Both the 2D and the 3D program allow for input of volumetric strain in x,y and z direction. Alternatively you may directly specify a volumetric strain (ε_vol) which basically means that the specified strain is equally distributed in x,y and z direction, i.e. ε_xx = ε_yy = ε_zz = ε_vol / 3.

When applying a volumetric strain to a cluster the following procedure is followed:

• first the program expands or contracts the relevant cluster according to the applied strain while maintaining the same stress level in this cluster;
• next the reaction stress/force coming from the surrounding soil and/or boundary conditions is determined based on this change in strain;
• then the unbalance due to this reaction stress/force is solved in the model introducing deformations which in the end may cause a lower strain to be calculated in the cluster with the applied volumetric strain;
• finally a stress equilibrium is found between all the relevant clusters and/or boundary conditions.

So realise that the stiffness ratio between the cluster with volumetric strain and its surrounding clusters determines the final deformations and stress levels.

In example: let’s assume a cluster with stiffness E1 which has a certain applied volumetric strain and the surrounding clusters with stiffness E2 with no applied volumetric strain.

Now for the following cases we find:

• if E1 >> E2: then ε_{vol,calculated} = (approximately) ε_vol,applied
• if E1 << E2: then ε_{vol,calculated} = (approximately) 0