--- title: "Causes of slope failure" subject: "Soil Mechanics" chapter: 11 key: "semester_4/soil_mechanics:slope-failure-causes" source: https://www.bctengineer.com/q/semester_4/soil_mechanics/slope-failure-causes site: BCT Engineering --- # Causes of slope failure Soil Mechanics · Chapter 11: Stability of Slopes - Asked in: 2073 Magh, 3 marks; 2075 Bhadra, 2 marks; 2075 Baisakh, 2 marks; 2079 Jestha (asked 4 times) - Video: [What Causes Slope Failures](https://www.youtube.com/watch?v=IbWiagy3q-w) — Blueprint to Reality - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/slope-failure-causes - Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-11 ## Question What are the causes of failure of earth slopes? (Write down the possible causes of increase in shear stress or decrease in shear strength in regard to slope instability.) ## Short answer Slope failure occurs when shear stress exceeds shear strength. Shear stress increases from toe excavation, surcharge, steepening, added water weight, earthquakes, and water-filled tension cracks. Shear strength decreases from rising pore pressure, rapid drawdown, weathering, strain softening, root decay, and liquefaction. ## Answer Slope failure occurs when the shear stress along a surface exceeds the shear strength of the soil. The causes can be grouped into those that **increase the shear stress** and those that **decrease the shear strength**. #### A. Causes that increase shear stress 1. **Removal of lateral support** by erosion, undercutting or excavation at the toe. 2. **Extra load on the top (surcharge):** buildings, stored materials, vehicles, or fill placed on the slope. 3. **Increase in slope steepness or height** by cutting or filling. 4. **Increase in weight of soil** from rain or seepage water (higher unit weight). 5. **Earthquakes and vibrations** from blasting, traffic or machinery, which add dynamic forces. 6. **Tension cracks filled with water,** which cause hydrostatic thrust on the potential slip surface. 7. **Transient loads such as wind or water pressure** acting on the slope. #### B. Causes that decrease shear strength 1. **Increase in pore water pressure** from rainfall, rising water table or seepage, which lowers effective stress. 2. **Rapid drawdown** of water against a slope (e.g. reservoir bank), leaving excess pore pressure inside the soil. 3. **Weathering and chemical changes** that weaken the soil or rock and reduce cohesion. 4. **Swelling of clay and loss of cementation** after wetting. 5. **Progressive failure and strain softening,** reducing strength from peak to residual. 6. **Decay of roots** and removal of vegetation, which reduces cohesion near the surface. 7. **Liquefaction or loss of strength** in loose saturated sand due to shaking. 8. **Freeze-thaw and cracking** that open fissures and weaken the mass. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Slope failure occurs when: A. effective stress exceeds total stress B. shear strength increases with water C. unit weight falls below gamma_w D. shear stress along a surface exceeds the shear strength Answer: D. Causes are grouped as those that increase shear stress or decrease strength. (Key point: Basic cause) 2. Which cause increases shear stress? A. Increase in pore pressure B. Weathering of the soil C. Surcharge on the crest or removal of support at the toe D. Strain softening Answer: C. Loads or loss of support increase driving forces; the others reduce strength. (Key point: Increase in shear stress) 3. Which is a cause of decreased shear strength? A. Placing a berm at the toe B. Rise in pore water pressure from rainfall, which lowers effective stress C. Flattening the slope D. Reducing crest load Answer: B. Strength depends on effective stress, which falls as pore pressure rises. (Key point: Decrease in strength) 4. Why is rapid drawdown dangerous for a reservoir bank? A. Water leaves faster than pore water, leaving excess pore pressure and removing support B. Water adds extra weight to the slope C. It increases suction and strength D. It reduces slope angle Answer: A. Excess pore pressure remains while water pressure support is lost. (Key point: Rapid drawdown) ## Related in Soil Mechanics - [Role of effective stress in shear strength](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Arole-effective-stress-shear-strength) — Soil Mechanics, chapter 6 - [Slope failure types and prevention](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-failure-types-prevention) — Soil Mechanics, chapter 11, asked 2× - [Types of slope failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-failure-types) — Soil Mechanics, chapter 11, asked 2× - [Remedial measures for slope failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-remedial-measures) — Soil Mechanics, chapter 11 - [Shear strength tests (lab and field)](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ashear-strength-tests-names) — Soil Mechanics, chapter 10, asked 3× - [Drained vs undrained shear strength](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adrained-vs-undrained-shear-strength) — Soil Mechanics, chapter 10 ## Related in other subjects - [Fault: causes, classification, slope effects](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afault-causes-classification-slope-effects) — Engineering Geology I, chapter 5 - [Gravity dam stability and shear stress at toe (2081)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-stability-2081-bhadra) — Hydropower Engineering, chapter 4 - [Overturning and sliding check with shear strength](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agravity-dam-overturning-sliding) — Hydro Power, chapter 4 - [Causes of foundation failure](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-failures) — Building Technology, chapter 2, asked 2×