Select the option that is appropriate regarding the following two statements labelled Assertion and Reason.

Assertion: Piping below the weir can be prevented by providing an impervious floor of sufficient length so that the path of percolation is increased and the exit gradient is decreased.

Reason: If the exit gradient is less than a certain critical value, the soil starts boiling and is washed away by percolating water.

Explanation:​

• Piping Phenomenon:
• Definition: Piping refers to the erosion of soil material from the foundation of a structure due to water flow, leading to the formation of channels or "pipes."
• Cohesionless Soil: This phenomenon is particularly prevalent in cohesionless soils, where high upward hydraulic gradients cause the soil to lose shear strength.
• Undermining: When the hydraulic gradient at the exit exceeds the critical gradient for the subsoil, soil particles move with the water flow, leading to degradation, cavity formation, and eventual structural failure. This process is also known as sand boiling or undermining.
• Prevention of Piping Failure:
• Piping failure in hydraulic structures can be prevented by the following methods:
• Increasing Stress:
• Enhance the stress due to the weight of the structure to counteract the hydraulic forces.
• Providing a Cut-Off Wall:
• A cut-off wall helps to block the flow of water and prevent erosion.
• Increasing the Flow Path:
• Use an impervious blanket to extend the flow path, reducing the hydraulic gradient and minimizing erosion.
• Impervious Floor:
• For structures like weirs, an impervious floor of sufficient length can be provided to increase the path of percolation, thereby decreasing the exit gradient and preventing piping.