Deconfinement vs Mstage

There are basically two different methodologies to simulate the construction process of NATM tunnels:

(1) Use unfinished staged construction by assigning SumMstage < 1.0;

(2) Use “deconfinement” option.

Traditionally, the 1^st method would be used. However, the deconfinement feature was introduced to allow for a more accurate simulation of the problem. Both methods can be included in the beta-methods category (i.e. load-reduction methods).

Method 1: unfinished staged construction by assigning SumMstage < 1.0 

Providing that a “plastic” calculation type is selected, a “Load advancement ultimate level” procedure is undertaken to solve the generally non-linear equilibrium equation inherent to the FE method. In particular, given the initial (i.e. results achieved in the previous construction phase) and the final configuration of the problem (i.e. configuration of the current construction phase), the software determines the total load increment to be applied in the current construction phase (given by the differences in loads and weights between the initial and final configurations). These loads are applied as unbalanced forces incrementally (i.e. in a series of load steps). SumMstage measures the progress of the unbalanced already applied. For instance, when SumMstage = 0.6, 60% of the unbalanced force was successfully applied.  

When using this option to simulate the construction of a NATM tunnel, the following steps are generally required: 

(a) De-activation of the tunnel clusters with SumMstage < 1.0. In this case, only a percentage of the self-weight of the materials (given by SumMstage = (1 – beta) < 1.0) is deactivated. The remaining 1.0 – SumMstage loads are stored as out-of-balanced loads and will be applied in the following construction phase.

(b) Activation of the tunnel lining with SumMstage = 1.0. The out-of-balance forces resulting from the previous construction phase, as well as those resulting from the activation of a new material (linnig) are applied at this stage.

Note that the methodology described above assumes that the cross-section of the tunnel is excavated in a single phase. A more complex sequence of operations may be required when the tunnel is constructed in parts (crown, benches and invert).

(2) Deconfinement value of 1 − β.

Along the same ideas, the deconfinement option assumes that only part of the stress change due to the excavation is applied to the unsupported tunnel, while the remaining part is applied to the supported tunnel. However, in this case, the calculation methodology is slightly different. Specifically:

(a) The soil clusters are completely deactivated; however, a part of the stresses corresponding to the self-weight of the deactivated soil clusters (equal to beta) is applied around the tunnel configuration as a support pressure; this means, that only part of the total stress change (1- beta) is effectively applied at this phase, as illustrated in the figure below.

(b) the lining is activated and the deconfinement of the deactivated soil clusters is set to 100 %, which means that the remaining load is applied (equal to beta) is applied at this phase. 

Once again, to ease the explanation, it is assumed that the cross-section of the tunnel is excavated in a single phase.

So for a more accurate simulation when you want different soil clusters to be in different stress state then you could use the deconfinement method.