Engineered Solutions for Sustainable Water Systems
Concrete pressure pipe (CPP) is engineered and manufactured to meet the specific needs of a project. To understand how to derive the greatest benefits of CPP for your project, it’s helpful to know about the many choices this pipeline system offers in design and construction, and the positive impact these choices can have on pipeline design, ease of installation and maintenance.
Concrete pressure pipe is produced with flexible, rubber-gasketed push-on joints, which are made with steel bell-and-spigot rings sized to extremely close tolerances. The spigot has a groove on its circumference to hold an O-ring rubber gasket. As the spigot is inserted into the bell, the gasket is compressed on all four sides, creating a watertight seal. O-ring sealed joints permit deflection for long radius curves and retain flexibility to permit minor settlement, expansion and contraction. These joints are routinely designed and tested for pressures commonly used in water transmission and distribution systems.
Unlike CPP joints, steel pipe joints are typically welded in the field, which limits the pipe’s ability to move with the soil. Moreover, field welding has significant quality control issues compared with factory welding in a controlled environment. Alternatively, steel pipe is sometimes produced with a roll-formed, O-ring sealed joint that remains as flexible and dimensionally unstable as the flexible steel pipe itself, making it difficult to properly align the pipe for good welding procedures.
Since the 1960’s, CPP mechanically-restrained joints, such as the Harness-Clamp and Snap-Ring, have provided the owner/contractor with a fast and easy method of thrust restraint. Mechanically-restrained joints can transfer axial thrust forces while providing a degree of flexibility.
CPP can also be manufactured to provide an air testable joint – another quality that ductile iron pipe and steel pipe do not share. Double-gasketed air-testable joints allow the contractor to pressure test individual joints immediately after they are field-assembled to verify the gasket seal. This aspect of CPP’s design adaptability is especially important when the pipe is laid in areas where leaks would be difficult to discover or repair, or when the pipe is otherwise inaccessible, such as subaqueous installations, deep cuts and pipe in tunnels. CPP’s testable joints are also valuable features when connecting a new pipe section to an existing pipeline, such as during relocations, when it is difficult to isolate the area to be tested. In these cases, shutdown time is usually limited; therefore, it is important to know that the joint is properly assembled before the line is put back into service.
FITTINGS AND SPECIALS
All CPP fittings and “specials” are engineered and manufactured in accordance with the manual of water supply practice, Concrete Pressure Pipe (M9), 3rd Edition and related AWWA Standards for Concrete Pressure Pipe to meet the same pressure and load conditions as the adjacent pipe. Fittings (bends, reducers, tees, etc.) are custom fabricated from steel plate on a project-by-project basis. The fittings are attached to the same joint rings used on pipe or to other end connections as needed to connect to appurtenances or other pipe materials. The assembly is lined and coated with concrete or cement mortar. Specials, such as outlets for lateral connections and air-release valve manholes can be incorporated into full lengths of pipe. In these applications, the openings are reinforced with steel wrappers, saddles or crotch plates.
The word “special” is an appropriate descriptor for concrete pressure pipe. CPP is special because of the extraordinary capability to manufacture virtually any configuration, geometry, length or angle. Unlike ductile iron pipe, CPP designs are not forced to “make do” with standard fittings, which often require the contractor to install 2, 3 or 4 different fittings to accomplish one section of pipeline alignment. Wyes, tees, bends and standard pipe with multiple connections can be routinely produced. While this design adaptability is helpful to the owner/contractor on every type of project, it is especially useful in the piping layout for water and wastewater treatment plants, pumping stations, power plants and other congested areas where it is virtually impossible to achieve the desired result with the multiple standard fittings required with competitive products.
RANGE OF SIZES AND LENGTHS
Concrete pressure pipe is offered in a wide range of diameters, up to and sometimes exceeding 144 inches. Typical lengths are 16 to 24 feet (in some cases even longer), depending on the type and diameter of the pipe. Competitor products, in contrast, are generally available in a smaller range of dimensions and lengths. For example, diameters of ductile iron pipe typically range up to and including 64 inches in lengths from 18 to 20 feet, while steel is generally produced only in lengths of 40 to 50 feet.
While 20 feet may be an optimal pipe length for most projects, there are occasions when the owner and/or site conditions do not allow the use of full-length pipe. For example, obstructions may necessitate the use of non-standard pipe sections or “shorts.” While every type of pipe can be manufactured in shorter lengths, only CPP manufacturers have shown a willingness and capability to provide non-standard laying lengths. Competitor manufacturers encourage owners/contractors to purchase full lengths that must be field-cut to fit their needs, a practice that results in inferior joints and increased waste/cost. CPP manufacturers regularly produce pipe lengths as required by the project conditions. They also provide a pipe layout schedule based on the engineer’s plan and profile drawings that illustrate where each pipe section should be installed – making it easy for the contractor to lay the pipe in accordance with the plan.
Another advantage that CPP’s design adaptability offers over competitor products is the capability to permanently join two or more pipe sections at the manufacturing facility (or in the field) to provide lengths of 40 to 60 feet. While lengths of this magnitude are unnecessary for most projects, they can significantly reduce the duration and risk of underwater work during subaqueous installations at intakes, outfalls and river crossings. They can also allow a designer to create an aerial pipeline with a greatly reduced number of pier supports.
Concrete pressure pipe can be engineered to withstand extreme earth loads and live loads because of its rigid structural wall. CPP’s load carrying ability is built in, providing up to 70 percent or more of the strength of the pipe-soil structure. Unlike competitor products, the inherent strength of CPP often allows the designer to use native bedding and backfill material. Varying soil conditions along the pipeline alignment, whether existing or as constructed, will not have a major influence on the pipe’s performance.
Flexible pipe materials, including ductile iron pipe and steel, are greatly dependent on the support of the surrounding ground to provide sufficient strength to the pipe-soil structure. In fact, the flexible pipe itself may provide as little as 15 percent or less of the necessary strength, relying on the contractor to construct the balance, a sometimes-elaborate structural soil envelope. This generally requires strict control of the trench width, special imported bedding and backfill material and careful compaction of the soil in the pipe zone – adding to the cost of construction and transferring the onus of performance from the pipe to the contractor and field inspector.
When CPP arrives on the project, substantial structural strength arrives in it. However, when steel or ductile iron pipe arrives on the project, the contractor must create the proper support structure around the pipe. Unfortunately, the only way to do so is to provide for continuous inspection and testing of the contractor’s work – an unnecessary and costly addition to the project.
CPP also provides an additional defense against hazards experienced by flexible pipe such as vacuum collapse: unlike low stiffness flexible pipe, rigid CPP has a substantial resistance to buckling.
In most cases, CPP projects can be accomplished using standard wall thicknesses and dimensions. Occasionally, extreme conditions require special designs.
- Substantial Earth Loads — CPP can be manufactured with an increased wall thickness, permitting it to be designed for substantial earth loads. This design adaptability allows CPP to be used in special applications, such as a spillway beneath a dam with an earth cover of 100 feet or more. Competitor products, on the other hand, are typically only available in a number of standard thicknesses not suitable for such extreme external loads.
- Non-Buoyant Pipe — Increased wall thicknesses also allow a designer to increase the weight of the pipe, making it non-buoyant. The weight of CPP generally precludes it from floatation during construction and when installed with shallow earth cover, unlike lighter competitor products. What’s more, some subaqueous applications require the pipe to be negatively buoyant, even when completely empty and not backfilled. CPP can be engineered and manufactured to satisfy this requirement, while competitor products must be artificially weighted.
- Direct Jacking and Microtunneling — The heavy wall and uniform O.D. of AWWA C300 reinforced concrete cylinder pipe and AWWA C302 reinforced concrete pipe noncylinder, make them superior pipes for direct jacking and microtunneling. Both are manufactured with thick, high-strength concrete walls, providing an inherent axial strength that is resistant to buckling. When used for jacking and microtunneling, both C300 and C302 are manufactured with the same flexible O-ring rubber-gasketed push-on joint (as in all CPP) or with air testable joints, making them easy to join and confirm that they are watertight. Competitor products generally require substantial welding, expensive machined joints and increased wall thickness in order to be jacked without buckling.