Cross bores are defined as an intersection of an existing underground utility or underground structure by a second utility resulting in direct contact between the transactions of the utilities that compromises the integrity of either utility or underground structure.
Trenchless construction techniques for utility installations provide a friendly alternative for reduced surface disturbances to homeowners and commercial business owners at a reduced cost. Horizontal directional drilling (HDD), percussion moles and plows are trenchless techniques used to install natural gas distribution lines. However, these installation methods do not allow the installer to observe if another utility was intersected; thereby, facilitating a high potential for creating cross bores unless proper procedures, locating of existing utilities and post construction verification procedures are used. Until the last few years, the industry was mostly unaware of the problem. In high risk, densely populated areas, as many as 3 cross bores per mile (of natural gas distribution mainline) have been found.
Sewer pipes in poor structural condition or obstructed by roots will often result in backups. In order to clear the lines, drain cleaners are called out by homeowners. If no exterior cleanout is present, the drain cleaner may pull a toilet or access an interior cleanout to run a rotating mechanical cutting tool through the sewer line to clear the blockage. If the blockage is due to a natural gas cross bore, the mechanical tool will effortlessly, yet unknowingly, cut through the plastic gas line. Since the gas is under pressure, it escapes the plastic line and flows via the path of least resistance, which is often through the sewer pipe and back into the house or structure. At this point, the structure is a ticking time bomb where any ignition source can cause a catastrophic explosion and result in significant damage and potential loss of life.
According to a 1976 National Transportation Safety Board (NTSB) report of an explosion due to a cross bore, which resulted in two deaths and four injuries, the NTSB recommended that “inspections [should be made]…. where gas mains and sewer laterals may be in proximity…” The highest claim paid for a single cross bore was reported at $30,000,000 and resulted in severe burns to two young girls. Cross bores can injure or cause death of installers, the public, service technicians and operating personnel; however, they are preventable with modern technologies and processes. Legacy cross bores can systematically be eliminated through inspection and elimination programs.
During the last several years, increased recognition of the injury, death and damage caused from cross bores has resulted in federal and state regulatory action. In August 2011, the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration’s (PHMSA) requirements of Distribution Integrity Management Program (DIMP) took effect. The goal is to increase the integrity of gas distribution systems.
It is generally agreed by distribution system operators that a cross bore of a gas line through a sewer is a considered a integrity fault of a gas system. All existing cross bores must be identified and new cross bores should be prevented. Fortunately, solutions for a high confidence cross bore elimination program are available.
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