Question:
When the net horizontal force at the base of the dam exceeds the frictional resistance at the base, then the dam fails by
When the net horizontal force at the base of the dam exceeds the frictional resistance at the base, then the dam fails by
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For dam stability, associate forces and moments with failure modes:
- Horizontal Forces $\rightarrow$ Check for Sliding.
- Moments (about the toe) $\rightarrow$ Check for Overturning.
- Vertical Forces / Stresses $\rightarrow$ Check for Crushing.
- Horizontal Forces $\rightarrow$ Check for Sliding.
- Moments (about the toe) $\rightarrow$ Check for Overturning.
- Vertical Forces / Stresses $\rightarrow$ Check for Crushing.
Updated On: Jul 1, 2026
- Crushing
- Sliding
- Overturning
- Buckling
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Question:
The question describes a specific failure scenario for a dam and asks for the name of this failure mode.
Step 2: Detailed Explanation:
The stability of a gravity dam is checked against several possible failure modes:
• (B) Sliding: The net horizontal force ($F_H$) acting on the dam (primarily from the water pressure) tends to push the dam horizontally. This is resisted by the frictional force ($F_f$) between the base of the dam and the foundation rock. The frictional force is given by $F_f = \mu \times W$, where $\mu$ is the coefficient of friction and $W$ is the total downward vertical force (weight of the dam). If the horizontal force exceeds the maximum possible frictional resistance ($F_H \gt F_f$), the dam will
slide along its base. This exactly matches the condition described in the question.
• (C) Overturning: The moment caused by the horizontal water pressure tends to rotate or overturn the dam about its "toe" (the downstream edge of the base). This is resisted by the moment caused by the dam's own weight, which acts to stabilize it. Failure occurs if the overturning moment exceeds the stabilizing moment.
• (A) Crushing: The combined effect of the dam's weight and the forces from the water creates compressive stresses at the base. If these stresses exceed the allowable compressive strength of the dam's material (concrete) or the foundation rock, the material will crush.
• (D) Buckling: This is a failure mode for slender compression members and is not relevant to a massive structure like a dam.
Step 3: Final Answer:
When the net horizontal force exceeds the frictional resistance at the base, the dam fails by sliding.
The question describes a specific failure scenario for a dam and asks for the name of this failure mode.
Step 2: Detailed Explanation:
The stability of a gravity dam is checked against several possible failure modes:
• (B) Sliding: The net horizontal force ($F_H$) acting on the dam (primarily from the water pressure) tends to push the dam horizontally. This is resisted by the frictional force ($F_f$) between the base of the dam and the foundation rock. The frictional force is given by $F_f = \mu \times W$, where $\mu$ is the coefficient of friction and $W$ is the total downward vertical force (weight of the dam). If the horizontal force exceeds the maximum possible frictional resistance ($F_H \gt F_f$), the dam will
slide along its base. This exactly matches the condition described in the question.
• (C) Overturning: The moment caused by the horizontal water pressure tends to rotate or overturn the dam about its "toe" (the downstream edge of the base). This is resisted by the moment caused by the dam's own weight, which acts to stabilize it. Failure occurs if the overturning moment exceeds the stabilizing moment.
• (A) Crushing: The combined effect of the dam's weight and the forces from the water creates compressive stresses at the base. If these stresses exceed the allowable compressive strength of the dam's material (concrete) or the foundation rock, the material will crush.
• (D) Buckling: This is a failure mode for slender compression members and is not relevant to a massive structure like a dam.
Step 3: Final Answer:
When the net horizontal force exceeds the frictional resistance at the base, the dam fails by sliding.
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