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Direct shear test

Direct shear test

(OP)
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 ?

RE: Direct shear test

I think your sample is in the highly consolidated state.

RE: Direct shear test

What's the soil type and strain rate?

f-d

ípapß gordo ainÆt no madre flaca!

RE: Direct shear test

(OP)
@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

RE: Direct shear test

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!

RE: Direct shear test

(OP)
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

RE: Direct shear test

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

RE: Direct shear test

(OP)
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

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