Introduction

A gravity dam is a massive civil engineering structure built primarily of concrete or masonry, designed to hold back large volumes of water by relying on its weight to resist external forces. A balance between the resisting forces governs its stability due to the dam’s self-weight and the destabilizing forces such as water pressure, uplift, silt pressure, and seismic activity. Because gravity dams are often located in seismically active or strategically significant regions, their performance under dynamic and extreme loads has become a critical area of study.

One of the most severe threats to the structural integrity of a gravity dam is an explosion load, which may result from accidental or intentional detonations. Explosions generate high-intensity shock waves that can cause localized damage, cracking, or even partial failure of the dam body. Unlike static water pressure, an explosion imparts sudden and impulsive forces, making the dam behave dynamically, with stress waves propagating through the structure and reflecting at material boundaries. This transient response can amplify damage and compromise the dam’s ability to resist subsequent loads.

Similarly, earthquake loads pose a significant challenge to the safety of gravity dams. During seismic events, the ground motion induces inertial forces within the dam body and the reservoir, leading to additional hydrodynamic pressures on the upstream face. The interaction between the dam, foundation, and reservoir water can result in complex dynamic responses, including rocking, sliding, or cracking of the structure. The design and safety evaluation of gravity dams,therefore, require careful consideration of seismic effects through dynamic analysis methods.

The combined study of explosion effects and earthquake-induced loads on gravity dams is crucial for modern dam safety assessment. Understanding the structural response under such extreme conditions not only enhances resilience against natural disasters but also prepares for man-made hazards.

In this package, some concepts like explosion, seismic loading, and crack growth during 6 practical tutorials are investigated.

Failure Modes of Gravity Dams under Extreme Loads

The stability of a gravity dam depends on its ability to resist overturning, sliding, and structural failure under various loading conditions. When subjected to explosions or earthquake-induced dynamic forces, the dam may experience one or more of the following failure modes:

1. Overturning Failure

• Description: Occurs when the resultant of all forces (hydrostatic, seismic, explosion-induced shock, etc.) falls outside the middle third of the dam’s base.

• Explosion effect: The sudden shock wave may create a localized uplift or impulse force, pushing the dam forward and causing it to rotate about its toe.

• Earthquake effect: Horizontal seismic inertia forces acting at the dam’s center of mass cause rocking, which may induce cracking at the heel or toe.

2. Sliding Failure

• Description: Happens when the horizontal forces exceed the frictional resistance at the dam–foundation interface.

• Explosion effect: High-intensity pressure waves can create instantaneous shear forces at the base, reducing stability against sliding.

• Earthquake effect: Ground shaking can weaken the contact surface, cause uplift pressure variations, and trigger sliding along weak foundation planes.

3. Structural Cracking and Tensile Failure

• Description: Concrete gravity dams are strong in compression but weak in tension. Under dynamic loading, tensile stresses may exceed the material’s capacity, causing cracks.

• Explosion effect: Shock waves generate rapid stress reversals, producing localized tensile cracks on the upstream or downstream faces.

• Earthquake effect: Seismic-induced hydrodynamic pressures increase tensile stresses at the dam crest and heel, leading to vertical cracking.

4. Foundation Failure

• Description: Stability relies heavily on the strength of the foundation rock. Weak foundations may fail before the dam’s body itself.

• Explosion effect: Repeated blast loads may cause joint opening, spalling, or weakening of rock beneath the structure.

• Earthquake effect: Seismic vibrations can induce liquefaction in weak foundation soils or amplify stresses in fractured rock masses, reducing shear strength.

5. Uplift and Piping Failure

• Description: Uplift pressure under the dam reduces effective weight and sliding resistance. Piping refers to progressive erosion beneath the dam due to seepage.

• Explosion effect: Blast waves can cause sudden uplift pressures, destabilizing the structure and worsening seepage paths.

• Earthquake effect: Seismic shaking may enlarge cracks or joints, increasing uplift pressure and promoting internal erosion.

6. Progressive Structural Collapse

• Description: A severe condition where local cracking or sliding leads to chain-reaction failure of large sections of the dam.

• Explosion effect: If damage is concentrated at a weak section (e.g., near spillways), partial collapse may propagate into a global failure.

• Earthquake effect: Combined hydrodynamic and inertial loads may exceed structural capacity, causing cascading cracking and block separation.