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)
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
Thank you Dennis,
In my model, since the critical circular slip surface occurred right at the filling material (drain) in Safety analysis not Consolidation one, the upheave of subsoil at the toe of the embankment will not affect FS. I also checked the excess pore pressure through a section perpendicular to the circular slip surface, it's surprising to see excess pore pressure above the groundwater as pictures below. Could you explain this?
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.
Answer Verified By: Nam Nguyen Duc
Hi Dennis,
Thank you for the reply, actually, I have the same case, an embankment on three soil layers, the first soil layer is silty clay wich is studied under undrained A, the second is sandy silt, undrained A and the third is alluvium in drained condition, the embankment is studied under drained conditions, the same problem, the displacement in safety analysis are higher after consolidation than before !!!
before there were displacements reaching 45 cm
after consolidation, the displacements reached 90 cm
NB: displacements are due to the safety analysis not consolidation analysis I made the option of reset displacement in safety analysis,
Do you have any explanation ?
Dear Ilias,
You say that the embankment is studied under drained conditions, but then you mention that you do a consolidation analysis. That seems to be contradictive, so I presume you mean the embankment is studied under undrained conditions.
Some explanations for the drop of the safety factor during consolidation are given in my earlier reactions. Could you indicate why those possible explanations are not correct for your case?