As director of a utilities division, I face a growing problem: Whenever the thirsty Texas soil drinks up moisture, it expands and cracks our aging clay sewer mains.
Texas is one of a handful of states that has “expansive” or swelling soils, which contain minerals—usually clay—that absorb water. The soil can swell up to 15 times its original size, causing damage to pavement and, in my community's case, cracks in sewer pipes. Tree roots then infiltrate these cracks and fill up the sewer mains, causing backups and overflows and demolishing pipe bells.
The shifting, heavy clay isn't our only problem. Our 100-year-old waste-water treatment system has been burdened by growth on the outskirts of its original infrastructure.
With a small staff and a limited budget to carry out repairs, we needed to perform a fast, easy, and relatively inexpensive overhaul.
After researching pipe materials and installation methods, I decided upon static pipe bursting. This trenchless technology is more affordable than the open-cut method, and requires less trench safety devices.
PNEUMATIC VS. STATIC
Pipe bursting involves replacing buried mains by fracturing the pipe and displacing the fragments outward while a new pipe is drawn in to replace the old pipe.
Pneumatic bursting, the most common pipe-bursting method, involves a combination of pulling the pipe with a winch system, while simultaneously operating a pneumatic piercing tool that's inserted into the bursting head. The pneumatic head is attached to the pipe to “pound” its way through the ground and fragments of old pipe. Pneumatic bursting, however, is only recommended when using a more flexible pipe than clay, like high-density polyethylene (HDPE), due to the force exerted on the connection point.
Static bursting is a pipe-bursting method that's gaining popularity with public works managers, engineers, and contractors. During the process, an expander—preceded by a cutting head—is pulled through the existing line by a hydraulically powered bursting unit. As the expander is pulled through, it splits the host pipe. An expander forces the fragmented pipe into the surrounding soil while pulling in the new pipe.
Static bursting is compatible with all pipe types that can be fused or locked together mechanically. The combination of static pipe bursting with restrained-joint polyvinyl chloride (PVC) pipe is a relatively new match, however. Restrained-joint PVC pipe provides improved mechanical properties, higher pressure ratings, and better flow performance than other thermoplastic materials. It also allows communities to stick to a proven long-lasting material that works seamlessly with existing infrastructure and simplifies installation.
HDPE VS. PVC
Last fall we rented a Grundoburst 800 G static pipe-bursting system, with 100,000 pounds of thrust and 200,000 pounds of pullback, from TT Technologies. With only four days of training by the company's personnel, we completed 14 projects between September 2007 and February 2008. During that time, we replaced nearly a mile (4,692 feet) of clay sewer mains, ranging in size from 6 inches to 10 inches.
To compare material performance during installation, we used 10-inch HDPE and 8-inch and 10-inch Certa-Flo GreenLine PVC pipe, a restrained-joint PVC pipe for trenchless and pipe-bursting applications. The crews performing the installation preferred the PVC to the HDPE for several reasons, including:
It takes a three-person crew eight hours to fuse the necessary amount of 10-inch HDPE pipe for an hour of pipe bursting. But the CertaFlo GreenLine doesn't need to be fused. It can be installed as the pipe-bursting machine pulls it into the ground, thanks to its unique spline-locking joint system tha utilizes an integral bell instead of coupling.
- For bursts of less than 6 feet where there's 50 feet or so behind the insertion pit, the restrained-joint PVC pipe can be assembled as fast as the machine can pull. Speed of the burst is hard to estimate because of differences in each project, but the actual installation of the main has been between 2 and 7 feet/minute.
Static pipe bursting not only allows us to replace 6-inch clay mains with 6-inch PVC, it also lets us increase the size of the main by two pipe sizes. We replaced 6-inch clay pipe with 8-inch and 10-inch pipes. Many of the small-diameter mains are currently at a minimum grade for their diameter:
Minimum recommended grade for 6-inch pipe is .5% or 6 inches/100 feet.
- For 8-inch pipe: .33% or 4 inches/100 feet.
- For10-inchpipe:.25%or3inches/100 feet.
By replacing a 6-inch main that was exceeding its designed capacity at minimum grade with an 8-inch pipe at the same grade, we got a new smooth pipe installed at a grade off of the minimum edge of the recommended slope—and with a significant increase in capacity. This allows us to accommodate past and future growth.
FIVE DISTINCT ADVANTAGES
Anybody who can do a traditional open-cut job can learn how to do a pipe-bursting job.
Another big advantage is that disturbance to surrounding areas is minimized. We burst mains under four-lane traffic roads, garages, chain link fences, manicured lawns, creeks, and through thickets of trees and soccer fields without disturbing the surface, except to re-tap services and excavate 20-foot x 5-foot pits at the start and finish points. Also, we didn't have to check the grade of the pipeline with a laser every few feet as required with open-trench projects because the new pipe maintains the grade of the original pipe.
Trenchless methods are safer for the staff, taking less of a physical toll on individuals. Plus, backfill materials and spoils are greatly reduced.
With this technology, we installed 400 feet of PVC in about eight hours. It took five to six workers about 1½ hours to dig pits; 1½ hours to set up equipment and put it back on the truck; two hours to insert rods, attach to pipe, and install; and three hours to connect to each end and backfill pits. Pipe is put together as it is pulled back into the ground, but the required taps and manholes take some additional time.
Finally, if mains are replaced in existing space, easements are not an issue. Increasing capacity by upsizing the main takes care of capacity and grade issues that may have been causing sewer overflows.
We compared costs associated with conventional open-cut methods, and found that pipe bursting is less expensive, in almost all instances, in the long run.
The up-front cost of pipe and pipe-bursting equipment—roughly $250,000—is actually higher than the initial cost of a traditional open-cut project. And restrained-joint PVC can cost up to three to four times as much as conventional PVC sewer pipe. But with trenchless PVC pipe installation, costs are offset by:
- Increased speed of installation, which lowers labor costs
- Reduced expenses for disposing sewer-contaminated spoil dirt
- Reduced volume of new embedment
- Reduced flex base and street patching.
And even though each tap on the old main must be located and re-tapped when the new main is installed, there's still a huge savings in time and material. Excavations are reduced and the need for destroying and replacing sidewalks, asphalt, trees, and grass is eliminated.
Plus, trenchless methods minimize the use of support equipment, which also means less fuel consumption and wear and tear. Because other equipment is not worked as hard nor as often after you convert to pipe bursting, it will last longer.
We were so impressed by the results of these projects, that we bought the Grundoburst 800G in March. Our crews are already using the new equipment to replace clay mains—with at least 50 more miles to go.
— Harris is director of utilities for the city of Brownwood, Texas.
Same culprit, different solution
An Indiana city replaces water mains under a three-lane street with minimal disturbances.
Soil was also the culprit that damaged cast-iron water mains in Indiana's second-largest city.
Installed in the 1930s in Fort Wayne's 200-home Belmont Addition neighborhood, the 11,000-foot water main suffered corrosion over the years, partially caused by the region's soil. Since the soil is mostly composed of clay, it soaks up and holds moisture, becoming a corrosive environment for unprotected pipes.
Because a busy street runs through the area, Fort Wayne City Utilities wanted to replace the damaged water main with minimal disturbances. The answer was directional drilling. City contractor S&S Directional Boring chose to replace the corroded pipe with 6-inch Certa-Lok C900/RJ restrained-joint polyvinyl chloride (PVC) pipe—which stands up better against corrosion than cast iron and is not as expensive as other materials.
“Directional drilling has proven itself to us over the years as the most cost-effective method of replacing water and sewer mains in busy or congested areas, but this is one of the first times we've used PVC pipe for a directional drilling project,” says Matthew Wirtz, Fort Wayne's assistant manager of planning and design. “Since the project was taking place in a residential area, we wanted to avoid having several hundred feet of fused pipe strung out for several days. The fact that the pipe is assembled as it goes into the boreholes was a huge advantage.”
S&S Directional Boring began drilling last September with crews ranging from eight to 12 workers and finished the job in April. After making adjustments to optimize drilling through the extra-hard clay, the crew completed the project with minimal difficulties.
“It turned out well,” Wirtz says. “We've received positive feedback from the contractor and our maintenance workers.”
Since completing the project, the utilities division has used PVC pipe in other directional drilling projects.
— Steven Gross, PE, is director of marketing with Valley Forge, Penn.-based CertainTeed Corp.'s Pipe Business.
To view a slideshow of how the PVC pipe was installed, click here!