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PLAXIS | SOILVISION Wiki Deconfinement method gives unexpected results in case of pore water pressure change in PLAXIS 2D
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            • 1D Site Response Analysis does not work in non-English versions
            • Deconfinement method gives unexpected results in case of pore water pressure change in PLAXIS 2D
            • 2D 2017.01: Drawing area not updated with small tunnel arc segments
            • 2D AE.00: A project may fail to load when the model contains imported geometry
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            • Ignoring suction after a fully coupled flow-deformation analysis in Safety analysis gives incorrect stresses in PLAXIS 2D [Solved]
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            • Interpolate pore pressures with an inclined top cluster boundary does not work correctly in 2DAE.00
            • Interpolated pore pressures in interfaces might be incorrect
            • Invalid saved views gives error in Output
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            • Layer dependent skin resistance of embedded beam row ignores spacing
            • Line and surface contraction value [Solved]
            • Linear distribution for line loads and prescribed line displacements defined in 2DAE's Tunnel Designer do not work
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            • Maximum value bulk modulus water not correct when using non-metric units (relevant for undrained, consolidation and flow analysis)
            • Mix up of state parameters in Output with multiple (user defined) soil models
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            • Multiple geogrid materials can cause incorrect Plaxis data
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            • Objects names with special Unicode characters may fail in meshing [Solved]
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            • Plastic behaviour in free field elements in PLAXIS 3D AE.00 may lead to a severe divergence error
            • Plastic Points History plots cannot be added in report generation
            • PLAXIS 2012 and Windows 8
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            • Point load not taken into account on top of embedded pile row if surrounding soil is deactivated
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            • Points outside of geometric boundaries may have strange behaviour in 2D AE and 2D 2015
            • Pore pressures in interfaces may be incorrect with Interpolate pore pressures
            • Possible data loss due to the Output report generator for user defined report folders
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            • Prescribed displacements are taken as additional phase displacements
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            • Re-applying same material in stages by a command reports: Tried executing, but failed
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            • Report generator on Japanese and Chinese Windows versions generate corrupted RTF
            • Results may not be correct for undrained excavations while using a cavitation cut off in PLAXIS 2D2011.02
            • Safety analysis after fully coupled flow-deformation analysis uses incorrect pore pressures
            • Safety analysis: Ignore undrained behaviour cannot be deactivated
            • Saturation is 0% for phreatic level calculations in PLAXIS 3D 2011.00
            • SciTE issues when running a Python script
            • Sensitivity analysis does not give results for the criterion Factor of Safety
            • Splitting dynamic signal over multiple phases may give wrong results when using compliant base boundary
            • Staged construction/Flow condition settings might not be loaded correctly in 2D 2017
            • Steady state groundwater flow results may be wrong for models with seepage boundaries and low permeable soils
            • Structural forces from automatic centerline might be incorrect in PLAXIS 2D 2016.00
            • Structural results are not included in 2D2011.00's generated report
            • Structural tables for a safety value show no values in PLAXIS 2D 2016.00
            • Switching between Classical and Advanced mode switches Suction cut-off on/off in an unexpected manner
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            • Thermosyphon boundary option is not fully documented in 2D 2016/2D 2017
            • Time interval specified for K0 phase (using Gravity loading first) incorrectly influences dynamic results
            • Total pore pressures and total stresses plot may not be correct for undrained excavations in PLAXIS 2D2012.00
            • Transient groundwater flow analysis is reset to steady state after changing the calculation type
            • Tunnel designer loses material assignments after loading project in 3D 2018
            • Tunnel properties seem lost when opening a project created in 2D 2015 or earlier
            • Tutorials 15 and 16 produce slightly different results with PLAXIS 2D 2016.01
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            • Unable to activate interfaces in water conditions mode for Undrained B materials
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            • Updated mesh in 3D Safety analysis incorrectly updates the geometry
            • Updated water pressure does not work in 2DAE.00
            • Use of "Line Search" option may give incorrect results
            • Use pressures from previous phase-option gives wrong pore pressures after changing the previous phase
            • User defined soil model dll not recognized in PLAXIS 2D AE Soiltest
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    Deconfinement method gives unexpected results in case of pore water pressure change in PLAXIS 2D

    Application PLAXIS 2D
    Issue status Open
    First Affected Version PLAXIS 2D 2018.00
    Found in Version PLAXIS 2D CONNECT Edition V20 Update 2

    Fixed and Released in Version PLAXIS 2D CONNECT Edition V20 Update 4
    Issue # 379697
    Date created 1 July 2020
    Date modified 30 September 2020

    Problem description

    The Deconfinement (1-β) method is used to simulate the three-dimensional soil arching behaviour that occurs around an unsupported tunnel face when using a 2D model, like PLAXIS 2D.

    There are two ways to set this up in PLAXIS 2D:

    • using the method of partial staged construction (setting the phase’s general ΣMstage value to 1 – β);
    • or using the Deconfinement option for a deactivated soil cluster.

    A deconfinement value can be entered during staged construction as 1-β in the model explorer for any selected and deactivated soil cluster (see figure below). This method intends to give similar results to those obtained when using the partial staged construction (ΣMstage) method while bringing additional flexibility since different deconfinements can be applied to different tunnels or different sections of the tunnel in the same phase.

    However, this Deconfinement option may give unexpected results in PLAXIS 2D CONNECT Edition V20 Update 3 or earlier when pore water pressure changes in the tunnel, for instance when dewatering the tunnel during the excavation process. When using the Deconfinement method, the effective stress changes are solved according to the defined deconfinement value, while all pore water changes are fully solved in the same phase.

    PLAXIS 2D: Deconfinement

    Fig 1. Deconfinement (1-β) approach in Selection explorer

    Solution

    This is now solved in PLAXIS 2D CONNECT Edition Update 4.

    In older versions, when you have a situation where you want to apply this Deconfinement method in PLAXIS 2D and you define pore water changes (e.g. dewatering the tunnel), please use this workaround:
    Use the approach of setting the multiplier ΣMstage in the Phases window to (1-β) to model the deconfinement of a tunnel in PLAXIS 2D. When a value of ΣMstage less than 1 is defined, the out-of-balance force is partly resolved. The remaining out-of-balance forces must be solved in the next calculation phase.

    For example, for a phase with ΣMstage = 0.3, only 30% of the out-of-balance forces are solved in this phase and the remaining 70% of the out-of-balance forces are solved in the next phase when keeping the default setting of ΣMstage = 1.0.

    Note: When pore pressures do not change, both approaches give the same result when only considering the tunnel deconfinement.

    • pore pressure
    • PLAXIS
    • deconfinement
    • Tunnel
    • PLAXIS 2D
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    • Micha van der Sloot Created by Bentley Colleague Micha van der Sloot
    • When: Wed, Jul 1 2020 9:34 AM
    • Micha van der Sloot Last revision by Bentley Colleague Micha van der Sloot
    • When: Wed, Sep 30 2020 8:44 AM
    • Revisions: 3
    • Comments: 0
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