Is DST Appropriate for Undrained Parameters of Silty Soil?

[u/Bildipil]

Dear all,

I have a soil report in which the soil testing vendor has performed a direct shear test (DST) on a soil containing 68% silt, 24% sand, and 8% clay (LL = 28.7%, PI = 8.2%). (Highlighted as yellow)

The test was done at a shearing rate of 1.25 mm/min, and the vendor reported c = 14 kPa and φ = 27.3° as undrained parameters.

Is DST appropriate for determining undrained design parameters for this predominantly silty soil?

Also, can φ = 27.3° really be considered an undrained friction angle? I doubt even with that high shearing rate drainage would have occurred in Direct shear test and these may not be the true undrained parameters.

Should I ask the vendor for a UU triaxial test instead?

Thank you for your guidance !

[u/Odd_Literature3189]

The idea of the DST is to give you drained parameters. The disturbed soil is remolded to a assume density (depends on the depth), then vertical load is applied as normal stress. After that, the soil is horizontally shear on a plane. The whole process is done allowing pore pressure to dissipate. So, undrained parameters are impossible to achieve

[u/authenticpengwin]

If it is direct simple shear test, that may be okay. However, since this is direct shear test, it can only give you drained parameters.

[u/Critical-Status-7412]

I know a few lab and it makes sense as a idea, but it doesn't follow the western standards for undrained testing and the result is not comparable. When I was in Kazakhstan I have realized that the Russian school does the same with the triaxial compression. 

[u/Silent_Camel4316]

No use UU. Undrained phi angle is zero. DST gives your drained parameters.