Active Earth Pressure

Definition

Active earth pressure is the lateral pressure exerted by soil on a retaining structure when the soil is allowed to expand, often due to wall movement away from the soil. This condition typically occurs when the wall moves outward or when there is an increase in soil volume, leading to a reduction in stress against the wall. Understanding this concept is crucial for designing various types of retaining structures, analyzing soil behavior, and ensuring stability.

5 Must Know Facts For Your Next Test

1. Active earth pressure is calculated using either Rankine's or Coulomb's earth pressure theories, which provide methods for determining the lateral force exerted by the soil.
2. The coefficient of active earth pressure (K_a) can be influenced by factors like wall friction and the angle of internal friction of the soil.
3. In cases where walls experience no movement, at-rest earth pressure conditions prevail, leading to higher lateral forces compared to active conditions.
4. Active earth pressure conditions can significantly affect the design of gravity walls and cantilever walls, requiring careful consideration during construction.
5. A key assumption in determining active earth pressure is that the wall moves far enough away from the soil to allow the soil to reach its active state without additional restraints.

Review Questions

• How does active earth pressure differ from at-rest and passive earth pressure, and what are the implications of these differences for wall design?
  • Active earth pressure occurs when a retaining wall moves away from the soil, allowing it to expand and exert lateral pressure. In contrast, at-rest earth pressure exists when there is no movement of the wall, resulting in greater lateral forces. Passive earth pressure happens when a wall moves towards the soil, increasing stress against it. These differences are crucial for wall design as they determine how much force must be countered by structural elements such as reinforcement and drainage systems.
• Evaluate how Rankine's theory assists engineers in calculating active earth pressure and its relevance in designing retaining walls.
  • Rankine's theory provides a straightforward method for calculating active earth pressure based on simple geometric and frictional principles. It assumes a level backfill with no wall friction, making it easy to apply. This simplicity helps engineers quickly determine the forces acting on retaining walls during preliminary design phases, ensuring that structures are adequately reinforced to withstand active pressures under normal conditions.
• Critically assess the role of wall movement in transitioning from at-rest to active earth pressure states and how this influences stability analysis for retaining walls.
  • The transition from at-rest to active earth pressure states is primarily governed by wall movement; as a wall moves outward, it allows the soil to reach its active state, resulting in lower lateral pressures. This shift can dramatically influence stability analysis since calculations for resistance must account for changes in force dynamics. Engineers must consider potential movements due to external factors like seismic activity or excavation work, ensuring that designs accommodate these changes while maintaining stability against both active pressures and possible failure modes.

Related terms

Retaining Wall: A structure designed to support and retain soil, preventing erosion and collapse while managing lateral earth pressures.

Passive Earth Pressure: The lateral pressure exerted by soil on a retaining structure when the wall moves towards the soil, resulting in an increase in stress against the wall.

Mohr-Coulomb Failure Criterion: A model that describes the relationship between shear stress and normal stress in soil mechanics, helping predict failure conditions under various loading scenarios.