Short comings of direct shear test are 1.Volume changes cannot be measured 2.Can’t be used for gravels. 3.Difficult to control drainage 4.Direction of failure plane is fixed Out of these statements,
- A1, 2 and 3 are correct
- B2, 3 and 4 are correct
- C3, 4 and 1 are correctCorrect
- D1 and 2 are correct
Solution
Concept
Limitations and advantages of the direct shear test apparatus in geotechnical engineering.
Step-by-step solution
1. Analyze statement 1: Volume changes during shear cannot be accurately measured in a standard direct shear box test. (Correct) 2. Analyze statement 2: Direct shear tests can actually be conducted on coarse-grained soils like gravels using large-scale shear boxes. (Incorrect) 3. Analyze statement 3: Drainage control is difficult because pore water pressure cannot be measured or controlled effectively. (Correct) 4. Analyze statement 4: The failure plane is artificially forced to occur along a predetermined horizontal plane between the two halves of the box. (Correct) 5. Therefore, statements 1, 3, and 4 are correct, which corresponds to option (C).
Explanation
In a direct shear test, volume changes cannot be easily measured during shear because drainage is not fully controllable for saturated soils in a standard quick test, and the failure plane is forcibly predetermined along the horizontal shear plane. However, it can be used for gravels (with larger box sizes), making statements 1, 3, and 4 correct.
Why the other options are wrong
A: Statement 2 is incorrect since direct shear tests can be performed on gravels using large boxes. B: Statement 1 is correct, volume changes are not measured. D: Statements 3 and 4 are also correct.
Shortcut
Remember the forced failure plane: shear failure *must* happen along the horizontal split of the box, which is a major drawback.
Common mistake
Assuming statement 2 is correct; large shear boxes exist specifically to test gravels and coarse aggregates.
Correct answer: C) 3, 4 and 1 are correct