Bi-linear Soil Stiffness Model

For a buried piping system the soil around the pipe acts to restrain the piping. The stiffness of the soil, in other words, how much restraint the soil provides for a given pipe displacement is determined by the compaction of the soil. In the figure below the difference in resistance between loose soil and compacted soil is shown.

For smaller pipe displacements, the larger stiffness of the compacted soil results in a larger soil reaction force when compared to loose soil. However, the figure also shows that with a large pipe displacement, when soil fracture occurs, for both loosely compacted and compacted soil the ultimate resistance that can be generated by the soil is the same.

In a piping stress analysis the soil resistance underground is typically simulated using a bi-linear assumption, shown by the dotted line in the figure. In the largely linear region, a stiffness value dependent on the compaction of the soil is used whilst in the soil fracture region a constant equal to the ultimate soil resistance is used.

To learn more about this soil stiffness model please see our self-paced course on soil/pipe interaction for piping systems or our learning path on buried piping engineering.