Shear rate of CD Direct shear 2

[GTNOP]

I am going to run CD Direct shear test but I do not have data of consolidation for calculate Shear rate. Can I assume Cv from Plastic limit? Who have any suggestion?

thankyou

 

[Ashayeri]

it really depends on your soil type if you have fine grained soil like CL or ML type the rate of test should not be more than 0.5 mm/min, for grater normal stresses (like 300 kpa normal stress) use smaller shear rate, the value of 0.3 to 0.2 mm/min is good. before starting the consolidation phase be sure of complete saturation of your samples this can be obtained by placing the sample and the container ring in water for 2 or 3 days, don't forget to prevent the sample from swelling and expantion by suitable surcharge.

 

[dirtman]

Part of performing a CD direct shear test is to consolidate the specimen prior to shearing it. You should monitor the consolidation with a dial gage and construct a consolidation versus log of time curve and determine t50 for the soil. The time to failure for the direct shear test then should be 50 times this t50 time. If for instance the t50 time were 300 minutes, the time to failure should be 1500 minutes, or slightly more than one day. You would have to estimate what the strain would be at failure ( 10 percent perhaps ) and set the speed of the machine to achieve that deformation in the allotted time. See the ASTM for more.

 

[azurite]

has anyone ever challenged this topic for overconsolidated clays or bedrock samples? it seems impractical to run a direct shear test for over an hour on any sample, yet that is the time necessary when determining rate by consolidation data. Is there any publication proving time does or does not make that big of a difference in peak or residual strength of cohesive soils? Is it standard to use .5mm/min for all soil types or just sand. There seems to be a lot of inconsistency or unfinished literature.

Thanks

 

[Focht3]

Hmmm,

[blue]azurite[/blue] opined,
it seems impractical to run a direct shear test for over an hour on any sample...

We've been doing these kinds of tests for about 50 to 60 years now. Tedious, boring, time consuming - yes. But impractical? No. Given the availability of relatively inexpensive data acquisition system (DAS) hardware and software - and dirt cheap PCs, dedicated DAS is more practical than ever. If you don't want to sit up all night taking readings, install a DAS. You can do it for less than $1,500. The payback period is pretty quick at that price -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[azurite]

impractical was the wrong word, I apologize. I am really just trying to figure out what kind of difference the extra time makes, in particular, for overconsolidated cohesive samples. Is there literature stating a maximum time or should 50t50 or 12t90 always be used to determine strain rate?

 

[Focht3]

In short, yes. But I don't remember the citations. Hopefully someone else will -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DirtMD]

...AZURITE, Have you found any literature that has addressed your comment regarding overconsolidated clays or bedrock samples. I really think there are a lot of consultants running the test much faster than ASTM, and I'm not aware of many failures being traced back to the rate at which a direct shear test was run. This issue has caught my curosity many times, especially when faced with a bunch of long tests. It will be by shear luck if you actually see this post.

 

[Focht3]

Experienced eyes will notice when a test is run significantly faster than the recommended value. Yes, a few do cheat. But that number is smaller than you might think. Most simply choose to not run the test at all...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"