# Initial Stresses following a Model Change

There may be situations in which one material model for deformable blocks is used in the process of reaching a desired stress distribution, but another model is used for the subsequent simulation. Models can be changed for zones using the block zone cmodel assign command. If one model is replaced by another non-null model, the stresses in the affected zones are preserved:

Initial stresses following a model change

model new
model large-strain on

block create brick 0,5 0,5  0,5
;
block zone generate edgelength 1.0
;
block zone cmodel assign elastic
block zone prop  dens 2000 bulk 3e8 shear 2e8
;
model gravity 0 0 -10
;
block gridpoint apply vel-x = 0.0  vel-z = 0.0 range pos-z 0.0
;
history mechanical unbalanced-maximum
model cycle 250
;
block zone cmodel assign mohr-coulomb
block zone prop  dens 2000 bulk 3e8 shear 2e8 friction 34


At this point in the run, the stresses generated by the initial elastic model still exist and act as initial stresses for the region containing the new Mohr-Coulomb model.

Two points should be remembered. First, if a null block is created (via the block excavate command) in any part of the model (even if it is subsequently replaced by another non-null block), all stresses are removed from the null block. Second, if one material model is replaced by another and the stresses should physically be zero in the new model, then a block insitu command must be used to reset the stresses to zero in this region. This situation would occur if rock is mined out and replaced by backfill; the backfill should start its life without stress.