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Effects Of Pipe Bursting On Nearby Utilities: Prediction, Prevention And Management
5th In A Series From The IPBA
If a 12-inch OD expander head were selected, the potential impact zone would be 12 inches minus 8-inches equals four inches; then multiply by 10-inches which equals 40 inches. Forty inches will be a baseline for determining the amount of separation from the existing pipe center line to adjacent utilities. The example for an eight-inch, size-on-size burst would be 10 inches minus eight inches equals two inches times 10-inches equals 20 inches.
In addition to the simple math that is required to determine amount of oversize for the burst head, a more important but complicated formula is to determine the volumetric displacement of the amount of earth moved in the burst to allow passage of the head through the original pipe ID. For example, the volumetric displacement would be greater for replacing a 10-inch pipe with a 12-inch pipe than it would be for replacing a six-inch pipe with an eight-inch pipe. Either of these scenarios is often referred to as a “one upsize” in pipe bursting lingo. Whereas a six-inch to 6-inch would be a “size-on-size” and a six-inch to a 10-inch would be a “double upsize.”
As stated earlier, geotechnical conditions play a critical role and may impact this potential by as much as double. Sands, loose gravel and small cobble move easily as the burst head travels through the ground and will have smaller potential impact. However, large cobble and some dense soils may have a greater potential impact area. This information should be provided on actual trench conditions and not soil borings provided outside of the original pipeline trench as is common with HDD pre-design.
Direction of force
It is very important to remember that the force exerted during a burst is focused from the center point of the existing pipe upwards and outwards. This helps in designing the utility coordination and mark-out procedures which is where 90 percent of the pre-burst risk management will occur.
Often times on a project, a utility will be found to be in the potential impact area but will be marked as “actual location of water main to be field determined” which can, in most cases, be done through a small keyhole style hydro-excavation. Once actual invert, pipe size and type is found, it can be noted and either taken off line, removed prior to the burst, or safely passed by the bursting head depending on its proximity to the burst path.