Correction Factors when using split spoon samples for direct shear testing? 5

[BSC55]

Hello!

The company I currently work for conducts direct shear tests on cohesive split spoons samples. I am aware this is a no-no in the industry because it can cause the sample to consolidate when being pushed out of the tube, which can lead to skewed results, and every document I've found states these samples are not to be used for strength/shear testing.

I was curious if a correction factor was applied to the strength properties to account for the disturbance?

I have researched numerical and empirical equations to relate the N60-value to estimated cohesion and friction angles values and found the study completed by Ranjan Kumar, Kapilesh Bhargava, and Deepankar Choudhury in 2016 called "Estimation of Engineering Properties of Soils from Field SPT Using Random Number Generation" is currently most promising.

Any help is appreciated as I'm at a loss besides bugging them to switch to Shelby tube sampling!

-BSC55

 

[oldestguy]

You are correct with this question. Sounds like a inexperienced engineer is asking for this sort of test sample. I can't visualize how any accuracy or dependability is there when using a highly disturbed of such a small diameter. Both rather stupid.

 

[GeoPaveTraffic]

Any direct shear shear test ran on an SPT sample should be considered worthless. No engineer worth their salt should agree to run such a test. Just DON'T DO IT.

Mike Lambert

 

[EireChch]

Yes, complete nonsense using a disturbed sample of cohesive material for strength testing. Correlate Cu based on SPT N60. Cross check it with some UU or CUs

 

[fattdad]

a big no-no!

f-d

ípapß gordo ainÆt no madre flaca!

 

[MTNClimber]

Ask for an explanation from your boss as why they do this. If it doesn’t make sense, which I can’t imagine it will, consider finding a new company to work for. If they are doing things like this I’m going to guess they do other things that are incorrect. You never want to be a young engineer learning from a bunch kooks.

That said, there are dry cohesive soils that can’t be sampled with a Shelby. That’s when you could switch to a California sampler but you still get a disturbed sample, just not as bad as SPT sample.

 

[Ron]

If that is a premise of the company you work for, you need to find a different company! What other idiotic procedures do they promote?

 

[LRJ]

The direct shear test is used to obtain friction angle data - either internal friction angle or interface friction angle. Given these are cohesive samples, I'd expect you'd be more interested in the interface friction angles. Contrary to the widespread outrage here, there is nothing inherently 'wrong' about testing these disturbed samples to infer an interface friction angle provided that the sample is reconstituted properly. The Imperial College Pile (ICP) design method for driven piles makes recommendations for similar interface shear testing on disturbed cohesive samples, albeit they recommend ring shear tests rather than direct shear tests to ensure you reach the ultimate state - which is what needs to be measured.

Essentially it all depends on what the ultimate use of these tests is and what you are aiming to replicate.