PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

RE: PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

Mohamad Raad — Fri, 16 Jun 2023 08:00:20 GMT

Thanks a lot Dennis!

Do you have any literature that you can share regarding (3)? I've seen that Hansen and DeBeer have proposed methods to determine the ultimate stress from Load-Settlement curves, and a third one based on Settlement/Base width = 10%, however I can't find the referenced articles.

Again, your assistance is much appreciated!

Mohamad

RE: PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

Dennis Waterman — Thu, 15 Jun 2023 07:46:11 GMT

Dear Mohamad,

1) That is the correct way to do it. I'm puzzled about your last remark though ... because it's not possible to get a failure load out of a Safety analysis so why would one try.

2) The plotted curve suggests softening behaviour after reaching the peak strength. To model that in PLAXIS you would have to use a constitutive model that includes softening behaviour. If softening is not included in the constitutive model, it cannot show in the results. Unfortunately softening is not so easy to calculate in finite elements, so therefore most of the models we have do not include softening and the ones that do are not the easiest models to work with, like the Hypoplastic model and the OC-Clay model. Hence, it's probably not worth trying just to simulate a literature graph.

3) Local and punching shear are often quite mesh dependent, hence when and how they occur depends on the element size around the footing. So it is indeed not easy to determine - there are some graphical methods described in literature (also used to determine preconsolidation pressure from oedometer tests, for instance) that you could look into, but it will always have an amount of uncertainty.

With kind regards,

Dennis Waterman

RE: PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

Mohamad Raad — Mon, 12 Jun 2023 13:06:44 GMT

Dear Hung, can you please elaborate on how the ultimate load should be extracted in a load control test?

1- For my Master's thesis, I'm modelling a plate with a huge point load at it's top (10,000 kN for example) to ensure shear failure of soil, then the ultimate load is extracted from the Mstage vs Uz (Mstage multiplied by 10,000 kN). No safety phase (Msf) is used to get the load!

2- Secondly, It's seems not possible to mitigate the load-displacement curve of the General Shear Failure for high dense sand similar to the one proposed by Terzaghi, as from my model the load becomes perfectly horizontal when it reaches the ultimate capacity.

3- Finally, I'm having a hard time locating the ultimate load for the cases of loose soil where Local and Punshing Shear Failures take place. Visually, the point where the curve becomes more erratic is not clear so I'm currently using the second derivative as an indication. Do you have any recommendation?

Thanks a lot in advance!

Mohamad

RE: PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

Hung Ho Manh — Thu, 25 May 2023 02:19:12 GMT

If you can make sure the same behavior or mechanism failure, the ultimate bearing capacity should be the same

RE: PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

Alvin Lim — Wed, 24 May 2023 07:40:11 GMT

Can I check that for both displacement control and load control, given the same boundary conditions and soil type, the load-settlement response will be different but the ultimate bearing capacity generated will be the same?

RE: PLAXIS 3D Bearing Capacity (Prescribed Load vs Prescribed displacement)

Hung Ho Manh — Wed, 24 May 2023 07:05:57 GMT

Hi Alvin,

When a prescribed displacement (displacement control) is applied, it is considered for a rigid footing, or the surface to be applied displacement has the same displacement. Therefore, when you use the load (load control), please make sure that the surface has also the same displacement, so, use very stiff footing for example. If not, it is a flexible footing, resulting in different load-settlement response. So, basically both approaches are fine but should be consistent when comparison.

When you apply a plate (or volume) for the load control, it should be very stiff to make sure it is a rigid footing.