5.2 Class Activity: Mohr’s circle, shear strength of soil: unconfined compression test, direct shear test, triaxial compression test, types of triaxial tests
In this activity, I want you to share your learning and reflections in the comment section below. Don’t worry about being 100% correct — this is all about exploring, practicing, and expressing your own understanding.
👉 How to participate:
1. Read or watch short videos on the questions given.
2. Answer the questions in your own words.
3. Add links to resources you reviewed.
4. Post your answers in the comments below this blog.
Mohr’s Circle
1. In your own words, what do you understand by Mohr’s circle after reading/watching videos? (Tip: Think about how it shows stresses in soil). Also, paste link you reviewed.
2. Draw a rough sketch of Mohr’s circle and label at least two things you learned (example: principal stress, shear stress).
Unconfined Compression Test (UCC)
1. Look at a diagram of UCC test and write down what you think is happening step by step.
2. Why do you think this test is mainly done for clay and not sand?
3. Write one advantage and one limitation of this test in your own words.
Direct Shear Test
1. Paste/Draw a simple sketch of the direct shear test apparatus after reviewing it. Label the main parts. Why do you think engineers like this test even if it has limitations?
Triaxial Compression Test
1. After reading about triaxial test, write down what you found about the setup.
2. List the three types of conventional triaxial tests you learned (UU, CU, CD). Write one line about how each is different.
3. Which test do you think is fastest? Which test do you think is slowest? Explain why you think so.
4. Why do you think the triaxial test gives more accurate results than the direct shear test?
💡 Remember: This is not an exam — it’s a chance to explore, reflect, and share your ideas. Be creative, use examples, and don’t be afraid to make mistakes. Your unique perspective makes the discussion valuable!
✍️ Drop your answers in the comments section below ⬇️
Mohr' Circle: Mohr' Circle is like a picture that shows how normal stress and shear stress change on different planes within soil. Instead of calculating stresses on every possible plane, you can just look at the circle and instantly see: the principal stresses, the maximum shear stress. and it helps engineers visualize the stress state in soil and predict failure.
ReplyDeleteUnconfined compression test (UCC)
1. What's happening
- A cylindrical between clay sample is prepared (no lateral support)
-It is placed between two loading plates in compression machine.
-Load is applied vertically, and the sample shortens in height.
-Because there is no confining pressure, the sides bulge freely.
-The test continues until failure (the soil cracks or bulges excessively)
-Stress -strain curve is drawn to fine unconfined compressive strength(qu)
2.Why mainly for clay and sand:
- Clay is cohesive; it can stand by itself in a cylinder shape
-Sand is non-cohesive; it falls apart without a container.
That's why UCC is practical for clay, not reliable for sand.
3.Advantage and limitation:
- Advantages are very quick and simple test, no need for complicated apparatus.
-Limitation: Only valid for cohesive soils(clay), cannot be used for sand and gravel.
Triaxial Compression Test
1.Setup: A cylinder soil specimen is placed inside a rubber membrane. The specimen is surrounded by a cell filled with water/oil (confining pressure is applied equally around it). An axial load is then applied from the top (simulating in-situ soil conditions). Finally, Drains and pressure sensors can be connected to measure pore water pressure and volume change.
2. Three types of conventional triaxial tests:
2.1 UU (Unconsolidated Undrained): No drainage allowed during both consolidation and shearing. Quick test.
2.2 CU (Consolidated Undrained): Soil is first allowed to consolidate (drain under confining pressure), then sheared without drainage. Pore pressure measured.
2.3 CD (consolidated Drained): Soil is consolidated and then sheared very slowly with full drainage (no pore pressure buildup)
3. Fastest VS Slowest
UU test is fastest test because no drainage or waiting for consolidation and CD is slowest test because drainage must occur during both consolidation and shearing which takes a long time.
4. The reasons why triaxial test is more accurate that direct shear test is that direct shear test has the failure plane is forced (predetermined), not natural. Stress distribution is not uniform. But Triaxial test, the soil finds its own failure plane, stresses are applied uniformly, and pore pressure can be measured. so this make triaxial result more realistic and closer to field conditions