable membrane (e.g., concrete, asphalt, or clay) on the upstream face to prevent seepage.

3.     Composite Dams:

o    Combine both earth-fill and rockfill materials.

o    Designed to optimize the use of available materials and site conditions.

Types of Earth Dam

     •         Homogenous Embankment type
•         Zoned embankment type
•         Diaphragm type

Homogeneous Earth-fill Dam:

• Description: Made of a single type of material, typically impermeable soil like clay or silt, throughout the dam body.

·         Characteristics:

• Simple design and construction.
• Relies on the entire mass of the dam to resist seepage.
• Suitable for low to moderate heights.
• Limitations: Prone to seepage and requires careful control of water flow to prevent internal erosion (piping).

 

 

Zoned Earth-fill Dam:

• Description: Composed of different zones or sections, each with specific materials to optimize stability and seepage control.

·         Common Zones:

• Core: Impermeable material (e.g., clay) at the center to prevent seepage.
• Shells: Permeable materials (e.g., sand, gravel) on the upstream and downstream sides for stability and drainage.

·         Characteristics:

• More efficient than homogeneous dams.
• Suitable for higher dams and sites with varying material availability.

Diaphragm Earth-fill Dam:

• Description: Features a thin, impermeable barrier (diaphragm) made of materials like concrete, steel, or asphalt within the dam body to control seepage.

·         Characteristics:

• The rest of the dam is made of permeable materials.
• The diaphragm acts as the primary water barrier.
• Suitable for sites with limited impermeable materials.

·         Types of Diaphragms:

• Concrete diaphragm.
• Steel diaphragm.
• Asphalt diaphragm.

If the thickness of the diaphragm at any elevation is less than 10 meters or less than the height of the embankment above the corresponding elevation, the dam embankment is considered to be of “Diaphragm Type”. But if the thickness equals or exceeds these limits, it is considered to be of zoned embankment type.

 Methods of Construction

     ·        Hydraulic-fill Method
·        Rolled-fill Method

Hydraulic Fill Dam:

A hydraulic fill dam is a type of earthen dam constructed by transporting and depositing soil materials using water. This method involves mixing soil with water to form a slurry, which is then pumped or transported to the dam site. The slurry is deposited in the desired location, and the water is drained, leaving behind the settled soil to form the dam structure.

Key Features:

1.     Construction Method:

o    Soil is excavated and mixed with water to create a slurry.

o    The slurry is transported through pipelines or channels to the dam site.

o    The slurry is deposited in layers, and the water is allowed to drain, leaving the soil to settle and compact naturally.

2.     Material Composition:

o    Typically uses fine-grained soils like silt or clay, which can easily form a slurry.

o    Coarser materials may settle faster, while finer materials take longer to consolidate.

3.     Compaction:

o    Unlike rolled earth-fill dams, hydraulic fill dams rely on natural settling and drainage for compaction.

o    This results in a looser structure with lower density and shear strength compared to mechanically compacted dams.

Advantages:

1.     Efficient Material Transport:

o    Slurry can be easily transported over long distances using pipelines.

2.     Reduced Labor:

o    Less manual effort is required compared to traditional earthfill methods.

3.     Suitable for Certain Sites:

o    Useful in areas where mechanical compaction is difficult or impractical.

Disadvantages:

1.     Lower Strength and Stability:

o    Natural settling results in lower density and shear strength, making the dam less stable.

2.     Prone to Seepage and Piping:

o    The looser structure increases the risk of internal erosion (piping) and seepage.

3.     Longer Construction Time:

o    Requires significant time for the slurry to settle and drain.

4.     Limited to Specific Materials:

o    Works best with fine-grained soils, limiting its applicability.

Applications:

• Historically used for smaller dams or in remote areas where traditional construction methods were challenging.
• Rarely used today due to advancements in rolled earth-fill techniques, which offer better stability and control.

 

Rolled Fill Dam:

A rolled fill dam is a type of earthen dam constructed by compacting successive layers (lifts) of soil or earth material using heavy machinery, such as rollers. This method ensures high density and strength, making it one of the most common and reliable techniques for building earthen dams.

Key Features:

1.     Construction Method:

o    Soil or earth material is spread in thin layers (typically 15–30 cm thick).

o    Each layer is compacted using rollers, vibratory compactors, or other heavy equipment to achieve the desired density.

o    The process is repeated layer by layer until the dam reaches its full height.

2.     Material Composition:

o    A wide range of materials can be used, including clay, silt, sand, gravel, and rock fill.

o    Materials are selected based on availability and engineering properties (e.g., impermeability, shear strength).

3.     Compaction:

o    Mechanical compaction ensures high density, strength, and stability.

o    The degree of compaction is carefully controlled to meet design specifications.

Advantages:

1.     High Strength and Stability:

o    Proper compaction results in a dense, stable structure capable of resisting external forces.

2.     Controlled Construction:

o    Each layer is carefully placed and compacted, allowing for quality control.

3.     Versatility:

o    Can be used to construct both homogeneous and zoned dams.

4.     Durability:

o    Provides long-term performance with minimal maintenance.

Disadvantages:

1.     Labor and Equipment Intensive:

o    Requires heavy machinery and skilled labor for compaction.

2.     Time-Consuming:

o    Layer-by-layer construction can be slower compared to other methods.

3.     Material Requirements:

o    Requires suitable materials with the right gradation and moisture content for effective compaction.

Applications:

• Widely used for constructing modern earthen dams of all sizes.
• Suitable for water storage, flood control, and hydroelectric projects.
• Can be adapted to various site conditions and material availability.

 

Causes of Failure of Earthen Dam

    ·        Hydraulic failures
·        Seepage failures
·        Structural failure

Hydraulic Failure in Dams:

Hydraulic failure refers to the failure of a dam caused by the uncontrolled action of water, which can lead to structural damage or complete collapse. This type of failure is often associated with the following causes:

1. Overtopping:

• Description: Overtopping occurs when water flows over the crest of the dam, exceeding its design capacity.
• Causes:
• Inadequate spillway capacity to handle floodwaters.
• Extreme rainfall or flood events beyond design limits.
• Blockage of spillways or outlet structures.
• Effects:
• Rapid erosion of the downstream face.
• Potential breach and catastrophic failure of the dam.
• Prevention:
• Design spillways to handle maximum probable floods.
• Regular maintenance to ensure spillways are clear and functional.
• Use of erosion-resistant materials on the downstream slope.

2. Erosion of Upstream Face:

• Description: Erosion of the upstream face occurs when water action wears away the material on the reservoir side of the dam.
• Causes:
• Wave action from the reservoir.
• Lack of protective covering (e.g., riprap or concrete) on the upstream face.
• Effects:
• Loss of material weakens the dam structure.
• Can lead to sloughing or sliding of the upstream slope.
• Prevention:
• Use of riprap (stone armor) or concrete slabs to protect the upstream face.
• Regular inspection and repair of damaged areas.

3. Erosion of Downstream Face:

• Description: Erosion of the downstream face occurs when water flows over or through the dam, washing away material on the downstream side.
• Causes:
• Overtopping or seepage through the dam.
• Poor compaction or inadequate slope protection.
• Effects:
• Loss of material weakens the dam.
• Can lead to progressive failure if not addressed.
• Prevention:
• Proper compaction during construction.
• Use of vegetation, riprap, or other protective measures on the downstream slope.
• Installation of drainage systems to control seepage.

4. Erosion of the Toe of the Dam:

• Description: The toe of the dam (the base of the downstream slope) can erode due to water flow or seepage.
• Causes:
• Concentrated seepage or piping at the toe.
• Overflow or spillway discharge hitting the toe area.
• Effects:
• Undermining of the dam's foundation.
• Loss of support for the downstream slope, leading to potential sliding or collapse.
• Prevention:
• Proper design of drainage systems to control seepage.
• Use of riprap or concrete aprons to protect the toe.
• Regular inspection and maintenance of the toe area.

Seepage Failure:

Seepage failure occurs when water infiltrates through the embankment body or foundation of a dam, leading to uncontrolled movement of water. If not properly managed, seepage can cause significant damage and even catastrophic failure. Below is a detailed discussion of the key issues related to seepage failure:

1. Piping (Internal Erosion):

• Description: Piping occurs when seepage water erodes soil particles within the dam or its foundation, creating channels or tunnels.
• Causes:
• High seepage velocity or pressure.
• Poorly graded or cohesionless soils.
• Cracks, voids, or discontinuities in the dam or foundation.
• Effects:
• Formation of erosion pathways weakens the dam structure.
• Can lead to sudden collapse if piping progresses unchecked.
• Prevention:
• Use of well-graded filters to prevent particle migration.
• Proper design of drainage systems to control seepage.
• Regular monitoring for signs of internal erosion.

2. Uplift:

• Description: Uplift occurs when seepage water creates upward pressure beneath the dam or within its foundation, reducing the effective weight and stability of the structure.
• Causes:
• High water pressure in the foundation or embankment.
• Inadequate drainage or cut-off measures.
• Effects:
• Reduces the dam's resistance to sliding or overturning.
• Can lead to foundation failure or structural displacement.
• Prevention:
• Installation of relief wells or drainage blankets.
• Use of cut-off walls or grout curtains to reduce seepage pressure.
• Proper foundation treatment during construction.

3. Sloughing:

• Description: Sloughing refers to the sliding or crumbling of the downstream face due to saturation and loss of soil strength caused by seepage.
• Causes:
• Prolonged seepage saturating the downstream slope.
• Poor compaction or steep slopes.
• Effects:
• Loss of material from the downstream face.
• Can progress to larger failures if not addressed.
• Prevention:
• Proper drainage to control seepage.
• Use of flatter slopes and well-compacted materials.
• Regular inspection and repair of saturated areas.

4. Conduit Leakage:

• Description: Conduit leakage occurs when water seeps through or around conduits (e.g., pipes, spillways) embedded in the dam.
• Causes:
• Poor construction or sealing around conduits.
• Cracks or deterioration of conduit materials.
• Effects:
• Increased seepage and erosion around the conduit.
• Potential for piping or structural failure.
• Prevention:
• Proper design and installation of conduits.
• Use of watertight seals and durable materials.
• Regular inspection and maintenance of conduits.

Structural Failure:

Structural failures in earth dams occur when the dam or its foundation cannot withstand the forces acting upon it, leading to instability or collapse. These failures are often related to shear failures, which cause sliding or displacement of the embankment or foundation. Below is a detailed discussion of the types of structural failures in earth dams:

1. Slides in Embankments:

• Description: Slides occur when a portion of the embankment loses stability and slides along a failure plane.
• Causes:
• Steep slopes or inadequate compaction.
• Saturation of embankment materials due to seepage.
• Poor material quality or improper construction.
• Effects:
• Displacement or collapse of the embankment.
• Potential breach of the dam.
• Prevention:
• Use of flatter slopes and well-compacted materials.
• Proper drainage to control seepage.
• Regular monitoring and maintenance.

2. Foundation Slides:

• Description: Foundation slides occur when the underlying soil or rock beneath the dam fails, causing the dam to slide or tilt.
• Causes:
• Weak or unstable foundation materials.
• High pore water pressure in the foundation.
• Inadequate foundation preparation or treatment.
• Effects:
• Displacement or tilting of the dam.
• Potential catastrophic failure.
• Prevention:
• Thorough site investigation and foundation treatment.
• Use of cut-off walls or grout curtains to stabilize the foundation.
• Proper drainage to reduce pore water pressure.

3. Liquefaction Slides:

• Description: Liquefaction occurs when saturated, loose soils lose strength and behave like a liquid during seismic activity or rapid loading.
• Causes:
• Saturation of loose, granular soils.
• Earthquake-induced shaking or rapid loading.
• Effects:
• Sudden loss of strength in the embankment or foundation.
• Rapid sliding or collapse of the dam.
• Prevention:
• Avoid building dams on loose, saturated soils.
• Use of densification techniques (e.g., compaction, vibro-flotation) to stabilize soils.
• Proper drainage to reduce saturation.

4. Failures Due to Earthquakes:

• Description: Earthquakes can cause ground shaking, leading to cracking, sliding, or liquefaction of the dam or foundation.
• Causes:
• Seismic activity in the region.
• Poor design or construction for earthquake resistance.
• Effects:
• Cracking, sliding, or collapse of the dam.
• Potential release of reservoir water.
• Prevention:
• Design dams to withstand seismic forces.
• Use of flexible materials and proper compaction.
• Avoid building dams in highly seismic zones.

5. Failures Due to Holes Caused by Burrowing Animals:

• Description: Burrowing animals (e.g., rodents, beavers) can create holes or tunnels in the dam, leading to increased seepage and potential failure.
• Causes:
• Presence of burrowing animals in the dam area.
• Lack of preventive measures.
• Effects:
• Increased seepage and erosion.
• Potential for piping or structural failure.
• Prevention:
• Regular inspection and removal of animal burrows.
• Use of barriers or deterrents to prevent animal activity.

6. Failures Due to Holes Caused by Leaching of Water-Soluble Salts:

• Description: Leaching of water-soluble salts (e.g., gypsum, halite) from the dam materials can create voids or weaken the structure.
• Causes:
• Presence of soluble salts in the dam materials.
• Prolonged exposure to water.
• Effects:
• Formation of voids or cavities in the dam.
• Reduced strength and potential collapse.
• Prevention:
• Avoid using materials with high soluble salt content.
• Proper site investigation and material testing.
• Use of impermeable layers to reduce water infiltration.

Design Criteria for Earth Dams

1.  No Overtopping – 

a.      Sufficient freeboard should be included to account for the effects of waves, frost, wind setup, and seismic activity.

b.      Appropriate allowances for shrinkage must be considered during the construction process.

c.      The dam's height should incorporate a suitable margin to accommodate potential settlement.

2.  No Seepage Failure-

a.      The phreatic line or seepage line must stay sufficiently inside the downstream face to prevent any sloughing or erosion of the downstream slope.

b.     Proper measures should be implemented to control seepage through the dam's body, foundations, and abutments.

c.      The dam and its foundation must be designed to ensure safety against piping failure.

3. No Structural Failure –

a.        The design of the upstream and downstream slopes must ensure their stability both during and right after the construction process.

b.       The upstream slope must remain stable in the event of a sudden drawdown.

c.        The downstream slope should be designed to remain secure under steady seepage when the reservoir is at full capacity.

d.       The entire dam structure must be capable of withstanding earthquake forces.

e.        The slopes on both the upstream and downstream sides should be sufficiently gentle to provide an adequate base width at the foundation level. This ensures that the maximum shear stress generated is significantly lower than the soil's maximum shear strength, thereby maintaining an appropriate safety factor.

4. Proper Slope Protection –

a.      The upstream slope must be safeguarded against erosion caused by wave action. To prevent erosion, rip-rap should be installed along the entire upstream slope as well as near the toe of the downstream slope, extending slightly above the tailwater level.

b.     The downstream slope and crest (top) should be protected from erosion due to rain and wind. To achieve this, turf should be applied to the downstream slope.

5.  Proper Drainage –

a.      The downstream section of the impervious core should be effectively drained using an appropriate horizontal drain filter, toe drain, chimney drain, or other suitable drainage system.

6. Economic Section –

a.      Whenever possible, locally available materials near the dam site should be utilized to minimize costs.

All these factors must be met to ensure the safe design and construction of an earth dam.

 

Components of An Earthen Dam

Shell, Upstream Fill, Downstream Fill, or Shoulder:

·        These parts of an earthen dam are built using permeable or semi-permeable materials on either the upstream or downstream side of the core. The upstream section is referred to as the upstream shell, while the downstream section is called the downstream shell.

Upstream Blanket:

·        This is a layer of impermeable material placed on the upstream side of an earthen dam when the underlying ground is permeable. Its purpose is to reduce seepage and lengthen the flow path, thereby decreasing both seepage and excess pressure on the downstream side. A natural blanket consists of naturally occurring low-permeability soil.

Drainage Filter:

·        This is a layer of permeable material built at the base on the downstream side of an earthen dam. It allows seepage to discharge safely and helps prevent piping failure.

Cutoff Wall or Cutoff:

·        This is a wall, collar, or similar structure designed to reduce water seepage through porous layers. It is installed in or on the dam's foundation.

Riprap:

·        This involves placing broken stones or rock pieces on the dam's slopes, especially the upstream side, to protect against water erosion, particularly from wave action.

Core Wall, Membrane, or Core:

·        This is a relatively impermeable wall located at the center of the dam. It controls water flow through the dam and can be made of compacted clay, masonry, or concrete.

Toe Drain:

·        This is a drain installed at the downstream slope of an earthen dam to collect and remove seepage water gathered by the drainage filters.

Transition Filter:

·        This is a component of an earthen dam that sits between the core and the shells. It consists of intermediate-grade material that acts as a filter, preventing fine material from the core from moving laterally.