Causes of Earth Dam Failure

Earth dams, also known as embankment dams, are constructed by compacting layers of soil, sand, clay, and rock. Because they are built from natural materials rather than solid concrete, they are particularly susceptible to erosion and structural degradation. Understanding the primary causes of earth dam failure is crucial for civil engineers, dam inspectors, and water resource management specialists.

Historically, failures are categorized into three main physical mechanisms: Hydraulic failures, Seepage failures, and Structural failures. These modes are often interconnected, meaning one minor defect can initiate a chain reaction leading to catastrophic failure.

1. Hydraulic Failures

Hydraulic failures are caused by the uncontrolled movement of water over or around the embankment, resulting in severe surface erosion. The major types of hydraulic failures include:

• Overtopping: This is the most common cause of earth dam breaches. It occurs when floodwaters exceed the storage capacity of the reservoir and spill over the crest of the dam. Since earth dams are highly vulnerable to flowing water, overtopping quickly washes away the downstream slope, leading to a complete breach within hours.
• Wave Erosion: Wind-driven waves on the reservoir can erode the upstream face of the dam if proper riprap protection is absent or degraded.
• Toe Erosion: High-velocity water exiting from spillways or tailwater channels can erode the downstream toe of the dam, undermining its structural foundation and causing slope collapse.
• Gullying: Heavy rainfall directly falling on unprotected downstream slopes can wash away soil, creating deep channels (gullies) that weaken the overall embankment thickness.

Historical Note: The Queensland Floods Commission of Inquiry (Australia) extensively evaluated the structural and hydraulic safety margins of the Wivenhoe Dam, referencing similar earth dam failures globally to establish robust design flood models.

2. Seepage Failures (Internal Erosion & Piping)

While all earth dams allow a minor, controlled amount of water to seep through their embankments, uncontrolled seepage is extremely dangerous. It accounts for nearly one-third of all dam failures.

• Piping (Internal Erosion): Piping occurs when seeping water carries fine soil particles out of the embankment body or its foundation. As these particles are washed away, a hollow channel or “pipe” is formed. This channel progressively grows backward towards the reservoir. Once the pipe reaches the reservoir, water flows through it under high pressure, leading to rapid washing out of the core and eventual failure.
• Piping through the Foundation: If the foundation soil consists of permeable materials like coarse sand or gravel, water can bypass the grout curtain or cutoff wall, causing piping from underneath the dam.
• Conduit Seepage: Concentrated seepage frequently occurs along the boundary between the soil embankment and concrete outlet pipes or spillway structures. Poor compaction around these interfaces creates easy paths for piping to initiate.

3. Structural Failures

Structural failures involve the physical sliding, cracking, or settlement of the dam embankment or its foundation, independent of direct water flow. They are usually caused by internal forces and soil characteristics:

• Slope Failure (Slides): Instability in either the upstream or downstream slope can lead to mass sliding of the soil. This is often triggered by sudden changes in reservoir levels (rapid drawdown), which leaves the upstream slope saturated and unsupported, or by high pore water pressures within the downstream slope.
• Foundation Settlement: Excessive or differential settlement of the foundation soil (especially soft clay) can cause vertical or diagonal cracking across the dam crest, creating pathways for water to flow through and initiate piping.
• Earthquake Liquefaction: Seismic activity can cause saturated cohesionless soils (like loose sand) within the embankment or foundation to lose shear strength and behave like a liquid, leading to sudden slope collapse.

Summary of Earth Dam Failure Mechanisms

References & Technical Reading

• United States Bureau of Reclamation (USBR) – Design of Small Dams.
• Federal Emergency Management Agency (FEMA) – Federal Guidelines for Dam Safety.
• Queensland Floods Commission of Inquiry – Exhibit 543: Report on Wivenhoe Dam safety guidelines.