Ductile Iron Proves Effective with HDD

Ductile Iron Proves Effective With HDD, Pipebursting

by Jeff Griffin
Senior Editor
Underground Construction

In many ways, plastic pipe and horizontal directional drilling seem to be made for one another.

Of course, plastic pipe was introduced many years before the development of the horizontal directional drilling (HDD) machines designed for utility construction, but as directional drills took over an increasing share of the underground construction workload during the 1990s, plastic pipe was what these trenchless machines installed most often. Continuous runs of HDPE pipe for natural gas distribution lines and duct and conduit for telecommunications and power cable became routine HDD installations. Indeed, much of the world's fiber optic cable is protected inside HDPE ducts.

Today, directional drills are installing more and more PVC and HDPE pipe in segments of potable water and sanitary and storm sewer systems.

Even as plastic pipe suppliers aggressively promote their products to the public works market, many water and sewer system operators continue to prefer and specify ductile iron pipe for new construction and pipe replacement projects.

"The use and demand for ductile iron pipe by the public works segment continues to be strong," says Ralph Carpenter, marketing specialist at American Ductile Iron Pipe Co., Birmingham, AL. "In many parts of the country, owners and engineers -- for a variety of reasons -- prefer the inherent strength, toughness and versatility of ductile iron pipe for their systems."

In addition, says Carpenter, ductile iron pipe is just as suitable for installation by directional drilling and pipe bursting procedures as is HDPE and PVC pipe, and in some respects may be even better suited for trenchless installations than plastic materials.

However, he adds, some in the industry are not aware of this.

"We find water and sewer system operators, design engineers and directional drilling contractors who do not know that ductile iron pipe can be effectively installed by trenchless methods," says Carpenter. "Many incorrectly assume that all ductile iron pipe joints will pull apart during a HDD or pipe bursting installation."

Wrong impression

Carpenter and others in the ductile iron industry want to correct this misconception. Flexible restrained joints effectively hold ductile iron pipe together during the pull-in process of both directional drilling and pipe bursting, while still allowing joints to deflect as a bore path changes direction.

Says Carpenter: "With the growing trend toward trenchless construction in areas where open-cut construction is disruptive or impossible, we believe that it is important for those who design, construct and operate public works systems to know that they can cost-effectively place ductile iron pipe in the ground by proven trenchless construction methods."

American Ductile Iron Pipe, the Ductile Iron Pipe Association (DIPRA) and others are actively promoting ductile iron pipe's appropriateness for trenchless installation. To obtain data to support these efforts, American, DIPRA, Advanced Boring Technologies of Gilbert, AZ, and the Arizona State University (ASU) Del E. Webb School of Construction conducted tests in Phoenix to document whether ductile iron pipe can be effectively installed by directional drilling.

Phoenix was selected for the tests in part because the city does not use plastic pipe in its water mains; the majority of the 5,500 miles of mains are ductile iron (see the October  2002 issue of Underground Construction for details).

"The study confirms that the use of restrained flexible joint ductile iron pipe permits alternative HDD installation techniques for pulling ductile iron pipe into the pilot bore hole," says Dr. Samuel T. Ariaratnam, associate professor at ASU's School of Construction and director of the university's part in the study.

Field research for the study was conducted in Phoenix. Two hundred feet of 8-inch diameter American Flex-Ring joint ductile iron pipe was installed to a depth of 6 feet with a Vermeer 24x40 HDD machine.

There are two methods of installing restrained joint ductile iron pipe with HDD equipment.

"Installation can be accomplished by either a cartridge or assembled joint method," writes Ariaratnam in ASU's report on the study. "The cartridge method involves connecting the joints during installation, one at a time, and is preferred in locations where there is limited right-of-way or easements which are needed to string out the pipe on the ground prior to pull-back. This method is made possible due to the ability of flex-ring [pipe] to be assembled quickly with a limited amount of intervention by the installation crew.

"The assembled joint method involves connecting all of the joints for the entire installation length and stringing out the pipe prior to installation. Both HDPE and welded steel pipe use the assembled joint method due to time required for joint fusion and welding, respectively."

With both HDD installation techniques, pipe sections are pulled into the bore hole with spigots ahead, allowing drilling fluid and slurry of excavated material to flow easily over the smooth contour of the bells.

Restrained joint the key

Clearly the restrained joint is the key to successfully installing ductile iron pipe by directional drilling and, by implication, pipe bursting.

The Flex-Ring joint was developed by American Ductile Iron Pipe in the early 1980s for purposes unrelated to directional drilling. The joints are used in buried piping systems when axial thrust restraint is required for restraining bend and other fitting configurations.

"The joint," the report explains, "is designed to provide liberal joint deflections, up to 4 degrees for 14- through 36-inch pipe, while maintaining full thrust capabilities. When deflection or articulation of the joint occurs, the 'wedge-shaped' flex-ring segments slide up or down the mating 'wedge-shaped' slope of the inside of the flex-ring bell maintaining joint restraint and effectively redistributing the thrust or pulling force around the face of the bell. It is this functionality that makes flex-ring ideal for trenchless applications including horizontal directional drilling and pipe bursting of water and sewer lines. In 4-inch through 12-inch Flex-Ring, a similar flexible 'wedge-shaped' design principle is employed allowing an even more generous joint deflection of 5-degrees. Finite element analysis of the Flex-Ring joint has confirmed the ability of the joint to effectively redistribute the thrust or pulling loads. Confirmation of this analysis was made possible by hydrostatically testing, to failure, Flex-Ring pipe in straight and fully deflected alignment. When taken to failure, the deflected configuration failed within 10 to 20 psi of the pipe in straight alignment with virtually identical failure modes.

"Joint analysis confirmed stresses were redistributed through the quarter section of joint and into the pipe barrel. This redistribution is critical in HDD installations in that it dramatically reduces any concentration of stress around the joint and pipe barrel during pull-back, unlike some other joints, and also unlike continuously fused HDPE or steel pipe lines. When HDPE or steel welded-joint pipes are installed using HDD technology, in the typical shallow arc, additional tensile stresses are induced into the pipe wall on the long side of the arc, while compressive stresses are introduced in the pipe wall on the short side of the arc."

One impediment to consideration of attempting HDD installations of ductile iron pipe has been the belief that greater pulling force is necessary than is required with plastic pipes.

The study challenges this view.

"Case histories," comments the report, "have given indications that pulling loads were less for ductile iron pipe than they typically are for similar size HDPE pipe. One reason for this is that the typical bulk density of typically empty ductile iron pipe is normally closer to that of the soil/fluid slurry than it is with lighter pipe materials. Therefore, there may often be very little normal force to result in increased friction against the walls of the bore hole as the pipe is pulled back."

High quality

During the Phoenix installation, recorded hydraulic pullback pressures were less than 100 psi, which is minimal for the equipment used, the installation length and pipe diameter installed.

To affirm the quality of the assembled pipeline, a successful in situ hydrostatic test of 200 psi was conducted a week after installation during which the target pressure was maintained for two hours without loss.

The test installation also evaluated the effectiveness of various polyethylene encasement materials and investigated methods of creating electrically continuous pipelines for the purpose of corrosion monitoring.

After four months in the ground, the test pipe was uncovered to observe the performances of the two types of encasements. Exposed pipe and surrounding soil were exposed to constant temperature in excess of 100 degrees F.

"It is appropriate to note," states the report, "that the strength and performance of ductile iron pipe are not compromised by this elevated in situ temperature . . . unlike plastic pipes where strength and performance capability of [plastic] material is reduced as temperatures rise above the published 74 degree F threshold." (Concern over plastic pipe's potential susceptibility to heat is one reason cited by Phoenix water officials for not using plastic pipe in the city's water distribution system.)

Field tests

In addition to the Arizona tests, recent projects confirm the compatibility of ductile iron pipe and directional drilling and pipe bursting, says Carpenter.

In Pawleys Island, SC, the county water and sewer district wanted to use HDD to install approximately 500 feet of 20-inch diameter pipe through environmentally-sensitive marshlands and under several creeks. Because of the pipe's proximity to a rail line and concerns about external loading induced by rail traffic, planners also wanted to use ductile iron pipe.

Although project owners were unaware ductile iron pipe could be installed by directional drilling, the contractor explained to them that a trenchless HDD installation could, in fact, be made. The installation was completed with a Ditch Witch JT7020.

In Tulsa, OK, approximately 2,500 feet of 8-inch vitrified clay pipe was replaced with 8- and 10-inch ductile iron pipe by pipe bursting. Site conditions made open-cut construction either impossible or prohibitively expensive. Pipe bursting was divided into three segments of 1,000, 1,100 and 400 feet. Hydrahaul H110XS equipment was used.

The cooperative test in Arizona concludes its report with this summary:

"Successful horizontal directional drilling installations, such as the project for GCWSD [Pawleys Island], firmly establishes restrained flexible joint ductile iron pipe as a viable, and probably superior in many instances, trenchless pipe option. Furthermore, the research, finite element analysis and verifying hydrostatic testing of the flex-ring joint assures owners, engineers and installing contractors that the demands of HDD installation on ductile iron pipe are well within the capabilities of the material."

Carpenter says that Flex-Ring ductile iron pipe is available in diameters from 4 to 36 inches.

"The quality and superiority of ductile iron pipe, combined with the benefits of horizontal directional drilling, offer project owners and engineers many advantages," he says.

"It is apparent that with the increased demand for water and wastewater infrastructure rehabilitation and the growing resistance to open-cut construction of the general public, trenchless installation using HDD or pipe bursting technology will most certainly play an increasing role.

"And in many parts of the world, specifying project owners and engineers prefer to use ductile iron pipe for their systems. We want public works people and contractors to know they have the option of installing ductile iron pipe with these trenchless technologies."

A long history of service
Summarized from information provided by the Ductile Iron Pipe Research Association.

Gray cast iron pipe -- ductile iron pipe's predecessor -- has a service history unequaled by any other type of piping material.

The first documented cast iron pipe installation was in 1455 in Siegerland, Germany. In 1664, French King Louis XIV ordered the construction of a cast iron pipe main extending 15 miles from a pumping station at Marly-on-Seine to Versailles to supply water to the fountains and town. This pipe served the palace gardens for more than 330 years.

Cast iron pipe was introduced to the United States as early as 1817 when it was installed in the Philadelphia water system. Today, more than 565 utilities in North America have had cast iron mains in continuous service for more than 100 years. Additionally, at least 16 utilities have had cast iron mains in continuous service for more than 150 years.

Ductile iron retains all of cast iron's qualities of machinability and corrosion resistance and also provides additional strength, toughness and ductility. Although its chemical properties are similar to cast iron, ductile iron incorporates significant casting refinements, additional metallurgical processes and superior quality control.

Since its introduction in 1955, ductile iron pipe has been recognized as an industry standard for water and wastewater systems. More than four decades of field experience have proven its strength, durability and reliability for transporting raw and potable water, sewage, slurries and process chemicals.

Ductile iron's high degree of dependability is due primarily to its high strength, durability and impact and corrosion resistance. Ductile iron has minimum strength requirements of 60,000 psi tensile strength, 42,000 psi yield strength and 10 percent minimum elongation.

Designed and manufactured to stringent standards, ductile iron pipe resists damage during shipping and handling, and after installation, withstands demanding operating conditions, including water hammer, frozen ground, deep trenches, areas of high water table and heavy traffic, river crossings, pipe on supports, rocky trenches, areas of shifting and expansive and unstable soils. Numerous laboratory and field tests have proven that ductile iron's corrosion resistance is equal to or greater than that of cast iron.