How can I calculate the strain in the concrete tunnel lining in RS2?

The approximated axial strain in the liner element can be calculated from the axial stress and the bending moment.

The axial strain equals ε=1/E (σa ± Mc/I), where σa is the axial stress, M is the bending moment, c is the perpendicular distance to the neutral axis, and I is the second moment of area. 

Note that the above strain formulation is not valid in the plastic regime.  In RS2, the liner is divided into a number of layers.  Stresses are assumed to be constant over the layer.  If stresses exceed the peak stress they drop down to the residual stress.  When plasticity occurs, since the stress is no longer linearly distributed, the strain equation is no longer valid.