Settlement Analysis of Footing in undrained soil

Depends on what type of analysis (Plastic, fully coupled) you are doing and what do want to achieve like do you want to study the effects of suction in the short or long term? Have you defined the SWCC of the soil or are you relying on the default unsaturated parameters of PLAXIS?
Also where is the water table located at?

I have clayey silt followed by sand till 20.0m then sandy soil till 75.0m and WT is found at 20.0m. I'm studying only settlements of raft under undrained case. And for SWCC am using the default values. I have hardening soil model and undrained A for all the soil layers. 

I have clayey silt followed by sand till 20.0m then sandy soil till 75.0m and WT is found at 20.0m. I'm studying only settlements of raft under undrained case. And for SWCC am using the default values. I have hardening soil model and undrained A for all the soil layers. 

Inclusion of soil suction can have a significant impact on the bearing capacity in undrained conditions. In PLAXIS, the pore pressure in a plastic analysis is composed of the steady-state pore pressure (Psteady), the excess pore pressure (Pexcess), the pore water pressure (Pwater) and the active pore pressure (Pactive). Psteady is a direct generation of water pressure with respect to the existing hydraulic conditions. Pexcess is a result of the undrained analysis when material drainage type is set to Undrained (A). These two components become Pwater . Furthermore, Pactive is composed of Pwater and the effective degree of saturation Seff.

Pactive = (Psteady + Pexcess).Seff

Now lets look at what happens with and without the inclusion of soil suction.

No suction

In case of the material set Undrained (A), the saturation is uniform and is equal to 100 % above the phreatic level. So the pore pressure will be:
Pactive = (Psteady + Pexcess).1

The excess pore pressures are high in the zone above the water table as saturation is high.

With suction

If suction is enabled in the numerical analysis, the degree of saturation can be redefined by means of a specified hydraulic model. The difference now is, the suction above the water level is computed. Hence, the saturation decreases with respect to the SWCC of the soil material. Furthermore the effective degree of saturation is reduced as well. Thus the active pore pressure above the phreatic level is computed lower compared to the case with no suction, which means that the effective stresses will be higher and bearing capacity will be higher as well:
Pactive = (Psteady + Pexcess).Seff

The excess pore pressures are lower in the zone above the water table compared to the case with no suction because now the saturation is computed as a function of suction. Lower excess pore pressures above water table will mean lower active pore pressures and higher effective stresses which will in turn increase bearing capacity.

Hope this helps.

I clearly understood now. Thank you.

I have one more query can i apply hardening soil model for clayey silt, sand and clayey sands. Since undrained condition gives my immediate settlements. Can I use the undrained A condition for all the soil layers which I have mentioned above. 

Settlements are less when I ignored suction. Which is not the way you said. Can you please explain.

My colleague didn't mention settlements, so your statemen "that is not the way you said" is not correct.

He mentioned that when having suction effective stresses are higher. And higher effective stresses generally give higher settlements.

With kind regards,

Dennis Waterman

Thank you for your clarification.