--- title: "Limitations of direct shear test" subject: "Soil Mechanics" chapter: 10 key: "semester_4/soil_mechanics:direct-shear-limitations" source: https://www.bctengineer.com/q/semester_4/soil_mechanics/direct-shear-limitations site: BCT Engineering --- # Limitations of direct shear test Soil Mechanics · Chapter 10: Shear Strength of Soil - Asked in: 2075 Baisakh, 2 marks - Video: [LIMITATION OF DIRECT SHEAR TEST II SHEAR STRENGTH OF SOIL](https://www.youtube.com/watch?v=B60xsu4IsdQ) — Concept Civil - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/direct-shear-limitations - Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10 ## Question What are the limitations of the direct shear test? ## Short answer Drainage cannot be controlled, pore pressure unmeasurable; failure plane forced horizontal; non-uniform stress and strain; contact area reduces; principal planes rotate; small specimen size; side friction; difficult to apply other stress paths. ## Answer Limitations of the direct shear test: 1. **Drainage cannot be controlled.** Pore pressure cannot be measured, so the test cannot give true undrained or effective stress parameters for clays. 2. **Failure plane is fixed** (horizontal). The soil may not fail along its weakest plane. 3. **Non-uniform stress and strain distribution** over the failure plane, with stress concentration at the edges and progressive failure. 4. **Area of contact decreases** as the upper box moves, so the shear stress is not exactly $T/A$ and a correction is needed. 5. **Principal planes rotate** during shear and the major principal stress direction is not known; a full Mohr circle cannot be drawn. 6. **Small specimen size** limits use for coarse-grained soil and gives a small, non-representative sample. 7. **Side friction** between the box and the soil, and the effect of the rigid boundaries, introduce errors. 8. Difficult to apply other stress paths, and the volume change is hard to measure accurately. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can direct shear not give true undrained or effective stress parameters for clays? A. The sample is too large B. The normal stress cannot be varied C. Shear is applied too slowly to measure D. Drainage cannot be controlled and pore pressure cannot be measured Answer: D. Without drainage control or pore pressure measurement, only drained-type behaviour is estimated. (Key point: Drainage) 2. What is the limitation of the fixed failure plane? A. The failure plane is always inclined at 45° B. The plane rotates freely to the weakest orientation C. Failure is forced on the horizontal plane, which may not be the weakest plane D. The plane is curved and hard to measure Answer: C. The soil cannot choose its own weakest plane. (Key point: Fixed plane) 3. What problem arises regarding principal stresses? A. Principal stresses equal the normal stress throughout B. Principal planes rotate during shear and directions are unknown, so a full Mohr circle cannot be drawn C. Sigma_3 is always zero D. The Mohr circle is always a point Answer: B. Because stress state is only known on the shear plane, full circles are not obtained. (Key point: Principal planes) 4. Why is shear stress not exactly T/A? A. Contact area decreases as the upper box moves, so a correction is needed B. Contact area increases with displacement C. Side friction is ignored so area is doubled D. Area is zero at failure Answer: A. Overlap of the two halves reduces as displacement grows. 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