--- title: "Draw plasticity chart with symbols" subject: "Soil Mechanics" chapter: 3 key: "semester_4/soil_mechanics:draw-plasticity-chart-symbols" source: https://www.bctengineer.com/q/semester_4/soil_mechanics/draw-plasticity-chart-symbols site: BCT Engineering --- # Draw plasticity chart with symbols Soil Mechanics · Chapter 3: Soil Identifications and Classification - Asked in: 2077 Chaitra, 2 marks; 2074 Bhadra, 3 marks (asked 2 times) - Video: [Geotechnical Eng'g 1 (Soil Mechanics) - Plasticity Chart](https://www.youtube.com/watch?v=2kPUw-412Rk) — Marvin Bartido Channel - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/draw-plasticity-chart-symbols - Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3 ## Question Draw neatly the plasticity chart (IS / USCS) and label the group symbols of the various soil regions in the chart. ## Short answer Plasticity chart plots Ip against wL with A-line Ip=0.73(wL-20), U-line, and vertical lines at 35% and 50%. Above A-line are clays CL, CI, CH; below are silts/organics ML, MI, MH; hatched zone gives CL-ML. ## Answer The plasticity chart (Casagrande) plots **plasticity index $I_p$ (vertical)** against **liquid limit $w_L$ (horizontal)**. It is used for the fine-grained fraction (passing 75 micron). #### Lines - **A-line:** $I_p = 0.73(w_L - 20)$, separating clays (above) from silts and organic soils (below). - **U-line:** $I_p = 0.9(w_L - 8)$, the upper limit for natural soils. - A vertical line at $w_L = 50\%$ (USCS) separates low and high plasticity. IS adds a line at $w_L = 35\%$, giving low (L), intermediate (I) and high (H) plasticity. ```text Ip (%) 50 | | | : U-line / 45 | | | : / A-line 40 | | |: CH / 35 | | :| / 30 | | : | / 25 | : | / 20 | : |CI /| 15 | : | / | MH or OH 10 | : CL /| | 7 | :-----/ | MI | <- CL-ML band (Ip 4 to 7) 4 | :----/ | OI | 0 | : / ML | | (ML or OL below A-line) +------------------------------------ wL (%) 0 20 35 50 70 ``` (`/` = A-line, `:` = U-line, `|` = vertical lines at $w_L$ = 35% and 50%; the vertical scale is not uniform near the bottom.) #### Group symbols | Region | Symbol | Name | |---|---|---| | Above A-line, $w_L < 35$ (IS) / 50 (USCS) | CL | Inorganic clay of low plasticity | | Above A-line, $35 < w_L < 50$ (IS) | CI | Inorganic clay of intermediate plasticity | | Above A-line, $w_L > 50$ | CH | Inorganic clay of high plasticity | | Below A-line, $w_L < 35$ / 50 | ML (OL) | Inorganic silt (organic silt) of low plasticity | | Below A-line, $35 < w_L < 50$ | MI (OI) | Inorganic silt (organic soil) of intermediate plasticity | | Below A-line, $w_L > 50$ | MH (OH) | Inorganic silt of high plasticity (organic clay) | | Hatched zone, $I_p$ 4 - 7 above A-line | CL-ML | Silty clay | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The plasticity chart plots A. Ip on vertical axis against wL on horizontal axis B. wL vertical against Ip horizontal C. w vertical against dry density D. Ip against plastic limit Answer: A. Casagrande's chart: Ip (y), wL (x). (Key point: Chart axes) 2. The A-line, Ip = 0.73(wL-20), separates A. gravels from sands B. clays above from silts and organic soils below C. silts above from clays below D. low from high plasticity Answer: B. Above = C; below = M or O. (Key point: A-line) 3. In IS system, medium plasticity (I) corresponds to wL A. below 35% B. above 50% C. 35 to 50% D. 20 to 35% Answer: C. IS uses wL = 35% and 50% as boundaries. (Key point: IS plasticity divisions) 4. The hatched zone above A-line with Ip 4-7 is A. CH B. MH C. OL D. CL-ML Answer: D. It is the borderline silty clay. (Key point: Hatched zone) ## Related in Soil Mechanics - [Systems by size and plasticity; ISSCS chart](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aissc-plasticity-chart-systems) — Soil Mechanics, chapter 3 - [Usefulness of plasticity chart](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aplasticity-chart-usefulness) — Soil Mechanics, chapter 3 - [USCS classification, 75 micron 8%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-75micron-8-percent) — Soil Mechanics, chapter 3 - [USCS classification from PSD curves](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-psd-curves-a-b) — Soil Mechanics, chapter 3 - [Systems using size and plasticity](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclassification-size-plasticity-systems) — Soil Mechanics, chapter 3 - [USCS versus AASHTO](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-vs-aashto) — Soil Mechanics, chapter 3 ## Related in other subjects - [Pile capacity in sand, group effect, load test](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-sand-group-load-test) — Foundation Engineering, chapter 8 - [Negative skin friction on 16-pile group](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aneg-skin-friction-group-16-piles) — Foundation Engineering, chapter 8 - [Footing 2 m × 3 m, chart factors, eccentricity](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Afooting-2x3-eccentric-skempton) — Foundation Engineering, chapter 6