In the estimation of the axial strain due to seismic wave propagation, the strains computed from
Equations 3.1 and 3.2 are compared. Until slippage occurs between pipeline and soil, Equation
3.1 is valid, however, once the slippage occurs, the strain of the pipeline cannot exceed the
maximum strain value due to the frictional forces exerted on the pipeline by the surrounding soil.
For this situation Equation 3.2 is valid. That is why the smaller one is used as the seismic strain
due to wave propagation effects, εwp, to be summed up with the other strains due to the loads
considered in the combined strain analysis.