Shear failure in Dense Sand

Dear Mohamad,

The calculation didn't stop at 1) and 2) because there was no failure detected yet.
Failure is considered if the maximum amount of unloading steps is exceeded while the Current Stiffness Parameter (CSP) < 0.01. Apparently that condition was not met there.
With respect to your second question: at none of the 3 points. The load-displacement curve gives no indication that failure is really occurring  - the behaviour seems to be almost linear elastic.

PLAXIS will try to increase the load and solve the stress increments due the load increase. After doing so, the new stresses must be in equilibrium with the load applied so far. In case this equilibrium cannot be reached, PLAXIS may decide to unload the soil (hence decrease the load a bit) to find equilibrium. If this is necessary for Nmax consecutive steps the calculation stops. If CSP < 0.01 PLAXIS will give the message "Soil body seems to collapse", otherwise the message will be "Load advancement procedure fails".
CSP is a measure for how much of the generated strain increment in a load step is elastic. If CSP = 1 the strain increment is fully elastic, if CSP = 0 the strain increment is fully plastic. If strains resulting from a load step are (almost) fully plastic it's likely to assume failure is occurring.

With kind regards,

Dennis Waterman

Dear Mohamad,

The calculation didn't stop at 1) and 2) because there was no failure detected yet.
Failure is considered if the maximum amount of unloading steps is exceeded while the Current Stiffness Parameter (CSP) < 0.01. Apparently that condition was not met there.
With respect to your second question: at none of the 3 points. The load-displacement curve gives no indication that failure is really occurring  - the behaviour seems to be almost linear elastic.

PLAXIS will try to increase the load and solve the stress increments due the load increase. After doing so, the new stresses must be in equilibrium with the load applied so far. In case this equilibrium cannot be reached, PLAXIS may decide to unload the soil (hence decrease the load a bit) to find equilibrium. If this is necessary for Nmax consecutive steps the calculation stops. If CSP < 0.01 PLAXIS will give the message "Soil body seems to collapse", otherwise the message will be "Load advancement procedure fails".
CSP is a measure for how much of the generated strain increment in a load step is elastic. If CSP = 1 the strain increment is fully elastic, if CSP = 0 the strain increment is fully plastic. If strains resulting from a load step are (almost) fully plastic it's likely to assume failure is occurring.

With kind regards,

Dennis Waterman

Dear Mr. Dennis, 

I’m confused now regarding the second point. Are you saying that I can’t rely on a Load-displacement curve because the criteria of failure (MC) for the soil is different from the one of PLAXIS’s (combination of Nmax and CSP), hence I can’t take point 3 as the ultimate load?

Thanks a lot!

Mohamad

Dear Mohamad,

I didn't say anything like that. There is a big difference between soil failing at one single stress point according to the M-C criterion of a whole geotechnical structure failing because there are too many single stress points where the soil fails. The determination of whether the whole geotechnical structure fails is based on Nmax and CSP. 

If the whole geotechnical structure fails one normally sees excessive displacement increase without load increase, which your graph doesn't demonstrate. Therefore based on your graph it cannot be determined what the ultimate load is.

With kind regards,

Dennis Waterman

Dear Mr. Dennis,

Please note that for the presented graph above, Nmax is increased to 15, and at point 3 at the top, there are actually 15 points aligned horizontally indicating failed attempts to increase the load while the displacement is increasing. They might look like a singular point in the presented graph above as the points are closely spaced. 

Reached CSP is 0.007159.

Is it needed to change something else for the curve to show shear failure?

Does this CSP correspond exclusively to the last step in the graph or is it possible that the step just before that have a CSP<0.01?

Thanks again for your kind help

Mohamad

Dear Mohamad,

At 1) and 2) after these small horizontal parts it continues to increase the load again, which clearly indicates that the ultimate load was not reached because if the ultimate load was reached, it couldn't increase the load any further. So how do you know for sure that it won't be able to increase the load further after the small horizontal part at 3 as well?  Load-displacement curves are generally curved and flattening whereas in your case it's quite straight and linear and suddenly goes horizontal (3 times, actually). This doesn't look like the typical load displacement curve hence based on just the curve it cannot be decided the ultimate load and thus failure has been reached.
Now if you see based on other data that indeed failure was reached, that is different. But you have that data and I don't, I only have the graph and based on the graph it cannot be determined that the failure load was reached.

With kind regards,

Dennis Waterman

Dear Mr. Dennis,

Would you recommend to always increase Nmax to a large number, e.g. 1000 for such analysis? This way it should always yield a flattened part, however, the ultimate load will be depending on CSP alone in that case.

Your help and assistance is much appreciated!

Mohamad