Why does the soil uplift？

Similar question posted here:Piled Raft Foundation

The structure is connected to the surrounding soil via the interface element. Therefore you should expect that depending on the level of bondage that the soil would follow the movement of the structure. 

Hi，

Do you mean that the upward movement of the concrete structure causes the upward movement of the surrounding soil? But in my model, I'm not putting any force on the concrete structure. I do not believe that the concrete structure can move upward due to the unloading rebound of the soil.

How can I explain such a small uplift？I'm really confused.

There are many obvious causes for the uplift that I presume you have already investigated?

• Did you use interfaces? If not, the excavation bottom lift also lifts the wall.
• And what is the weight of the wall compared to the weight of the excavated soil? If the wall is relatively light, it will lift
• What is the ratio of soil stiffnesses and is the soil normally consolidated or overconsolidated?
  • If it's (heavily) overconsolidated the HSsmall model will behave pretty much elastic and you'll have the same problem as with M-C.
  • If it's normally consolidated then the wall lowers (at installation) mostly depending on Eoed an E50 but the soil rises during excavation depending on Eur and E50. If the ratios Eur/E50 and Eur/Eoed are not so high than the soil may rise enough to push the wall up, especially if no interfaces have been used.

You can always send your project to our support department to have a further look.

With kind regards,

Dennis Waterman

Thank you for your reply.

First of all, I used the interface in all models and the soil was normally consolidated. The unit weight of the wall is 25, and the unit weight of the soil is 18. I have made quite a few attempts according to your suggestions.
1) By increasing the ratios Eur/E50 and Eur/Eoed, the uplift value of the concrete wall does decrease, but the reduction is small enough to be negligible.
2) By testing HSS, HS and MC respectively, the results of wall uplift are Uz (MC) >Uz (HS) >Uz (HSS).

However, no matter how it is changed, the soil and the wall will settle during the installation of the wall (which is reasonable). But during excavation, the walls always uplift. For example, during the installation phase, the total displacement UZ=-2mm, and then during the excavation phase, the total displacement UZ=0.1mm.

In fact, I just want to understand how to avoid this kind of situation in the program that is impossible to happen in real engineering.

I have to give up now, I found that soil uplift is impossible to change.

I'm sure there is a simple explanation and solution for it. That is why I suggested to send the project to our support department, which I presume you haven't done yet.

I'm sure there is a simple explanation and solution for it. That is why I suggested to send the project to our support department, which I presume you haven't done yet.

This is a fairly simple model. I really want to know what the problem is and how to modify it so that the soil does not uplift. Here is the command flow.

##~ running Plaxis3DInput.exe 22.1.0.452
_setproperties "Title" "" "Company" "only@ ::LAVteam::®" "Comments" "" "UnitForce" "kN" "UnitLength" "m" "UnitTime" "day" "WaterWeight" 10 "ReferenceTemperature" 293.15 "LiquidSpecificHeatCapacity" 4181.3 "LiquidThermalConductivity" 0.0006 "LiquidLatentHeat" 334000 "LiquidThermalExpansion" 0.00021 "LiquidTemperature" 293.15 "IceSpecificHeatCapacity" 2108 "IceThermalConductivity" 0.00222 "IceThermalExpansion" 5E-5 "VapourSpecificHeatCapacity" 1930 "VapourThermalConductivity" 2.5E-5 "VapourSpecificGasConstant" 461.5 "UseTemperatureDepWaterPropsTable" False "ModelType" "Full" "ElementType" "10-Noded"
_initializerectangular SoilContour -150 -150 150 150
_borehole 0 0
_set Borehole_1.Head -0.5
_soillayer 0
_soilmat "SoilModel" "Mohr-Coulomb"
_set SoilMat_1.SoilModel "HS small"
_set SoilMat_1.Identification "soil"
_set Soil.DrainageType "Undrained A"
_set Soil.gammaUnsat 18
_set Soil.gammaSat 18.6
_set Soil.E50Ref 5000
_set Soil.G0Ref 45000
_set Soil.gamma07 0.0002
_set Soil.cRef 10
_set Soil.phi 20
_set Soil.InterfaceStrengthDetermination "Manual"
_set Soil.Rinter 0.7
_set Soillayer_1.Zones[0].Bottom -150
_set Soillayer_1.Soil.Material Soil
_gotostructures
_gotosoil
_soilmat "SoilModel" "Mohr-Coulomb"
_set SoilMat_1.SoilModel "Linear Elastic"
_set SoilMat_1.Identification "concrete"
_set Concrete.gammaUnsat 25
_set Concrete.gammaSat 25
_set Concrete.DrainageType "Non-porous"
_set Concrete.ERef 30000000
_set Concrete.nu 0.2
_gotostructures
_polycurve 0 0 0 1 0 0 0 1 0
_reset Polycurve_1 "Arc" 0 180 6 "Arc" 0 180 6
_set Polycurve_1.Offset2 -6
_polycurve 0 0 0 1 0 0 0 1 0
_reset Polycurve_2 "Arc" 0 180 6.4 "Arc" 0 180 6.4
_set Polycurve_2.Offset2 -6.4
_surface -8 8 0 9 8 0 9 -8 0
_addpoint Polygon_1 -10 -7 0
_intersect (Polygon_1 Polycurve_2 Polycurve_1)
_delete (Surface_3)
_extrude (Surface_1 Surface_2) 0 0 -1.5
_delete Geometry (Surface_1 Surface_2)
_decomposesrf Volume_1
_delete Geometry (Surface_Volume_1_3 Surface_Volume_1_4)
_delete Geometry Surface_Volume_1_1
_delete Geometry Surface_Volume_1_6
_posinterface (Surface_Volume_1_5 Surface_Volume_1_2)
_delete Geometry (Polycurve_6 Polycurve_5 Polycurve_4 Polycurve_3)
_gotomesh
_refine (BoreholeVolume_1_Volume_1_1 BoreholeVolume_1_Volume_2_1)
_mesh "Coarseness" 0.05 "UseEnhancedRefinements" True "EMRGlobalScale" 1.2 "EMRMinElementSize" 0.005 "UseSweptMeshing" False
_gotostages
_phase InitialPhase
_set Model.CurrentPhase Phase_1
_set Soil_1_Soil_2_1.Material Phase_1 Concrete
_activate Interfaces Phase_1
_phase Phase_1
_set Model.CurrentPhase Phase_2
_set Model.CurrentPhase Phase_1
_set Model.CurrentPhase Phase_2
_deactivate (Soil_1_Soil_3_1) Phase_2

I'm sorry to say, but the forum is not for support cases but to answer general questions. Support cases must be sent in through https://bentleysystems.service-now.com/csp

With kind regards,

Dennis Waterman