Question

A possible slope failure is shown in the figure. Three soil samples are taken from different locations (I, II and III) of the potential failure plane. Which is the most appropriate shear strength test for each sample to identify the failure mechanism? 

P: Triaxial compression test 
Q: Triaxial extension test 
R: Direct shear (shear box) test 
S: Vane shear test 

 

• A. I--Q, II--R, III--P
• B. I--R, II--P, III--Q
• C. I--S, II--Q, III--R
• D. I--P, II--R, III--Q

Hint:

Near free faces/crests $\Rightarrow$ extension paths (TXE).
Along suspected slip planes $\Rightarrow$ direct shear.
Deeper confined zones $\Rightarrow$ compression paths (TXC).

Answer 1

The Correct Option is A

Step 1: Identify stress states at the three locations.
Location I (near slope face/toe): The soil mass close to the free face tends to unload laterally and undergoes a reduction in minor principal stress, producing a tensile/extension stress path. Hence, behaviour is best captured by a triaxial extension test. $\Rightarrow$ I $\rightarrow$ Q.
Location II (along potential failure plane): This is the sliding interface. The shear strength mobilized along such a plane (including large-displacement/residual behaviour if needed) is most directly measured in a direct shear (shear box) test. $\Rightarrow$ II $\rightarrow$ R.
Location III (deeper beneath crest): Soil is under higher confining pressure and predominantly compressive stress state. Strength and dilation are appropriately assessed using a triaxial compression test. $\Rightarrow$ III $\rightarrow$ P.

Step 2: Match with options.
Mapping I--Q, II--R, III--P corresponds to option (A).
\[ \boxed{\text{I--Q,\; II--R,\; III--P}} \]