Drain and Undrain Condition

Question

If the Water table is very deep or no water table such that it doesn't affect the behavior of deep excavation (with diaphragm wall) and the soil is clayey type. If one time I used the undrained A condition and another time drain condition (Long term situation) with the same soil properties, results of deflection should be the same or different?

Stefanos Papavasileiou · Answer

Dear Shashwat, 
 Whether the soil will behave as drained or undrained, depends on the material itself, the loading condition and the time is given. 
 In theory (to give an exaggerated example) a clay material can be handled as drained material if you load it very slowly over the course of 1000 years. The same for sand, given the short time and a huge load applied, it can behave as undrained material. 
 So, the question you should ask is whether your conditions are considered to be drained or undrained and pick the correct drainage type. Note that if you are not sure, you can also run a Fully coupled flow deformation analysis or a Consolidation analysis in which only the permeabilities, the boundary conditions and time are considered. 
 In any case, the water table is a separate part of the calculation as to consider the presence of water and thus affect the effective stresses of the soil. 
 I would refer you to our detailed chapter on Modelling undrained behaviour on our Reference manual.