Passive pressure is the maximum horizontal resistance that a soil mass can offer to the movement of a retaining structure pushing into it before the soil yields and fails in shear along a defined wedge.
In simpler terms, imagine you're pushing a wall into a soil bank. The soil resists that push. Passive pressure is the maximum amount of resistance the soil can provide before it gives way and a wedge of soil begins to slide upwards. It's the soil's ultimate "pushback" force.
Understanding Passive Pressure
Here's a breakdown of key aspects:
- Direction: Passive pressure acts horizontally, opposing the movement of the structure pushing into the soil.
- Magnitude: It's significantly larger than active pressure (the pressure the soil exerts on a stationary retaining structure). This is because it represents the soil's resistance to being compressed.
- Development: Passive pressure develops when a retaining structure moves into the soil mass, compressing the soil. The required movement is usually larger than that required for active pressure to fully develop.
- Failure Mechanism: As the structure moves, the soil behind it is compressed. Eventually, the soil reaches its shear strength, and a wedge of soil fails along a slip plane, sliding upwards and away from the structure.
- Calculation: Passive earth pressure can be calculated using various theories, such as Rankine's theory and Coulomb's theory. These theories consider soil properties like cohesion, angle of internal friction, and unit weight.
Factors Affecting Passive Pressure
Several factors influence the magnitude of passive pressure:
- Soil Properties:
- Angle of Internal Friction (Φ): A higher angle of internal friction results in greater passive pressure.
- Cohesion (c): Cohesive soils exhibit higher passive pressure.
- Unit Weight (γ): Increased unit weight of the soil leads to higher passive pressure.
- Soil Condition: Water content and compaction can greatly influence soil properties, thus impacting passive pressure.
- Wall Characteristics:
- Wall Friction Angle (δ): Friction between the wall and soil increases passive resistance.
- Wall Inclination: Inclined walls affect the geometry of the failure wedge and, consequently, the passive pressure.
- Surcharge: Any additional load applied to the soil surface behind the wall (surcharge) increases passive pressure.
Importance in Geotechnical Engineering
Passive pressure is crucial in the design of various geotechnical structures:
- Retaining Walls: It provides resistance against sliding and overturning forces.
- Sheet Piles: Passive pressure on the embedded portion of the sheet pile provides stability.
- Anchors: Anchors rely on passive resistance to provide pullout capacity.
- Foundations: It contributes to the bearing capacity of shallow foundations.
In conclusion, passive pressure represents the soil's maximum resistance to compression by a moving structure, playing a critical role in ensuring the stability of geotechnical constructions.
