Accidental inaccurate results in PLAXIS 3D with large incremental multiplier Mstage

Micha van der Sloot — Wed, 18 Dec 2019 15:47:05 GMT

Current Revision posted to GeoStudio | PLAXIS Wiki by Micha van der Sloot on 12/18/2019 3:47:05 PM

Application	PLAXIS 3D
Issue status	✔ Solved
First Affected Version	PLAXIS 3D 2010
Found in Version	PLAXIS 3D 2010
Fixed and Released in Version	PLAXIS 3D 2013.00
Issue #	15783
Date created	23 September 2011
Date modified	24 October 2016

Problem description

In some cases the use of the default parameters for the iterative procedures in PLAXIS 3D may lead to inaccurate results. This may happen in case the last step of a Staged Construction phase is a big step, in which the (total) multiplier SumMstage goes from somewhere below 0.5 to 1.0.

In the worst case, it could be concluded that a construction stage is stable whereas it is not.

For example, if tunnel heading stability is analized, it might be wrongfully concluded that the tunnel heading is stable.

Solution

Since PLAXIS 3D 2013.00 this is resolved: in the phase's numerical control parameters there is a new parameter called Max load fraction per step. This parameter is set to 0.5 by default, and this means that any calculation step will have a Mstage value smaller than 0.5.

For 3D Version 2012 and earlier

In a situation as described above occurs it is recommended to recalculate the phase with a reduced tolerated error (e.g. 0.005 or lower instead of the default value of 0.01) and to make sure that the increment of SumMstage (i.e. Mstage) in the last step is smaller than 0.5.

To check this value for Mstage after the calculation, open the results of a phase in the PLAXIS Output program and open the menu item Project > Calculation info. Here you can find the incremental value Mstage under Staged construction:

Alternatively, it can also be found under Project > Calculation info per step. Here you can see the incremental multiplier value for Mstage in the tab Staged Construction. Note that this will show the values for all steps. Please only check the last step of each staged construction phase.

Tags: PLAXIS 3D, calculations, iterative settings, Staged Construction, phases, error, Calculation, Stability, PLAXIS, Tunnels, Tunnel, stages, calculation error