Reason for this type of failure when doing safety analysis

Dear Nitha,

This might be related to the fact that the excess pore pressures increase quite a lot for the undrained material. Have you considered the following article in your modelling?
https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45955/safety-analysis-and-undrained-behaviour

It seems relevant to your situation.

Dear Mr. Stefanos, 

Thanks for that material and by providing ignore un-drained behaviour in the safety analysis which provided nice failure surface when compare with the previous unsuitable failure surface. 

By reading that article, I encounter some unclear matters. Please help to clarify these.

01. Since the soil i have adopted here is cohesive soil with phi=0, so here I'm eager to know how excess pore water pressure influence in the un-drained shear strength of the soil?

02. In the above case (circular excavation), previous calculation I didn't provide ignore the un-drained behaviour, that time in safety analysis i cut the section near to and see the current cohesion and the value came as like this (10.91E3 max) and excess pore water pressure also increased from 164kPa (plastic calculation) to 1xE3 (safety analysis). How it is possible? 

03. In that articles why it is mentioned as considering excess pore pressure depends on the situation? Here in safety analysis we are checking what is the safety of the system under this condition (means same amount of excess pore water pressure from the plastic analysis). So changing the excess pore pressure in the safety analysis is not good approach in my point of view. Please correct me if I'm wrong. 

Thanks. 

Dear Mr. Stefanos, 

Thanks for that material and by providing ignore un-drained behaviour in the safety analysis which provided nice failure surface when compare with the previous unsuitable failure surface. 

By reading that article, I encounter some unclear matters. Please help to clarify these.

01. Since the soil i have adopted here is cohesive soil with phi=0, so here I'm eager to know how excess pore water pressure influence in the un-drained shear strength of the soil?

02. In the above case (circular excavation), previous calculation I didn't provide ignore the un-drained behaviour, that time in safety analysis i cut the section near to and see the current cohesion and the value came as like this (10.91E3 max) and excess pore water pressure also increased from 164kPa (plastic calculation) to 1xE3 (safety analysis). How it is possible? 

03. In that articles why it is mentioned as considering excess pore pressure depends on the situation? Here in safety analysis we are checking what is the safety of the system under this condition (means same amount of excess pore water pressure from the plastic analysis). So changing the excess pore pressure in the safety analysis is not good approach in my point of view. Please correct me if I'm wrong. 

Thanks. 

Hi Nitha

Q1: When you set your material to adopt an Undrained B or Undrained C approach, (so with Undrained Shear strength s_u as direct input, and no friction angle), the Undrained Shear Strength does not change with the excess pore pressure development. Please be careful when using this approach, especially when using Undrained B, as the stress path developed under elastic conditions (e.g. when using Mohr Coulomb) might give incorrect excess pore pressures. When using Undrained A, make sure to check the Undrained Shear Strenght profile as mentioned here: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/46010/how-to-check-undrained-shear-strength-using-undrained-a 

Q2: For that specific behavior, we would need to investigate the model to see what is happening here. Please submit a Service Request here and send over your PLAXIS model: apps.bentley.com/.../ProductSupport 

Q3: Allow or not allow more excess pore pressures to be generated. As mentioned in the article:

In the end the choice for using the option “Ignore undrained behaviour” in a Safety analysis is something for the user to decide.

you indeed should make a choice on how to evaluate the safety factors and how excess pore pressures play a role in this.

Finally, I see some odd shape of the waterlevel.

Please have a look at this webinar that deals with this: PLAXIS CONNECT Edition: Excavation Dewatering and Common Pitfalls

Dear Mr. Micha, 

Thanks for the reply, will check 

and for odd shape I have created this post =>  https://communities.bentley.com/products/geotech-analysis/f/plaxis-soilvision-forum/215689/urgent-help-please-water-table-during-shaft-excavation-using-axisymmetric-model

because for that axisymmetic shaft excavation (without embedment depth) I don't know how to provide it. Your webinar contains structure with embedment depth but my case no embedment depth. 

Thanks.