--- title: "General, local and punching shear failure" subject: "Foundation Engineering" chapter: 6 key: "semester_5/foundation_engineering:modes-shear-failure" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/modes-shear-failure site: BCT Engineering --- # General, local and punching shear failure Foundation Engineering · Chapter 6: Bearing Capacity and Settlement of Shallow Foundations - Asked in: 2081 Baisakh, 4 marks; 2078 Bhadra, 4 marks; 2075 Chaitra, 2 marks; 2076 Asoj, 2 marks (asked 4 times) - Video: [Types of Shear Failure - Bearing Capacity of Shallow Foundations - Geotechnical Engineering - II](https://www.youtube.com/watch?v=LT38UrJ_NaA) — Ekeeda - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/modes-shear-failure - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-6 ## Question Differentiate between general shear failure, local shear failure and punching shear failure with neat sketches. How do you ascertain whether the soil is likely to fail in local or general shear? ## Short answer General shear occurs in dense/stiff soils with a continuous failure surface reaching the ground, clear heave, and a sharp peak in load-settlement. Local shear in medium soils has a failure surface fading out, slight heave, and no clear peak. Punching shear in loose/soft soils shows only vertical shear, no heave, and continuously rising settlement. Ascertain by relative density or SPT N (N>30 general, N<10 punching/local), stress-strain tests (peak at small strain indicates general shear), or plate load test (defined breaking point). For local shear, Terzaghi reduces c' = 2/3 c and tanφ' = 2/3 tanφ. ## Answer Terzaghi and Vesic described three ways in which a soil fails below a footing. They differ in the shape of the failure surface and in the load–settlement curve. | Point | General shear | Local shear | Punching shear | |---|---|---|---| | Soil type | Dense sand, stiff clay | Medium dense sand, medium clay | Very loose sand, soft clay | | Failure surface | Continuous, reaches the ground surface | Well defined only below the footing, dies out in the soil | Vertical shear around the footing only | | Heaving at surface | Clear heave on both sides | Slight heave | No heave, the footing sinks | | Load–settlement curve | Sharp peak, then a drop | Peak not clear, steady slope | No peak, continuously rising | | Failure | Sudden and tilting | Gradual | Gradual, with large settlement | | Strain at failure | Small (below 5% in clay) | Medium | Large (above 20%) | ```text General shear Local shear Punching shear [== footing ==] [== footing ==] [== footing ==] heave /\ || /\ heave slight /\ || /\ slight | || | ______/ \ || / \______ ______/ \ || / \______ heave | || | no heave \ \ || / / \ \||/ / | || | `--.\/.--' `-.\/.-' |_____||_____| failure surface reaches failure zone fades out vertical shear only the ground surface in the soil ``` **Ascertaining the mode of failure** - **From relative density** (sand): $D_r > 70\%$ gives general shear, $D_r < 20\%$ gives punching shear, and in between local shear occurs. From the SPT: $N > 30$ (dense) gives general shear and $N < 10$ (loose) gives punching or local shear. - **From stress–strain behaviour** (clay): use an unconfined or triaxial test. A well-defined peak at small strain (below about 5%) means general shear. A strain at failure of 20% or more, with no clear peak, indicates local or punching shear. - **From the plate load test:** a defined breaking point in the load–settlement curve means general shear. - If the soil is likely to fail in local shear, Terzaghi's design reduces the strength: $c' = \tfrac{2}{3}c$ and $\tan\phi' = \tfrac{2}{3}\tan\phi$, and the modified factors $N_c', N_q', N_\gamma'$ are used. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes general shear failure? A. Vertical shear around footing, no heave B. Failure surface that dies in the soil, steady slope C. Continuous failure surface to ground surface, clear heave, sharp peak, sudden tilting D. Gradual large settlement without peak Answer: C. General shear occurs in dense sand or stiff clay with a sharp peak in the load-settlement curve. (Key point: General shear features) 2. Which soil typically fails in punching shear? A. Dense sand B. Very loose sand or soft clay C. Stiff clay D. Rock Answer: B. Punching shear occurs in very loose or soft soil with no heave and no peak. (Key point: Punching shear) 3. By relative density, which mode is expected for D_r > 70%? A. Local shear B. Punching shear C. General shear D. None Answer: C. D_r > 70% gives general shear, < 20% punching, in between local shear. (Key point: Relative density criteria) 4. If local shear is likely, how does Terzaghi adjust the strength? A. c' = 2c/3 and tan phi' = 2 tan phi/3 with modified N factors B. Strength doubled C. phi' = phi D. c' = c/10 Answer: A. Reduced strength parameters (two-thirds) and modified N_c', N_q', N_gamma' are used. 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