Shear Strength MCQ with answer and solution

Concept

Pore Water Pressure in Undrained Shear

Formula

Δu = B × [Δσ3 + A × (Δσ1 - Δσ3)] (Skempton's Pore Pressure Equation)

Step-by-step solution

1. Identify the test conditions: In a Consolidated Undrained (CU) test, drainage is prevented during shearing. 2. Analyze soil behavior: Normally Consolidated (NC) clay tends to contract/volume-decrease when sheared. 3. Apply pore pressure concept: Prevention of volume decrease in undrained conditions converts the compression tendency into positive excess pore water pressure. 4. Conclusion: Excess pore water pressure increases in NC clay.

Explanation

In a Consolidated Undrained (CU) triaxial test, normally consolidated (NC) clay tends to compress or contract when sheared. Because the drainage valves are closed during the shearing phase, the water trapped inside cannot escape, leading to an increase in excess pore water pressure.

Shortcut

Remember: Normally Consolidated (NC) soil Contracts when sheared, causing Positive (+Δu) excess pore water pressure. Over-Consolidated (OC) soil Dilates, causing Negative (-Δu) pore pressure. (NC = Contract = +Δu)

Common mistake

Students often confuse normally consolidated (NC) clays with over-consolidated (OC) clays or dense sands, mistakenly selecting OC clay or dense sand because they confuse dilative (volume expansion / negative pore pressure) behavior with contractive (volume reduction / positive pore pressure) behavior.