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.

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.