Question

A field vane shear testing instrument ( as shown in figure) was inserted completely into a deposit of soft, saturated silty clay with the vane rod vertical such that the top of the blades were 500 mm below the ground surface. Upon application of a rapidly increasing torque about the vane rod, the soil was found to fail when the torque reached 4.6 Nm. Assuming mobilization of undrained shear strength on all failure surfaces to be uniform and the resistance mobilized on the surface of the vane rod to be negligible, what would be the peak undrained shear strength (rounded off to the nearest integer value of kPa) of the soil?

• A. 5 kPa
• B. 10 kPa
• C. 15 kPa
• D. 20 kPa

Answer 1

The correct option is B 10 kPa

Top of the blades are 500 mm below the ground surface. So shear failure will occur at both top and bottom

Shear strength,

${\tau }_f=\frac{T}{\pi d^2[\frac{h}{2}+\frac{d}{6}]}$

Torque, T = 4.6 Nm

h = 100 mm = 0.1 m

d = 50 mm

= 0.05 m

${\tau }_f=\frac{4.6}{\pi (0.05{)}^2[\frac{0.1}{2}+\frac{0.05}{6}]}$

$=10040N/m^2$

= 10 kPa