June 8, 2012Five years ago, David Harris, director of utilities for the City of Brownwood, Texas, was looking for a more practical, efficient way to update the city’s aging wastewater collection system. Many of the clay gravity-fed sewer mains were getting close to 100 years old and had become severely cracked over the years by heavy clay soil that expands and contracts in response to varying moisture conditions.
Over time, tree roots infiltrated these cracks and filled the sewer mains, leading to back-ups, overflows and the crumbling of pipe bells. These problems, combined with population growth on the outskirts of the original infrastructure, made apparent the need for the progressive replacement of several miles of pipe using the most efficient, budget-conscious means possible.
After researching and evaluating a variety of different pipe materials and installation methods, Harris ultimately chose static pipe bursting to replace the clay sewer mains with restrained-joint polyvinyl chloride (PVC) pipe. In September 2007, the City of Brownwood rented a TT Technologies Grundoburst 800 G static pipe bursting system with 100,000 lbs of thrust and 200,000 lbs of pullback power and got ready to work. After only four days of training from TT Technologies, the Brownwood crew was ready to begin its first pipe bursting project. Within the timeframe of just a few weeks, static pipe bursting proved to be a fast, low-cost method for getting the job done. For Brownwood’s municipal maintenance staff, which handles the bulk of the city’s water and sewer system maintenance, it was also easy to learn, as they quickly became highly proficient in the process.
At this point — five years later — Brownwood now owns a Grundoburst 800G and has used it in more than 80 projects to replace approximately four miles of clay sewer main, ranging in size from 6-in. to 10-in., with 8-in., 10-in. and 12-in. restrained-joint PVC pipe. The ongoing positive results of Brownwood’s static pipe bursting sewer projects have made Harris an outspoken advocate for this pipe replacement method and the use of restrained-joint PVC pipe.
“Static pipe bursting has worked out very well for us, as it allows us to get a lot of work done in a short time,” Harris says. “Plus, operating the equipment and assembling the pipe is not rocket science. That’s one of the main reasons why we’re doing pipe bursting — I don’t have to send my staff to school to get certified before they can start on a job.”
Static pipe bursting uses hydraulically powered machinery to fracture the existing sewer or water main, referred to as the host pipe, with a cutting head specifically designed for cutting through the material of the host pipe. The cutting head is followed by an expander, which disperses host pipe fragments and increases the diameter of the hole. Attached to the rear of the expander is the new replacement pipe, which is pulled in simultaneously. This differs from pneumatic pipe bursting, a more common pipe bursting method where the bursting of the host pipe is done by a pneumatic soil displacement hammer connected to a hydraulic variable speed winch. Due to the force exerted by the pneumatic hammer, pneumatic bursting is only recommended for use with a more flexible pipe material. Static pipe bursting, however, is compatible with virtually any type of replacement pipe material, including restrained-joint PVC.
For its sewer bursting projects, Brownwood uses CertainTeed CertaFlo Greenline restrained-joint PVC pipe, which is manufactured specifically for pipe bursting and other trenchless applications. Though somewhat new to the pipe bursting market, restrained-joint PVC offers several benefits over other thermoplastic pipe materials. For example, it provides improved mechanical properties, higher pressure ratings and better flow performance than high-density polyethylene (HDPE) pipe, and it allows municipalities to stick to a proven, long-lasting material that works seamlessly with existing infrastructure. And, equally as important, it makes pipe installation move faster with a simple assembly process. CertaFlo Greenline, for example, is assembled in 10- or 20-ft lengths with spline-locked integral bell ends as pullback continues, allowing pipe installation to continue at a steady pace.
When Brownwood began its pipe bursting operations, the maintenance staff used a mix of 10-inch HDPE pipe and 8-inch and 10-inch CertaFlo Greenline to compare the two materials. The crew members largely preferred the restrained-joint PVC pipe to HDPE, due to its ease of installation and ability to be assembled during pullback instead of before the pull.
“With HDPE, we found that it takes three people about eight hours to fuse the pipe and then it has to have a cool-down period,” Harris says. “Then, we had to have a place to put a 340-ft string of pipe. With restrained-joint PVC, however, we can actually put the pipe together as it’s being pulled into the ground, and it only takes us about 45 minutes to get all the rods in,” Harris says. “We don’t have to assemble the pipe the day before or show up a few hours early to do it.”
Having the option of both 10- and 20-ft lengths with the CertaFlo Greenline pipe helps the Brownwood crew to have less waste on the jobsite, and the 10-ft lengths of pipe can also be assembled in a trench box in situations where less space is available.
“If we have too much pipe left, we simply take the end piece off, put it back on the truck and use it next time,” Harris says. We generally order a few 10-ft joints, either to start with or to finish with in those types of situations.”
Through the use of static pipe bursting, the Brownwood crew is not only able to replace 6-in. clay mains with 6-in. PVC, but also can increase the size of the main by two pipe sizes. Over the past five years, the 10-person crew has replaced 6-in. clay with both 8-in. and 10-in. pipe. The minimum grade for 6-in. pipe is .5 percent, or a 6-in. drop every 100 ft. For 8-in. pipe, minimum grade is .33 percent or a 4-in. drop per 100 feet, and minimum grade for 10-inch pipe is .25 percent or a 3-in. drop per 100 ft. So, by replacing a 6-in. main that is exceeding its designed capacity at minimum grade with a larger-diameter pipe at the same grade, the city gets a better pipe installed at a grade off of the minimum edge of the recommended slope and a significant increase in capacity. This increases capacity to accommodate past and future population growth.
Pipe bursting pulls in Brownwood usually average around 300 ft, with the longest being close to 600 ft — the most allowed by the amount of rods the city owns. Each pull usually goes from one manhole to another, and the distance between manholes rarely exceeds 400 ft. No matter the length, CertaFlo Greenline pipe has always displayed excellent joint integrity during pulls, holding together very well. The Brownwood crew completed a 550-ft pull this past February with no problems whatsoever.
Reaping the Benefits
Through static pipe bursting with restrained-joint PVC pipe, the City of Brownwood has not only made sewer main replacements run more quickly and efficiently, but has also decreased disturbance to homes, businesses and roadways and significantly reduced costs. The amount of excavations needed is 84 percent less than an open-cut job, which leaves much of the landscaping unscathed and greatly reduces the need for backfilling and re-landscaping after a project. Homeowners and business owners have been pleasantly surprised at how non-invasive the pipe bursting projects have been.
“In every scenario, pipe bursting is less expensive and far less disruptive to our customers than open cut,” Harris says. “We’re able to burst under roads, flower gardens, fences, trees and other important landscaping, and people really appreciate that. It really shocks people when we finish and we haven’t even been there that long. They’re always asking how we could replace the pipe without leaving a hole in the street.”
The city also saves money by owning the pipe bursting equipment and supplying the labor, which enables it to run on its own schedule and control the quality of installation.
“We put one of our jobs out to bid and found that the contractor’s price is $65 to $100 per ft of pipe installed,” Harris says. “For us to take a 6-in. clay pipe and replace it with 8-in., it costs us $25 a ft and that’s for materials and labor. So, doing the sewer main bursts in-house saves us a considerable amount of money.”
Today, Harris continues to spread the word about the advantages of static pipe bursting with restrained-joint PVC pipe to other municipal sewer and water officials through occasional speaking engagements at industry conferences, where always offers an open invitation to visit Brownwood and observe one of his crews’ ongoing sewer main bursts. Visitors are frequently impressed by the demonstration and use that experience to begin talking seriously about giving static pipe bursting a try in their own towns and municipalities.
“We’ve had city engineers and municipal maintenance staff from all over Texas come out to observe our bursting projects and have their questions answered,” Harris says. “They all want to know how it works, and the best way for them to learn is to watch an actual project take place and witness how efficiently it runs. It gives us a good opportunity to teach people how static pipe bursting works. When I tell them we can do a sewer burst for $25 a ft and put 400 ft of pipe together in an hour, they don’t believe it. But, after you actually see it being done, there’s no way you can’t believe it. It’s something you just have to see.”
John Coogan is channel marketing and communications manager for the CertainTeed Pipe & Foundations Group.
All photos to City of Rowlett, Texas.