[Article Sharing] Flexible Pipe

Rigid and Flexible Pipes | Trenchless Technology Magazine

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Buried pipelines for the conveyance of potable water and sanitary sewers form the bedrock of our civilization. The very basic services that we take for granted, such as a running faucet with clean drinking water and the ability of our wastewater to be transported away from our homes and businesses for treatment and release into the environment, are possible because of the vast network of buried pipes that lie hidden beneath roads and the concrete jungles that define the large urban centers that are our cities.

These intricate networks consist of various types of pipe materials that range from metals to concrete to plastics to composites. Whatever the material type, all are expected to provide some minimum service qualities: prevention of leaks, which ensure that potable water isn’t compromised by contaminants entering a piping system and that wastewaters do not pollute soils and existing sources of groundwater, and most importantly, that the structural soundness of a buried pipe system results in a minimum service life of 50 to 100 years. The latter can only be ensured through a thorough understanding of and designing for the loads to which a buried pipe will be exposed, its response to the loading, and the interaction mechanism between the pipe and surrounding soils.

While an understanding of pipe-soil interaction is important for the sound structural design of pipelines, it should be noted that the concern for soil pressure on a pipe is limited to empty pipe or gravity flow pipelines where the conduit never flows full. In municipal pressure piping systems, the internal pressure is typically much greater than soil pressure on the pipe; internal pressure essentially supports the soil load when the line is placed into service.

The story of the formal study of buried pipe structures in North America begins in Ames, Iowa, at the turn of the 20th Century, when Dr. Anson Marston, then Dean of Engineering at the Iowa State College (now Iowa State University) and the first Chairman of the Iowa State Highway Commission, analyzed soil pressures on buried culverts in an effort to drain muddy rural roads. Research was also necessary as thousands of small wooden bridges were being replaced by concrete and clay pipes placed in stream beds underneath roadway embankments. It was necessary to design them properly so that they would not fail.

Buried pipe design, then, was inextricably connected to the development of highway systems in the United States. Marston was the first Chairman of the federal Highway Research Board. Our knowledge of pipe-soil interaction, as well as the development of new pipe materials and the improvement of traditional ones, has grown by leaps and bounds since those early days, leading to a multi-billion dollar global pipe materials industry. Another relatively new construction method in buried pipeline construction and rehabilitation is the advent of trenchless technology.

While trenchless technology is no longer in its infancy, the only known and practiced trenchless construction methods during Marston’s lifetime included jacking and tunneling, and little else. But a tremendous amount of research in the last two decades now provides design engineers and contractors with an understanding of the pipe-soil mechanics of pipelines built by methods other than traditional open-trench methods.

Classical Pipe-Soil Interaction Theory

In 1913, and 1930, Marston published his original papers “The Theory of Loads on Pipes in Ditches and Tests of Cement and Clay Drain Tile and Sewer Pipe” and “The Theory of External Loads on Closed Conduits in the Light of the Latest Experiments,” respectively, marking the earliest systematic approach of studying the structural mechanics of buried pipes. Thus was defined the Marston Theory of Loads on buried conduits.

This became, in part, the very foundation on which much of the later work around the world on earth loading technology of buried pipes was based. In 1941, Marston’s student Merlin Spangler, known today as “the father of buried flexible pipe design,” published another ground-breaking paper, “The Structural Design of Flexible Pipe Culverts,” in which he derived an equation, the Iowa Formula, for predicting the ring deflection of buried flexible pipes. Spangler would later become chairman of the Culvert Committee of the federal Transportation Research Board. In 1958, Spangler’s student, Reynold Watkins, published “Some Characteristics of the Modulus of Passive Resistance of Soil – A study in Similitude,” in which he solved a fundamental flaw in the dimensions of a modulus of passive resistance in Spangler’s Iowa Formula, defining a new modulus of horizontal soil reaction, E’, in the Modified Iowa Equation. In later years, other significant contributions came from A. Howard, J. Duncan, J. Hartley, F. Heger, T. McGrath, M. Zarghamee, and others that now enable the external load design of rigid and flexible pipes to be an even more exact science.

The Marston Load Theory

In his analysis of external loads on buried pipe, Marston defined two main types of loading conditions of buried pipes, a ditch conduit (referred to as trench load condition in present day nomenclature), and a projecting conduit (referred to as an embankment condition in present day literature), Table 1.

The basic concept of the Marston Load Theory is that the load on a buried pipe, because of the weight of the column of soil, or central prism, directly above the pipe, is modified by the response of the pipe and the relative movement of the side columns of soil, or external prisms (adjacent to the pipe, between the pipe and the trench walls on either side), to the central prism. The relative movement of the central prism and the side prisms result in shearing stresses or frictional forces, calculated using Rankine’s theory.

Rigid Pipe

Marston recognized that in a trench (generally, trench width = 2 x pipe diameter), when the side columns of soil or the external prisms are more compressible than the pipe due to its inherent rigidity, this causes the pipe to assume load generated across the width of the trench. The shearing stresses or friction forces that develop due to the differential settlement of the external prisms and the central prism are additive to the load of the central prism alone. Pipes that behave in this manner are referred to as Rigid Pipes. Generally, rigid pipes start showing signs of structural distress before being vertically deflected 2 percent. Rigid pipes include reinforced non-cylinder concrete, reinforced concrete cylinder, prestressed concrete cylinder, vitrified clay, polymer concrete, cast iron, asbestos cement and cast-in-place pipes.

Flexible Pipe

On the other hand, if a pipe is more compressible than the external soil columns, without any structural damage caused to the pipe as a result of its vertical deflection, allowing the central prism to settle more in relation to the external prisms, the actual load on the pipe is less than the load of the central prism due to the direction in which the shearing stresses or friction forces develop as a result of the differential settlement of the central prism in relation to the external prisms. Pipes that display this behavior when buried are referred to as Flexible Pipes. Use of the prism load is conservative because installers do not compact soil against the pipe. The pipe is embedded in a “packing” of less dense soil that serves the same way as does packing around an item in a shipping container. The pipe is relieved of part of the prism load which the soil then picks up in arching action over the pipe.

As a general rule, flexible pipes will deflect at least 2 percent without structural distress. Most flexible pipe material standards allow up to 5 percent deflection. Deflection is limited to 2 percent if the flexible pipe has a rigid lining and coating and 3 percent for a rigid lining and flexible coating. Flexible pipes include steel, ductile iron, thermoplastics such as Polyvinyl Chloride (PVC) and High Density Polyethylene (HDPE), thermosetting plastics such as fiberglass-reinforced polymer (FRP), bar-wrapped concrete cylinder pipe, and corrugated steel pipes.

Reference

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