Bending Stresses From External Loading On Buried Pipe

The pipeline industry has long been interested in evaluating the effects of external loading due to fill and surface loads, such as excavation equipment, 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.

The pipeline may also suffer corrosion or damage that requires excavation and repair. Heavy excavation equipment is often placed directly over a pipeline during repair work, as shown in Figure 1. Safety while excavating pressurized pipelines is a serious concern for operating companies. Both gas and liquid pipeline companies often specify reduced pressures while excavating and repairing in-service pipelines.

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 ALA Guideline for the Design of Buried Steel Pipe presents design provisions for use in evaluating the integrity of buried pipelines for a range of applied loads. (ref: “Guideline for the Design of Buried Steel Pipe,” American Lifelines Alliance/ASCE/FEMA, 2001.) Its methodology offers an approach for evaluating the fill and surface-loading effects on buried pipelines. This approach utilizes the deflection of the pipe, calculated using a version of the classic Iowa Formula, in estimating the wall-bending stresses in the pipe. The wall-bending stress is then combined with other calculated stresses to calculate the overall stress in the pipe.

Figure 2: Schematic of the deflection of a buried pipe due to vertical loading.

Smith and Watkins pointed out that the 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. (ref: Smith, G., and Watkins, R., “The Iowa Formula: Its Use and Misuse when Designing Flexible Pipe,” Proc. of Pipelines 2004 Int’l Conf., ASCE, 2004.) It is often used to estimate wall stresses, however, and determination of the total stress is important to safety calculations. In this article, the wall-bending stress calculation and some quirks in its behavior will be discussed.

Pipe materials are classified as being either flexible or rigid. A flexible pipe has been defined as being able to deflect at least 2% without structural distress. (ref: Moser, A.P. and Folkman, S., “Buried Pipe Design, 3rd Ed.,” McGraw Hill, 2008.) Materials such as steel and most plastics are considered flexible pipe. Concrete and clay pipes are considered rigid. The Iowa Formula was developed for use with flexible pipes.

Flexible pipes derive much of their load-carrying capacity from pressure induced at the sides of the pipe as they deform horizontally outward under vertical loading. Analysis of the effect of fill weight and surface loading is therefore a problem of interaction between the pipe and the soil. The Iowa Formula describes the interaction of the pipe and soil and the deflection that results from vertical loading.

Figure 3: Effect of wall thickness ratio on the normalized wall-bending stress

In his research of the performance of buried flexible pipes, M. G. Spangler observed that, compared to rigid pipes, flexible pipes provide little inherent stiffness and perform poorly in 3-edge bearing tests. However, flexible pipes performed better than predicted by these tests when buried. He reasoned that the source of strength of the flexible pipe is not the pipe itself, but is primarily the soil beside the pipe. (ref: “Insight into Pipe Deflection Predictions: An Interview with M.G. Spangler,” Sewer Sense No. 17, National Clay Pipe Association, 2004.) 

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