Passive pressure is the maximum resistance an in-situ soil can provide against the lateral movement of a structure, such as a retaining wall or a pile, before the soil fails by shearing. Essentially, it's the limit state condition where the soil is being compressed to its maximum capacity to resist movement.
Here's a breakdown:
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Definition: The greatest possible horizontal stress that a soil mass can sustain when it is compressed laterally. It represents the soil pushing back against a structure trying to move into it.
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Mechanism: As a structure (e.g., a retaining wall) moves into the soil mass, it compresses the soil. This compression generates a resisting force, which increases until the soil reaches its shear strength and fails along a rupture surface. This maximum resisting force is the passive pressure.
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Relevance to Retaining Structures: Passive pressure is a critical factor in the design of retaining walls. Engineers often utilize the passive pressure developed in front of a wall's toe to enhance its stability and resistance to sliding. However, it's crucial to remember it's a limit state and a large factor of safety is often applied.
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Applicability to Piles: While the principles are derived from retaining wall theory, passive pressure concepts can also be applied to analyze the lateral resistance of piles, particularly short, rigid piles.
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Important Considerations:
- It's a Limit State: Passive pressure represents the maximum pressure, right before failure. Therefore, it's not an allowable pressure for design. Designs must incorporate a significant factor of safety.
- Movement is Required: For passive pressure to develop, the structure must move significantly into the soil mass.
- Soil Properties Matter: The magnitude of passive pressure depends heavily on the soil's properties, including its angle of internal friction, cohesion, unit weight, and the groundwater table.
- Methods of Calculation: Several theories, such as Rankine's and Coulomb's, are used to calculate passive earth pressure. These theories make simplifying assumptions about the soil and the failure surface.
In summary, passive pressure is the maximum force a soil can exert against a structure that is moving into it, representing a limit state of soil failure. Its accurate estimation is vital for safe and reliable structural design, especially for retaining walls and piles.
