Depth of groundwater table is unknown

Question

Hello everyone! 
 I am trying to conduct the flow-deformation analysis for slope stability assessment under transient boundary conditions. However, the problem is that at the study site, we do not know the depth of the groundwater table. We are assuming that the groundwater table is quite deep based on the fact that there was no evidence of groundwater at the landslide when it occurred. Can anyone please suggest the best way to tackle such a situation? 
 Thank you in advance. 
 Best Regards, 
 Sam

Dennis Waterman · Answer

Dear Sam, 
 If the groundwater table is so deep, why would you want to do a flow-deformation analysis? There will be almost no flow as flow in the unsaturated zone is usually extremely small. If it's just the suction you're interested in, you could use a plastic analysis with the "ignore suction" option unchecked. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Sam, 
 I'm afraid that won't work either. If you would apply an initial degree of saturation it is indeed, as you say, an initial value. So the fully-coupled analysis will see this as a change of the underlying steady-state situation, hence the initial degree of saturation will lead to ground water flow and a slow return to what is considered the steady-state situation. And the steady-state situation is the saturation/suction distribution according to the SWCC. 
 To know the suction at any location in your model a phreatic level is required. If you don't specify one, PLAXIS will just assume it is at the bottom boundary of your model and calculate accordingly. While in reality probably your phreatic level is much deeper. 
 The option to specify an initial degree of saturation is meant for for instance embankment construction where soil with a different degree of saturation is used to create the embankment. In this case when activating the soil its degree of saturation can be specified and then in time water will flow in our out when the newly applied soil will slowly adapt its water content to the project area conditions. 
 However, if you have measurement date on suction, maybe saturation, and for sure hydraulic conductivity, it should be possible to make a reasonable estimate where the phreatic level is. Just pick a soil classification system and the soil from that system that matches best your soil. Based on the SWCC from that soil classification and your measurement data an estimate can be made how deep is the phreatic level and then just use that in your model. 
 With kind regards, 
 Dennis Waterman

Depth of groundwater table is unknown

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