Subsidence and Deflection

Subsidence, or the sinking of the soil underneath a pipeline, can be caused by the soil collapsing into a void or a mine opening under the surface. Subsidence subjects the pipes to new, often unexpected, vertical forces, or bending moments. These forces, will bend, or "deflect" the pipe, and if great enough, break it at some critical point of vertical displacement.

As pipe sections bend, they begin to exert axial, or stretching forces, on the pipeline. The size of the axial force that develops is dependent on the method of coupling lengths of pipe together. The coupling may allow the pipe to slide apart to an extent, while not completely disconnecting, which relaxes the axial stress in the pipe in the run as it settles.

If the subsided area spans one or more pipe joints, the lengths of pipe will often form a series of segments in an arch, which will act as shortened spans that may sustain a higher bending force than a longer segment. The analysis done here does not allow the pipe to elongate at its connections.

Comparison of Ductile Iron vs. PVC

When engineering pipe installations, we are seeking to minimize deflection, but at the same time need to choose materials that are able to withstand bending and stretching forces.

Ductile Iron pipe is a more rigid and brittle substance than PVC; rather than bending and stretching, it resists both bending moments and axial forces up to the point of failure, resulting in a crack. PVC pipe, by itself, could withstand greater vertical deflection. Assuming a fused length of PVC pipe, with the fusion having the same strength characteristics as the pipe, the increase in axial stress will elongate the pipe, allowing it to settle into the subsided area.

As long as the axial stress remains below an acceptable level, and the pipe wall thickness is not decreased to the point that the pipe falls into hoop stress due to the internal pressure of the pipe, the PVC pipe will not fail.