Problem description

In PLAXIS 2D it is possible to enter a Deconfinement value during staged construction (as 1- β) in the model explorer. Doing this enables part of the stresses (β) in the soil polygon inside the tunnel to be retained as a support pressure. 

Figure 1. Deconfinement feature in Selection explorer

However, when the general water level is below the tunnel where the deconfinement feature will be used, and the soil polygon is not set as “dry”, a wrong stress distribution will be generated in PLAXIS 2D CONNECT Edition V20 Update 4 and PLAXIS 2D CONNECT Edition V21, leading to inaccurate calculation results. 

The image below is a comparison of the stress distribution from a construction phase with a deconfinement of 30%, and with a construction phase with ΣMstage = 0.3. Since this only simulates the excavation of the tunnel, the stress distribution of both phases should be the same.

Figure 2. Computed cartesian stresses using ΣMstage and Deconfinement options

Note that a similar issue with the deconfinement method when a tunnel was subjected to pore water pressure changes in PLAXIS 2D is fixed since PLAXIS 2D CONNECT Edition Update 4.
For further information regarding this please refer to:
https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/50563/deconfinement-method-gives-unexpected-results-in-case-of-pore-water-pressure-change-in-plaxis-2d

It is important to highlight that the present issue only occurs when dry and semi-dry conditions are considered, and therefore it is not related to the aforementioned case.

Solution

This issue is resolved since PLAXIS 2D CONNECT Edition V22.00

As a workaround, for PLAXIS 2D CONNECT Edition V20.04/V21 it is required to manually set the deactivated soil polygon of the tunnel to “Dry” water conditions. This will enforce proper support pressure during partial deconfinement.

The image below illustrates how to set the selected soil polygon as “Dry” in Staged construction. Please note that it is required to check if these settings are correct in the subsequent phases.

Figure 3. Assigning "Dry" water condition to a soil polygon

By setting the water condition of the tunnel soil polygon to “Dry”, the same stress distribution is obtained.

Figure 4. Computed cartesian stresses with the suggested workaround