Ticking Time Bombs
Cross bores: increasing safety and reducing risk.
January 1, 2012 by Mark Bruce
Cross bores are the unintended intersections of utilities and are a threat to the safety of industry workers and the general public. In the case of gas lines intersecting sanitary or storm sewers, explosions, injury and death have resulted. Energized utilities, such as natural gas and electric distribution lines, have the potential for immediate and long-term danger.
HOW DOES THIS OCCUR?
Trenchless installation techniques are widely used. These methods provide minimal disruption to the surface, minimal disruption to traffic and potential economic efficiencies. However, use of plowing, percussive moles and horizontal directional drills do not provide visual confirmation of the location of the new utility.
In 1999, one of the earliest records of the significance of cross bores was recognized in a Kentucky Utilities Commission ruling. In this case, a sewer utility complained that the gas distribution utility had, and was, continuing to damage its sewer infrastructure. The gas utility countered by stating that the sewers locations were not marked. Simply put, the commission ruled that the gas utility must avoid damaging the sewers and the sewer owner is required to locate its sewers.
Unfortunately, where intersections of sewers have been created by installation of natural gas lines, the risk is often latent and undiscovered until a drain cleaning company is called to open a plugged sewer line. Blockages result in backups and overflows and, unless promptly dealt with, damage to the interiors of homes and businesses can occur from water or wastewater damage. The unpleasant interruption of the use of toilets, sinks and showers calls for immediate attention. The plumber may not take the time to call the utilities, or “One Call” line (if available).
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.
Typically, devices that have been used to open sewers include rotating cutters, which can be inserted in a plugged drain line. Often the problem may arise from foreign objects introduced to the system: roots in sewer lines, collapsed pipe and offset joints. Since the line is full of water and debris, the drain cleaner is working in the blind.
Root cutters are typically electrically powered “snakes” that have a steel cutter on the end that can open sewers. These cutters can cut through plastic gas distribution, communication, water and other utilities inadvertently installed through sewers.
If a cross bore is in a sewer line that is being opened by a root cutter, the utility line can be cut. If it is a gas line, gas will be introduced into the connected structure. In a matter of a few moments, gas/air concentrations can reach the explosive range. Sparks from electric motors and switches or pilot lights for water heaters and furnaces can ignite the gas/air mixture with catastrophic results, including explosions and complete destruction of structures and loss of life.
If electric lines are hit by the root cutter, shock of the operator can result in injury or death. Other utilities used for communications normally will not cause immediate injury, but communication with emergency services such as police or fire departments is impeded. Delayed response can then result in loss of property, injury or death. Push rod cameras are ineffective in identifying a cross bore if the line is already backed up with water/sewerage, since visibility is limited.
Regrettably, these scenarios are not a remote possibility. Reports from hundreds of miles of inspection projects to eliminate gas line cross bores from sewers has found a range of between two to three per mile of sewer and sewer laterals inspected. Each one is a “ticking time bomb” waiting to be energized when the home or business owner has a plugged sewer. In addition to homes, hospitals and schools have had natural gas cross bores in their sewer lines. If a home explosion does not get your attention, the catastrophic magnitude of a school or hospital cross bore explosion should.
Cross bores can be prevented. In the case of existing gas sewer cross bores, they can be found and eliminated. Many gas utilities now require an increased level of prevention during new construction activities. Post inspection is an alternative that is gaining favour in new construction of gas distribution lines. It is somewhat contrary to the “damage prevention” concept, but it has real advantages at times.
Where the existing sewers are being inspected for cross bores from earlier construction activities (legacy cross bores), identifying that a sewer is free from a cross bore is the goal. The equipment may be the same as used in pre-construction locates. Competently designed and run legacy programs provide a shelter from punitive damages while providing public safety. <>
Mark Bruce is president of the Cross Bore Safety Association and president of Can Clay Corp. He is vice president of Hydromax USA, a company that provides field data collection services for gas distribution, water, wastewater and dams and levees to owners, contractors and engineers. For more details visit www.crossboresafety.org.
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.
• Contact your local utility (or One Call if available) before starting work.
• If the local utility has a marking program for identifying sewer connections that have been inspected and cleared as not having cross bores, become familiar with the program.
• Recognize that gas utility lines are most often plastic. Notice if plastic is on the cutting tool when the cutter is withdrawn for inspection. Other types of utility lines may have similar characteristics to observe.
• If bubbles or gas are found to be escaping from the entry point of the cutting equipment, IMMEDIATELY exit the structure. Warn all inhabitants as well.
• Do not flip any switches or use any type of phone until clear of the structure by a safe distance.
• Since the structure can explode radially outward distances of 150 ft. or more, retreat to a safe distance.
• Do not go back into the structure under any circumstances.
• Once safely clear of the structure, call the emergency number, fire department, police and/or the local distribution utility.
• Safety personnel should proceed with due caution.
• When deemed appropriate and with concern for personal safety, notify adjacent property owners. In the case of a gas explosion, the force and debris can be lethal at a great distance.
• Adjacent structures may not be safe locations. In some cases, a cut gas distribution line can cause other structures in the vicinity to have gas pressurized into them.
• Do not return until professionals have cleared the area and structures.