Micro Pipe Bursting
- M. B. Johnson - Hazen and Sawyer
The City of Greensboro, NC has been contracting with KRG Utility, Inc for several years to rehabilitate their water distribution system. The selected method for the replacement and upsizing of 4-inch and 6-inch water mains has been to use the static pipe burting equipment supplied by TT Technologies. City personnel feel that water main replacement via pipe bursting provides the least disruption and social impact to the community. Following the installation of over 20,000 linear feet of 6-inch and 8-inch water mains the city of Greensboro decided to expand the diameters replaced via static bursting. Their first target was 2-inch cast iron mains serving residential neighborhoods that water models identified as being undersized. This previously unattainable feat was made possible by a two pass bursting system that used a unique combination of static bursting rods created by KRG Utility and TT Technologies. To date more than 1500 lf of 2-inch cast iron water main has been upsized with 6-inch fusible PVC. Pipe bursting pull lengths have ranged from 150 lf up to 400 lf. 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.
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 a 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. This first pass will cut and displace the existing 2-inch main but will only pull in the 3-inch rods which will set 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 Grundoburst unit. This process will take about one hour to accomplish. This larger machine is sized for the 3-inch rods and can supply a pulling force of over 170,000 pounds. 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 will be 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 exisiting main was cut and displaced on the first pass that installed the 3-inch rods. Both pits are now ready for the second pass.
The second pass will utilize the 800G hydraulic unit and the 3-inch rods to pull 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 provides 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 vial 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.
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