Cast Iron or PVC: Selecting the Right Pipe for Underground Drainage

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Choosing between cast iron or PVC DWV is a critical part of designing plumbing systems for commercial underground drainage applications in North America.

The decision becomes more complex when more choices are introduced, such as whether to use service, no-hub, or extra-heavy cast iron — or whether to approve foam core PVC when solid wall PVC has been specified.

“The short answer is that the perfect piping solution for all applications does not exist,” said Brian Helms, Training Manager at Charlotte Pipe and Foundry. “The selection of the correct material for an application is as unique a decision as the requirements for any specific project.”

This article provides plumbing engineers and designers with general information on the performance properties of PVC and cast iron soil pipe (CISP) drainage systems with an emphasis on underground applications. It also details applicable standards and aids for material selection.

What are the performance differences between Cast Iron Soil Pipe and PVC DWV?

Both PVC DWV and cast iron soil pipe have proven track records of excellent service over the life of a building. There are, however, distinct performance differences between PVC DWV and cast iron piping systems, including:

  • Maximum working temperature

  • Combustibility

  • Initial cost

  • Life expectancy

  • Structural strength

Examining Cast Iron Soil Pipe

The term “soil pipe” goes back to the origins of plumbing. The name, which predates the term “sanitary waste,” refers to a piping system designed to carry the “night soil” or wastewater. Although it may have been common practice to empty the night soil from chamber pots into the streets of medieval Europe, thankfully sanitary standards, as well as the quality of life, have improved significantly with the advancements in piping systems and system design.

Charlotte Pipe’s cast iron pipe and fittings are manufactured from 100% postconsumer recycled content. This is an important consideration for green building programs, such as LEED certification under the U.S. Green Building Council. Additionally, cast iron can be readily recycled at the end of its service life.

As a rigid piping system, cast iron soil pipe offers great structural strength, which significantly reduces hanger requirements and provides consistent fall.

“As a rigid piping system, cast iron soil pipe offers great structural strength, which significantly reduces hanger requirements and provides consistent fall,” Helms said. “In addition, the graphite content provides cast iron with natural corrosion resistance to sanitary effluents.”

Applicable Standards for Cast Iron Soil Pipe

  • CISP has a maximum working temperature of 212 F with ASTM C564 gaskets (higher for lead and oakum joints) and good corrosion resistance to sanitary effluent.

  • Cast iron soil pipe is manufactured to ASTM A74 for service and extra-heavy pipe and fittings and CISPI 301 and ASTM A888 for no-hub pipe and fittings.

  • Extra-heavy and service soil pipe and fittings are commonly joined with compression gaskets conforming to ASTM C564. However, they may also be joined with lead and oakum.

  • No-hub pipe and fittings are joined with shielded couplings conforming to CISPI 310, ASTM C1277, or ASTM C1540 and a thermoset gasket conforming to ASTM C564.

  • ASTM E136 classifies cast iron as a noncombustible material, making it acceptable for use in plenum spaces without restriction.

ASTM A888, ASTM A74, and CISPI 301 require the raw material to be 100% screened for radioactive material. The standards also require the iron to be subjected to spectrographic analysis to ensure it meets specified metallurgical content. Tensile strength testing is also required; test bars must meet or exceed 21,000 pounds per square inch.

Finally, cast iron soil pipe is required to be date-coded so that records for a production lot of pipe can be matched to quality control records. According to the standard, these records must be maintained for seven years and must be made available upon request.

Exploring PVC DWV

PVC has a high strength-to-weight ratio, a broad chemical resistance, and the unique ability to be solvent-welded, a process that forms a permanent cross-molecular bond between pipe and fittings. These performance characteristics, combined with its relatively low cost, make PVC the most widely used piping material in the world.

Charlotte Pipe’s PVC DWV products have excellent resistance to chemical and sanitary effluent and a maximum working temperature of 140 F,” Helms said. “However, as a flexible piping system, PVC requires greater support and care in underground applications than a rigid piping system, such as cast iron.”

Additionally, PVC will expand approximately 3.6 inches per 100 feet of pipe with a 100 F temperature change.

“Compensation for thermal expansion should be designed into systems and generally can be accomplished through the use of offsets or expansion joints,” Helms said.

Compensation for thermal expansion should be designed into systems and generally can be accomplished through the use of offsets or expansion joints.

Applicable Standards for PVC DWV

  • PVC DWV is manufactured to ASTM D2665, which is a system standard addressing solid wall PVC pipe, fittings, and solvent cement. Solid wall PVC is also commonly manufactured and listed to ASTM D1785 for pressure-rated pipe and is thus dual marked.

  • Foam core pipe is manufactured and listed to ASTM F891 and is suitable for non-pressure applications only. Cellular core PVC pipe is designed to be used with ASTM D2665 fittings.

  • Solvent cements shall conform to ASTM D2564. Primer is required to join PVC properly and shall conform to ASTM F656.

  • Pipe and fittings manufactured to ASTM D2665 or ASTM F891 are listed to NSF Standard 14. This establishes minimum physical and performance properties, testing frequencies, and health effect requirements for plastic piping systems. It is also the basis for third-party conformity assessment programs required in most model plumbing codes.

  • ASTM D2665 and ASTM F891 require pipe and fittings to be produced from virgin material. At the end of its service life, PVC can readily be recycled into other products.

What are the fire properties of PVC?

PVC is classified as a combustible material. Yet, with a low heat contribution of approximately 5,000 British thermal units per pound (Btu/lb) and a high limiting oxygen index of 40, it cannot support combustion in the Earth’s atmosphere.

However, because PVC will not pass an ASTM E84 or ULC S102.2 test with index values of less than 25 flame spread and 50 smoke developed, it will not meet the requirements for inclusion in plenum areas found in mechanical codes.

Finally, listed fire-stop devices or collars conforming to ASTM E814 must be used when penetrating a fire-rated assembly.

The Difference Between PVC DWV and CISP in Underground Applications

For underground installations, the most significant difference between PVC DWV and CISP is how they support or fail to support external loads.

When rigid piping systems, such as cast iron, fail under an earth or live load, they get crushed. For this reason, rigid piping systems can be evaluated with a ring crush test. Also, the derived values can be used to verify that the pipe will withstand the required external load.

Flexible piping systems, such as PVC and ABS, are considered to be in a failed condition when earth or live loads have caused the pipe to deflect 5% or more.

CISP in Underground Applications

Evaluating cast iron for required earth or live loads is relatively straightforward. The Cast Iron Soil Pipe Institute publishes maximum crush load data on no-hub, service, and extra-heavy cast iron soil pipe in the Cast Iron Soil Pipe and Fittings Handbook.

“According to the Handbook’s Maximum Crushing Load data, extra-heavy soil pipe can withstand up to a 58% greater crush load than service. This is useful to designers working with applications subject to demanding live loads or shear forces,” Helms said.

The Handbook also provides tables containing earth load and live load values for varying trench widths and depths. This data can be used in combination with maximum crush load tables to evaluate soil pipe for a specific application. Additionally, the Handbook publishes tables for maximum allowable trench depth, which indicates the maximum burial depths for soil pipe.

Overall, there are only a few special requirements for burying cast iron soil pipe:

  • The trench should be dug to a minimum width to accommodate the size of the pipe.

  • The trench bottom should be stable and provide uniform support for the pipe.

  • No special bedding is required.

  • CISPI 310 no-hub couplings may be used for below-ground installations.

  • Backfill requires no special procedures, except that it is free of large rocks.

“Because service and extra-heavy hub and spigot cast iron systems offer greater resistance to shear forces and can be easier to install in underground applications, it’s common to specify no-hub above ground and service or extra-heavy below ground,” Helms said.

PVC DWV in Underground Applications

When planning for the underground installation of PVC, a frequently asked question is: “What is the crush strength of PVC pipe?”

While many hope for a simple answer in tabular format, the complexity of the question often requires a more in-depth analysis.

As a flexible system, PVC pipe works with the surrounding soil to support an earth or live load, termed the “soil-pipe mechanism,” which is calculated by the Iowa formula. For that reason, the crush strength of a section of PVC pipe in a press without the support provided by the surrounding soil is not relevant.

The Uni-Bell PVC Pipe Association provides a calculator based on the Iowa formula that allows a designer to input variables specific to the application, including:

  • Maximum allowable deflection

  • Soil modulus

  • Load parameters

  • Additional live loads

  • Pipe diameter

  • Pipe stiffness

Pipe stiffness values for these equations can be taken from the applicable ASTM standards and are available from pipe manufacturers, including Charlotte Pipe upon request.

"It’s worth noting that stiffness is constant for each pipe diameter for pipe manufactured to a dimensional ratio, but varies by pipe diameter for IPS systems, such as Schedule 40 and 80 PVC,” Helms said. “For many piping systems, 5.0% to 7.5% deflection is commonly used to define the point of failure.”

PVC requires specific techniques for burying, as detailed in ASTM D2321. These include:

  • Excavating the trench width in direct proportion to the size of the pipe.

  • Carefully preparing the trench to provide a minimum of 4 inches of firm, stable, and uniform bedding material (6 inches if rock is encountered).

  • Providing critical haunch support for the pipe by backfilling the appropriate aggregate in 6-inch layers, with each layer sufficiently compacted 85% to 95%.

Flexible piping systems, such as PVC, are dependent on proper compaction and backfill for their ability to withstand an external load. Naturally, if these systems are not installed correctly, they will not be able to withstand the loads they were designed to carry and are considered to be in a failed state on day one of service.

Is foam core PVC an option for underground applications?

When foam core PVC pipe, or ABS, was introduced in the United States in the 1980s, it was touted as being lighter weight, lower cost, and quicker cutting. To produce foam core PVC, a layer of foam is sandwiched between a thin inner and outer layer of solid PVC.

“Essentially, foam core PVC has less PVC than solid wall PVC pipe,” Helms said. “Because of this, and several other factors, Charlotte Pipe cautions plumbing professionals on using foam core PVC in commercial applications.”

Finding the Right Solution

Due to the countless variables involved in designing commercial underground drainage applications, design engineers must have the tools and resources necessary to make decisions on the best material to use.

The differences in the performance properties of PVC and cast iron soil pipe drainage systems can be complex for even the most experienced plumbing designer or engineer. By using the available industry resources to weigh the requirements of an application against cost and ease of installation considerations, plumbing professionals can make the best determination for which material to use.

Still have questions? Contact Charlotte Pipe’s Technical Support team for assistance in determining the right piping solution for your commercial project.

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