Bending Stresses From External Loading on Buried Pipe

ABSTRACT:

The pipeline industry has long been interested in evaluating the effects of external    loading due to fill and surface loads on buried pipes.  This interest stems not only from the initial design of pipeline systems, but also from the need to evaluate changing loading conditions over the life of the pipeline.  Variations in loading conditions can arise due to the construction of roads and railroads over the pipeline and one-time events in which, for example, heavy equipment must cross the pipeline.  A common issue is determining what pressures are safe during excavation and repair procedures.  Design codes, regulations and industry publications offer little guidance on what factors should be considered to determine safe pressures during in-service excavation activities.  Surface loading conditions and soil overburden result in stresses that should be evaluated in determining safe excavation pressures near areas of damage or corrosion.  Large concentrated loads, like truck wheel loads, are of primary concern.

The classic Iowa formula was derived to predict the ring deflection of flexible culverts, and not as a design equation to determine the wall thicknesses of pipes.  It is often used to estimate wall stresses, however, and determination of the total stress is important to safety calculations.  The use of the Iowa formula to calculate the wall-bending stresses in a pressurized buried pipe can, under certain circumstances, lead to results that behave strangely, particularly for high vertical loading.  Inclusion of a pressure stiffening term in the stress equation appears to improve the behavior and remove some of the excessive conservatism inherent in the Iowa formula.  At high vertical loading pressures and low internal pressures, the wall buckling limit may be the dominant factor in the minimum allowable wall thickness.

Matta, Lawrence M., “Bending Stresses From External Loading on Buried Pipe”, Pipeline and Gas Journal, June 2011, Vol. 238 No. 6, pp. 69-71.