--- title: "Field identification of fine-grained soils" subject: "Soil Mechanics" chapter: 3 key: "semester_4/soil_mechanics:field-identification-clay-silt" source: https://www.bctengineer.com/q/semester_4/soil_mechanics/field-identification-clay-silt site: BCT Engineering --- # Field identification of fine-grained soils Soil Mechanics · Chapter 3: Soil Identifications and Classification - Asked in: 2073 Magh, 5 marks; 2074 Bhadra, 1 marks; 2077 Chaitra, 4 marks; 2078 Poush, 2 marks (asked 4 times) - Video: [Field Identification of Soil](https://www.youtube.com/watch?v=JvU-OXOyiTU) — Elementary Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/field-identification-clay-silt - Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3 ## Question Describe the field identification tests used to identify fine-grained soils and to distinguish between clay and silt. ## Short answer Dilatancy test: silt shows rapid water appearance on shaking and disappearance on squeezing, clay shows none. Dry strength: clay is high, silt crumbles. Toughness/thread: clay forms a strong, tough thread, silt forms a weak, crumbly thread. Clay feels smooth and sticky, silt feels gritty. ## Answer Field identification tests are quick hand tests on a small moist sample, used to tell fine-grained soils apart without laboratory equipment. They use the **dry strength, dilatancy, toughness, plasticity and appearance of the soil**, and are carried out on the fraction finer than 0.425 mm (No. 40 sieve). #### 1. Dilatancy (shaking) test Make a pat of moist soil (soft putty consistency) and shake it in the palm, then squeeze it. - **Silt:** water quickly rises to the surface and gives it a shiny look; it disappears on squeezing (rapid reaction). - **Clay:** no change (no reaction). #### 2. Dry strength (crushing) test Dry a moulded pat and crush it between the fingers. - **Clay:** high dry strength; it cannot be crushed easily. - **Silt:** low dry strength; it crumbles easily to powder. #### 3. Toughness (thread) test Roll the soil into a thread about 3 mm in diameter, then fold and re-roll it repeatedly until it crumbles at the plastic limit. - **Clay:** strong, stiff thread; tough lump after crumbling. - **Silt:** weak thread and soft lump; it loses coherence quickly. #### 4. Plasticity / stickiness Clay is plastic and sticky in the hand and cannot be washed off easily; silt feels gritty or floury and washes off easily. #### 5. Other tests - **Visual and touch:** clay feels smooth, while silt feels slightly gritty between the teeth. - **Shine test:** a cut or scratched surface of clay looks shiny; silt stays dull. - **Sedimentation test:** shaken in a jar of water, sand settles in 30 - 60 seconds and silt in about 15 - 60 minutes, while clay stays in suspension for hours or even days. | Test | Silt | Clay | |---|---|---| | Dilatancy | Rapid | None | | Dry strength | Low | High to very high | | Toughness | Low | Medium to high | | Feel | Gritty/floury | Smooth, sticky | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the shaking (dilatancy) test, how do silt and clay behave? A. Silt shows quick water rise and shine; clay shows no reaction B. Clay shows quick water rise; silt shows none C. Both show slow reaction D. Silt shows no change; clay turns dull Answer: A. Silt is permeable and dilatant; clay has no reaction. (Key point: Dilatancy test) 2. A dried pat that is hard to crush between fingers is A. Silt, having high dry strength B. Clay, having high dry strength C. Silt, having low dry strength D. Sand, having medium strength Answer: B. Clay has high dry strength; silt crumbles easily. (Key point: Dry strength) 3. In the thread test, a strong stiff 3 mm thread and tough lump near the plastic limit indicates A. Silt B. Sand C. Clay D. Organic silt only Answer: C. Silt threads are weak and the lump is soft; clay is tough. (Key point: Toughness test) 4. In the sedimentation test, which settles quickly? A. Clay settles in 30-60 s; silt stays suspended B. Both settle in a few seconds C. Both stay suspended for hours D. Silt settles in 30-60 s; clay stays suspended for hours Answer: D. Silt particles are larger so they settle quickly. (Key point: Sedimentation test) ## Related in Soil Mechanics - [Field tests for sand and clay](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afield-tests-sand-clay) — Soil Mechanics, chapter 3 - [CU tests on overconsolidated clay](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acu-overconsolidated-effective-parameters) — Soil Mechanics, chapter 10 - [Engineering vs index properties; index tests](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aindex-engineering-properties-index-tests) — Soil Mechanics, chapter 2 - [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× - [Lab and field permeability tests](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Apermeability-test-names) — Soil Mechanics, chapter 6 - [Clay 10 m, 2.54 cm sample, half settlement](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-10m-half-settlement-time-ratio) — Soil Mechanics, chapter 9 ## Related in other subjects - [Field identification criteria of fault](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afield-identification-criteria-of-fault) — Engineering Geology I, chapter 5, asked 7× - [Field identification criteria of fault and fold](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afield-identification-criteria-of-fault-and) — Engineering Geology I, chapter 5, asked 2× - [Identify the three rock types in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-the-three-rock-types) — Engineering Geology I, chapter 4, asked 5× - [Identify metamorphic rock in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-metamorphic-rock-in-the) — Engineering Geology I, chapter 4, asked 2×