--- title: "Why design for active pressure only" subject: "Foundation Engineering" chapter: 3 key: "semester_5/foundation_engineering:passive-pressure-neglect" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/passive-pressure-neglect site: BCT Engineering --- # Why design for active pressure only Foundation Engineering · Chapter 3: Lateral Earth Pressure Theories and Retaining Walls - Asked in: 2080 Baisakh, 2 marks; 2079 Bhadra, 2 marks; 2076 Chaitra, 3 marks; 2075 Asoj, 6 marks (asked 4 times) - Video: [Rankine Theory of Earth Pressure | Elementary Engineering](https://www.youtube.com/watch?v=3CnW3mHvJ2Q) — Elementary Engineering - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/passive-pressure-neglect - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-3 ## Question Why are retaining walls designed for active earth pressure? Justify the reasons for not considering/neglecting the passive earth pressure in the design and stability analysis of rigid retaining structures. ## Short answer Retaining walls yield slightly outward, mobilizing active earth pressure as the destabilizing force they must resist. Passive pressure on the toe is neglected because full mobilization requires large movement, the soil in front may be removed or softened, and ignoring it is conservative. ## Answer #### Why walls are designed for active pressure A rigid retaining wall holds soil on the back side. The retained soil pushes the wall outwards; the wall yields very slightly (about 0.001 to 0.005 H for sand, up to 0.01 to 0.02 H for clay) and the soil reaches the **active state**, the minimum lateral pressure. This is the destabilising force that causes overturning and sliding, so the wall must be designed against it ($K_a = \tan^2(45^\circ - \phi/2)$). #### Why passive pressure is neglected Passive pressure acts on the toe side of the embedded part of the wall and resists sliding. It is usually ignored, or only a fraction (about 50%) is used, because: 1. **Large movement is needed:** full passive resistance needs a movement of about 0.01 to 0.05 H toward the soil, much more than the small wall movement allowed at working load. At working movement only a small part is mobilised. 2. **Soil in front may be removed:** by scour, erosion, future excavation, utility trenches or other construction. 3. **Soil may soften or shrink:** wetting by surface water, seasonal moisture change, frost action and cracks reduce its strength; the top 0.5-1 m is particularly unreliable. 4. **Construction disturbance:** the toe backfill is often loose or poorly compacted. 5. **Compatibility of strains:** active state forms at very small strain, passive state at large strain, so both limit states cannot occur together. 6. **Safety:** neglecting passive resistance gives a conservative design, because $K_p$ is large and an overestimate is dangerous (unsafe). Where passive resistance is used (e.g. a shear key), it is taken only below the depth of possible disturbance and with a factor of safety of at least 2 on $P_p$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a rigid retaining wall designed for active earth pressure? A. Active pressure is the maximum pressure and the wall must resist it B. Active pressure is the minimum lateral pressure, reached after slight outward yielding, and it is the destabilising force C. Active pressure acts on the toe side and resists sliding D. Active pressure develops only when the wall does not move Answer: B. The retained soil pushes the wall outward; after small yielding it reaches the active state, the minimum lateral pressure, which causes overturning and sliding. (Key point: Active state) 2. Which is the main reason passive resistance is not fully relied upon at working load? A. It needs a very large movement toward the soil (about 0.01-0.05 H) compared with the small movement allowed B. It is always smaller than active pressure C. It acts on the backfill side of the wall D. It needs no movement but is hard to calculate Answer: A. Full passive resistance needs large movement; at working movement only a small part is mobilised. (Key point: Movement needed for passive) 3. Which is NOT given as a reason for neglecting passive pressure? A. Soil in front may be removed by scour or excavation B. Soil may soften or shrink with moisture change C. Toe backfill is often loose or poorly compacted D. Kp is smaller than Ka so passive pressure is negligible Answer: D. Kp is large, not small; neglecting passive is conservative because overestimating it is unsafe. (Key point: Soil in front unreliable) 4. If passive resistance is used (e.g. with a shear key), how is it taken? A. Fully, from the ground surface B. Only below the depth of possible disturbance with a factor of safety of at least 2 on Pp C. Doubled to account for Kp D. Only in clay soils Answer: B. It is used only below the disturbed zone and with a factor of safety of at least 2. (Key point: Use of passive resistance) ## Related in Foundation Engineering - [Flexible versus rigid retaining structures](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aflexible-vs-rigid-retaining) — Foundation Engineering, chapter 5, asked 3× - [Guidelines and design considerations, retaining walls](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aretaining-wall-guidelines) — Foundation Engineering, chapter 3 - [Stability checks of retaining walls](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Astability-checks-retaining-wall) — Foundation Engineering, chapter 3, asked 2× - [Active pressure on wall, cohesive soil](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acohesive-wall-pressure-diagram) — Foundation Engineering, chapter 3 - [Passive thrust on retaining wall back](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awall-passive-thrust-two-layer) — Foundation Engineering, chapter 3 - [Proportioning of retaining walls](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aproportioning-retaining-walls) — Foundation Engineering, chapter 3, asked 3× ## Related in other subjects - [Insertion gain/loss, design steps, active vs passive](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Ainsertion-loss-steps-active-passive) — Filter Design, chapter 1 - [Effect of neglecting earth on capacitance](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aearth-effect-horizontal-13m) — Power System Analysis I, chapter 4 - [Types of retaining walls](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aretaining-walls-types) — Transportation Engineering I, chapter 5, asked 5× - [Active vs passive; non-inverting (s+8)/(s+2)](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aactive-passive-difference-noninverting) — Filter Design, chapter 6, asked 5×