--- title: "Draw UCS and direct shear results" subject: "Soil Mechanics" chapter: 10 key: "semester_4/soil_mechanics:ucs-direct-shear-results-plot" source: https://www.bctengineer.com/q/semester_4/soil_mechanics/ucs-direct-shear-results-plot site: BCT Engineering --- # Draw UCS and direct shear results Soil Mechanics · Chapter 10: Shear Strength of Soil - Asked in: 2078 Baisakh, 2 marks - Video: [Unconfined compression Shear test](https://www.youtube.com/watch?v=W1VsZz2K-cU) — WIT Solapur - Professional Learning Community - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/ucs-direct-shear-results-plot - Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10 ## Question Draw final test results of the Unconfined Compression Test and the Direct Shear Test for the same soil so that the strength parameters of the soil could be obtained. ## Short answer For UCS, plot axial stress vs strain to get qu, then draw Mohr circle through origin with diameter qu; the horizontal tangent gives cu = qu/2. For direct shear, plot shear stress vs displacement for each normal stress, then plot peak shear strength vs normal stress to obtain c and φ from the best-fit line. ## Answer **Unconfined compression test (UCS)** Plot axial stress $\sigma_1$ against axial strain; the peak is $q_u$. The Mohr circle drawn on the $\sigma$-$\tau$ plane passes through the origin ($\sigma_3 = 0$) with $\sigma_1 = q_u$, and the envelope is a horizontal line tangent to the circle for $\phi_u = 0$: ```text UCS stress-strain Mohr circle s1 tau | ___ qu | ..--.. | / \ cu |--*' '*-- | / | / C \ +--------- strain -+-+--------+--- sigma 0 (s3=0) qu ``` **Direct shear test** Plot shear stress against horizontal displacement for each normal stress to get the peak $\tau_f$. Then plot $\tau_f$ against normal stress $\sigma$ for three or more tests and draw the best-fit straight line: ```text tau_f / | / <- slope = phi | / c |--------/ | . +--------------- sigma s1 s2 s3 ``` Intercept $= c$; slope angle $= \phi$. For the same clay, the UCS gives $c_u = q_u/2$ (total stress, quick), while the direct shear (slow) gives $c$ and $\phi$ under drained conditions. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. From the UCS stress-strain plot, what is q_u? A. The peak axial stress B. The axial stress at zero strain C. The strain at the peak D. The final stress at large strain regardless of peak Answer: A. q_u is the maximum (peak) axial stress reached, which is sigma_1 at failure. (Key point: UCS result) 2. How are c and phi obtained from direct shear tests at three or more normal stresses? A. Plot sigma against strain; slope gives c B. Plot tau against displacement; the peak gives phi C. Draw one Mohr circle and read c at the origin D. Plot peak tau_f against sigma, fit a straight line; intercept is c and slope angle is phi Answer: D. Each test gives one (sigma, tau_f) point; the best-fit line gives the Mohr-Coulomb envelope. (Key point: Direct shear envelope) 3. For the same clay, how do the two tests differ in what they give? A. UCS gives phi and direct shear gives only c_u B. Both give identical undrained parameters C. UCS gives undrained c_u = q_u/2 quickly; direct shear (slow) gives drained c and phi D. UCS gives drained c and phi; direct shear gives c_u Answer: C. UCS is a quick total stress test; direct shear is run slowly so that drained parameters result. (Key point: UCS vs direct shear) ## Related in Soil Mechanics - [UCS test soil type and parameters](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aunconfined-compression-soil-type-mohr) — Soil Mechanics, chapter 10 - [Limitations of direct shear test](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adirect-shear-limitations) — Soil Mechanics, chapter 10 - [UCS as special case of UU test](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aucs-special-uu-triaxial) — Soil Mechanics, chapter 10 - [Shear tests; direct shear calculation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ashear-tests-direct-shear-strength-calc) — Soil Mechanics, chapter 10 - [Triaxial tests by drainage; UCS vs cu](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-tests-by-drainage-ucs) — Soil Mechanics, chapter 10 - [Triaxial advantage over direct shear](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-advantages-over-direct-shear) — Soil Mechanics, chapter 10 ## Related in other subjects - [Characteristic strength from test results](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acharacteristic-strength-30-28) — Concrete Technology and Masonry Structure, chapter 5 - [Stress on oblique section with direct and shear stress](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Aoblique-section-stress-derive) — Strength of Materials, chapter 4 - [Diagonal shear test and flat-jack test calculations](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Adiagonal-shear-flat-jack-numerical) — Design of Timber and Masonry Structures, chapter 8 - [20 kVA 250/2500 V test parameters](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atransformer-20kva-250-2500-primary) — Electrical Machine, chapter 2, asked 2×