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.