--- title: "Stress-strain curves, consistency states" subject: "Soil Mechanics" chapter: 2 key: "semester_4/soil_mechanics:stress-strain-consistency-states" source: https://www.bctengineer.com/q/semester_4/soil_mechanics/stress-strain-consistency-states site: BCT Engineering --- # Stress-strain curves, consistency states Soil Mechanics · Chapter 2: Solids- Water –Air Relations and Index properties of soils - Asked in: 2079 Asoj, 2 marks; 2075 Bhadra, 3 marks; 2075 Baisakh, 2 marks; 2076 Baisakh, 2 marks; 2073 Magh, 3 marks (asked 5 times) - Video: [Graph Of Consistancy of Soil II Geotech Engineering](https://www.youtube.com/watch?v=LbG6tW2b0Iw) — 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/stress-strain-consistency-states - Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-2 ## Question Draw the stress-strain curves (graph) for soil at different consistency states. ## Short answer The stress-strain curve for solid soil shows a high peak strength with brittle failure; semi-solid is stiff and brittle; plastic soil yields and strain hardens with lower strength; liquid soil flows at almost zero strength, the stress-strain curve lying on the strain axis. As water content rises, peak strength falls and failure strain increases. ## Answer Soil changes its state with water content: **solid, semi-solid, plastic and liquid**, separated by the shrinkage limit ($w_s$), plastic limit ($w_p$) and liquid limit ($w_l$). The stress-strain behaviour differs in each state. #### Stress-strain curves ```text Stress | ___ Solid (brittle, peak | /\ then sudden drop) | / \______ Semi-solid | / ______ Plastic (yields, | / __/ strain hardens |/ _/ ......... Liquid (no |/.-' stress, flows) +-------------------- Strain ``` | State | Water content | Behaviour | |---|---|---| | Solid | $w < w_s$ | Very high strength, brittle, small strain at failure, no volume change on drying | | Semi-solid | $w_s < w < w_p$ | Stiff, brittle, crumbles when moulded, volume decreases when dried | | Plastic | $w_p < w < w_l$ | Can be moulded without cracking; strength falls and strain at failure rises; ductile (plastic flow) | | Liquid | $w > w_l$ | Flows like a viscous fluid; almost zero shear strength; stress is nearly independent of strain | As water content rises, the peak strength drops and the strain at failure increases. The brittle curve has a sharp peak with a post-peak drop, the plastic curve rises gradually and levels off, and the liquid curve lies on the strain axis. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which sequence of states is correct as water content increases? A. Liquid, plastic, semi-solid, solid B. Semi-solid, solid, plastic, liquid C. Plastic, solid, liquid, semi-solid D. Solid, semi-solid, plastic, liquid Answer: D. Limits ws, wp, wl separate solid, semi-solid, plastic and liquid states in increasing water content. (Key point: States and limits) 2. The solid-state stress-strain curve shows: A. Almost zero strength B. High strength, brittle with sharp peak and drop, small failure strain C. Gradual levelling off D. Flow like a viscous fluid Answer: B. Solid soil is very strong but brittle, failing suddenly at small strain. (Key point: Solid state behaviour) 3. In the plastic state (wp < w < wl): A. The soil is brittle and crumbles B. Volume decreases on drying with high strength C. Stress is independent of strain D. It can be moulded without cracking; curve rises gradually and levels off Answer: D. Plastic soil is ductile; strength falls and failure strain rises. (Key point: Plastic state) 4. As water content increases, peak strength and failure strain: A. Both increase B. Strength drops and failure strain increases C. Strength rises and strain drops D. Both stay the same Answer: B. More water weakens soil and makes it more ductile. 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