By M. Brent Johnson
The City of Greensboro, NC, has been contracting with KRG Utility for several years to rehabilitate their water distribution system using static pipe bursting to upsize 4 inch and 6 inch water mains. Following the installation of over 20,000 linear feet of 6 inch and 8 inch water mains the city decided to expand the diameters replaced via static bursting. The first target was 2 inch cast iron mains serving residential neighborhoods.
|Fusible PVC pipe from Underground Solutions was used for this pipe bursting project.|
This previously unattainable feat was made possible by a two-pass bursting system that uses a unique combination of static bursting rods created by KRG Utility and TT Technologies. To date more than 1500 linear feet of 2 inch cast iron water main has been upsized with 6 inch fusible PVC. Pipe bursting pull lengths have ranged from 150 feet up to 400 feet.
This new technology has provided the City of Greensboro with a trenchless replacement option for maintenance-heavy 2 inch water mains. The newly installed 6 inch lines provide increased flow and capacity for the community and also allow for the addition of hydrants for fire protection.
|After bursting the 2 inch mains, a second pass was needed to pull in the new 6 inch pipe.|
The first pass of this system involves the insertion of 1.4 inch diameter rods into the existing 2 inch water main. This is accomplished using TT Technologies' Grundoburst 400G which normally uses 2.13 inch rods. Modifying the hydraulic unit to fit the smaller 1.4 inch rods also causes a 60% reduction in the allowable pulling force used.
Once the 1.4 inch rods have been successfully inserted into the host pipe they are attached to cutting wheels sized for the 2 inch cast iron main. Also attached is a 4 inch expander head and 3 inch bursting rods that are used by the larger 800G Grundoburst hydraulic unit. The first pass cuts and displaces the existing 2 inch main as it pulls in the 3 inch rods, setting the stage for the second pass.
Before the second pass can begin the contractor must remove the 400G hydraulic unit from the bursting pit and install the larger 800G unit. This process takes about one hour to accomplish. The larger machine is sized for the 3 inch rods and can supply a pulling force of over 170,000 pounds.
|TT Technologies' Grundoburst systems were used to burst 2 inch cast iron mains and pull in new 6 inch PVC pipe.|
Once the 800G is in place the replacement pipe is placed at the entrance of the insertion pit. To allow for the insertion of the 6 inch replacement pipe a 9 inch expander head is attached to the 3 inch rods. A tracer wire can be attached to the expander head at this point and is inserted into the hole along with the replacement pipe to aid in future water main locating efforts. No cutting wheels are needed for this pass since the existing main was cut and displaced on the first pass.
During the second pass the 800G hydraulic unit pulls the 6 inch replacement pipe into place. To date the longest pull has been 400 feet. A shorter burst of 150 linear feet was accomplished by skipping the first pass of pulling in 3 inch rods and simply attaching the replacement pipe to the 1.4 inch rods and using the 400G to pull in the pipe.
Since the smaller rods limit the 400G to around 36,000 pounds of pulling force this does provide some limitations. The limited pulling capacity can be hampered by cast iron saddles for water services and pipe break repair efforts that involve bands or concrete poured around the pipe.
When possible the contractor can be greatly aided in his bursting efforts if a sample piece of pipe can be supplied to the pipe bursting equipment manufacturer. This will facilitate proper sizing of hydraulic units, cutting wheels and expander heads.
The City of Greensboro has seen many benefits of pipe replacement via static pipe busting. In addition to the social and environmental benefits it is also becoming more cost effective due to its growing popularity among municipalities and more widespread participation by contractors.
About the Author: M. Brent Johnson, P.E., is a Senior Principal Engineer with Hazen & Sawyer. He has extensive expertise in pipe condition assessment, distribution/collection system planning, I/I studies, pipeline rehabilitation, and trenchless technologies. For the past 12 years Mr. Johnson has focused on the rehabilitation of existing water distribution and wastewater collection systems with a special emphasis on the use of trenchless technologies to reduce costs, minimize impacts to the public and environment and to shorten construction times.