Direct shear test

[killswitchengage]

Hello
Our technician conducted Direct Shear box test on samples i gave him , the test is conducted with three normal Stresses (1 ,2 ,3 bars )
Thing is i find that sometimes the shear resistance of the sample for say 1 bars is greater than that of 3 bars or you may find the 3 bars shear strength in the middle between the other two !
how can we explain that ?

 

[L0k]

I think your sample is in the highly consolidated state.

 

[fattdad]

What's the soil type and strain rate?

f-d

ípapß gordo ainÆt no madre flaca!

 

[killswitchengage]

@lok well if it was highly consolidated , the stress strain curve must have had a peak , which does not !

problem is the odometer test concluded that it is over consolidated.

@Fattdad its a saturated clay with low plasticity , test is conducted until a stress of 5 mm is reached
the strain rate is 1 mm/min

 

[fattdad]

five minutes to failure is likely too quick a strain rate for a clay. There's a chance you are just seeing undrained shear strength and trying to package it as drained behavior. I'd check out the ASTM for the direct shear test. I think the default is 2 hours to max strain, which is 0.5 inches (0.05mm/min).

f-d

ípapß gordo ainÆt no madre flaca!

 

[killswitchengage]

actually i want the undrained strength but my question was about why would the 2 bars normal stress be higher than 3 bars doesnt make any sense !
i am guessing that the sample was modified during preparation

 

[SuperSandman]

Were all your samples cut from the same undisturbed block sample? I have had experience in this before, the samples were taken from a shelby tube with the same result as you explained. On further inspection, it was discovered that the three samples taken from one shelby tube varied in grading modulus and PI

 

[killswitchengage]

Of course from the same sample
Grading modulus ? as in soil sieve analysis ?
tbh i never verify my samples so does all engineer here but i will from now one , the guys at the lab can ruin samples for sure