Decrease in Factor of Safety in embankment during waiting period.

Question

Hi everyone. 
 I am simulating embankment with phased filling by applying consolidation analysis type. In some steps, the Factor of Safety (FS) of waiting phase without any additional filling height is lower than one of construction phase with filling height. This seems unreasonable as after waiting phase the strength of soil will increase as a result of consolidation and therefore FS should be higher. Modelling results are presented in the pictures below. Is there anyone experiencing this case? Please explain to me. Thank you. 
 construction phase FS=1.2864 (consolidation in one day) waiting phase FS=1.2848 (consolidation in five day)

Dennis Waterman · Accepted Answer

Hi, 
 The phreatic level is not more than the level where the pore pressures are zero and below which the soil is fully saturated. However, above the phreatic level the soil is not dry. If in the calculation suction is alllowed then the soil above the phreatic level is partially saturated according to the chosen Soil-Water Characteristic Curve (Retention Curve) in the material set. If in the calculation suction is ignored (which is the default) then Drained material sets considered dry above the phreatic level and Undrained A and B material sets are considered fully saturated above the phreatic level. And of course a fully or partially saturated soil can still develop excess pore pressures. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Hi, 
 Please note that the strength increases during consolidation if there are excess pore pressures that are being dissipated. However, in case there are excess pore tensions to be dissipated then the strength decreases during consolidation. A known phenomena for undrained embankments is that there is some upheave at the toe causing excess pore tensions and thus provide some extra stability. If that is consolidated out, indeed the factor of safety slightly decreases. Could that be the case for your project? Unfortunately from your pictures it is hard to say anything. 
 With kind regards, 
 Dennis Waterman

Decrease in Factor of Safety in embankment during waiting period.

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