Settlement Analysis of Footing in undrained soil

Question

what would the effect of suction in intial stage, excavation stage and loading stage? Should i uncheck or check that.

Dennis Waterman · Accepted Answer

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

Faseel Khan · Answer

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?

Faseel Khan · Answer

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.

Settlement Analysis of Footing in undrained soil

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