Normal Drain Vs Vacuum Drain in Plaxis 2D

Question

Hi all and Rushan Gizzatullin Stefanos Papavasileiou Dennis Waterman 
 
 Normal drain and vacuum drain both used to reduce the GWT but vacuum drain can reduce lower the provided head. I thought for normal drain we need to apply loading to reduce and for vacuum drain no need to apply external loading as it creates the vacuum which reduce the pore pressure and increase the effective stress (OBP + delta sigma), where delta sigma will be used as the external loadings. 
 But when i did the analysis using both, both provide same result without applying any external loadings as shown below. 
 
 Normal Drain:

Vacuum Drain:

Please tell the reason for this. 
 
 Thanks

Dennis Waterman · Answer

Hi, 
 From the figures you show it looks like you used drains in a steady-state groundwater flow analysis, in which case indeed there is no effect. Note that a normal drain and a vacuum drain will have the same final pore pressure distribution, the difference is that a vacuum drain will reach that final distribution faster than a normal drain. But for steady-state groundwater flow there is no time-dependent analysis. Hence, using vacuum drains only make sense for consolidation and fully-coupled analysis. And even then it may not make a difference if for instance the specified head doesn't represent an under pressure, or if the drainage capacity is limited by the soil permeability. With kind regards, 
 Dennis Waterman

Normal Drain Vs Vacuum Drain in Plaxis 2D

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