Concrete Pressure Pipe for Sewer Applications

As migrating population trends force our surroundings to expand and evolve into larger and more populated communities, public utilities are constantly being challenged to meet the needs of and the demands imposed upon the infrastructure which serves these communities. The facilities which convey sewage make up a critical and integral part of any community's growing infrastructure.

The laws which have been put in place by Congress, and implemented and enforced by environmental authorities, in particular the U.S. Environmental Protection Agency, have mandated more stringent water quality standards. As public utility systems come under closer scrutiny and more restrictive regulations, economic decisions must be made. Compliance might require the installation of new systems, or upgrading the existing facilities may be an option. In the specific case of a sewerage network, a "tie-in" to a regional wastewater or water reclamation district may also be a solution. More often a previously constructed regional sewerage system would be extended to accommodate the new demands.

In addition, environmental concerns and increased treatment costs are requiring public utilities and the civil engineering firms that support them to design conveyance systems with low infiltration/exfiltration characteristics. Designs must meet present and future needs as well as comply with all federal and regional standards. And it must be kept in mind that systems such as sewer outfalls and wastewater treatment plant effluent pipelines that terminate in rivers and oceans also function under pressure.

Design Considerations

System performance is dependent on a pipe product which will convey the wastewater as required, and have reliable watertight joints to prevent infiltration and exfiltration. Concrete pressure pipe is one type of pipe which works successfully in the initial collection and transmission phases of a sewer system. It can serve also as the "inside-the-fence" yard and process piping in the treatment plant. The pipeline system's hydraulics can be designed to utilize either gravity or pressure to carry the waste stream, although sometimes it is more economical to design a hybrid system.

Site conditions and design characteristics can vary considerably from project to project. The data from subsurface exploration might indicate areas that have a high water table, a low-compression organic soil or a rock stratum.

Long reaches of gravity sewer systems may require progressively deeper trench excavations and wider embankment installations. Such conditions also might require that the pipeline be elevated and operated under pressure with the use of pumps. The operation and maintenance costs of pumping can be easily offset through reduced construction costs as a result of shallower excavation, and less effort to transport and put in place the selected bedding material. A sewer force main constructed of concrete pressure pipe requires fewer manholes than gravity systems, and generally, the longer lengths of pipe installed result in fewer joints in the pipeline.

Performance Requirements

The joints used for connecting lengths of concrete pressure pipe in sewer force mains exhibit the same performance characteristics as those installed in drinking water distribution system mains. This property assures owners and engineers that the end result of sewerage installation project is a watertight system. However, the joints may need protection from aggressive fluids that may show up in the waste stream. Epoxy and zinc coatings are used for pipe diameters under 30 inches, while mortar is applied where the pipe is over 36 inches in diameter.

Regardless of whether the sewer systems being constructed are of the gravity type or are force mains, they may have to be installed in environmentally sensitive areas. For instance, the pipes might have to be laid adjacent to, through, under, or over natural waterways, aquifers or water mains. Also, these systems may have to traverse wetlands, which today are receiving substantially more attention than in the past in new regulations being prepared. Most engineers, when addressing these various conditions, recommend that sewers be designed and constructed to water pipe standards. The proven performance of the joint and barrel system used for concrete pressure pipe lends itself well to such designs.

An Engineered Product

To complement the concrete pressure pipe itself, an assortment of special pipe fittings is available. Examples are integral base-tee manholes and riser sections. The manholes can incorporate drops and deflections, or use elbows in lieu of manhole sections when access is not necessary.

A sewer force main operating under pressure can be designed to utilize restrained joints. Wherever an unbalanced force is anticipated due to dead-ends, such as bulkheads or tees, or changes in alignment such as elbows, the pipe joints can be restrained. That allows for a completely integral thrust restraint system which avoids the use of thrust blocks.

In areas where the soil quality is poor, pile-supported pipe is a viable alternative. Single or simple span pile supports can be used. The types of supports can vary and be designed with chocks or concrete pile cap supports, which can reduce costs.

Transient conditions are common and can be severe in sewer force mains installations, primarily because of the cyclical pumping which occurs in these systems. Concrete pressure pipe is designed for transient conditions and is capable of handling either negative or positive pressure surges. Also, this type of pipe is a rigid structure which is designed to resist not only internal pressures, but also the applied external loads. The reinforcing steel built into the wall of the pipe is protected by its highly alkaline surroundings, a condition which which passivates the steel. Portland cement is an excellent corrosion inhibitor in most soil types.

The Interior Environment

The control and prevention of hydrogen sulfide generation or its conversion to sulfuric acid, can be accomplished with proper engineering design, and has been extensively studied by the engineering community. When that is not possible, the use of recognized and approved lining systems can mitigate the effects of sulfuric acid on cementitious materials in sectors of the pipeline which do not flow under full conditions.

This article first appeared in TIPS, a technical information paper produced by the American Concrete Pressure Pipe Association (ACPPA). It was adapted with permission and edited for presentation in Water Online by Ian Lisk.

<%=company%> recommends the following publications as references to assist in developing design specifications for pipelines.

Sulfide and Corrosion Prediction and Control Design Manual, American Concrete Pipe Association.

Concrete Pipe Handbook, <%=company%> , 1988.

Design Manual, Odor and Corrosion Control in Sanitary Sewage Systems and Treatment Plants, Center for Environmental Research Information, U.S. Environmental Protection Agency, EPA/625/185/018, October, 1985.

<%=company%> is located at 11800 Sunrise Valley Drive, Suite 309, Reston, Virginia 20191; Tel. 703-893-4350; Fax. 703-821-3054.