CROSS BORES



What is a Cross Bore?

A cross bore is the intersection of an existing underground utility or underground structure by a second utility installed using trenchless technology. This results in an intersection of the utilities, compromising the integrity of either or both utility or underground structure.

Cross bores can occur between different types of underground utilities, including the intersection of a gas line or electric line and a sewer line or water line. These can be disruptive, costly and potentially very dangerous. Intersecting buried electric lines or gas lines can cause injury or even death. Intersecting buried communication cables can interrupt calls for emergency assistance.
 
How does a Cross Bore occur?

When a new utility line (gas, electric, telephone, or cable television) needs to be installed it is not uncommon for the utility providers to dig horizontally underground (called directional boring). Since this installation method does not involve digging a trench, it avoids damage to sidewalks, patios, landscaping, and existing utility lines marked after placing a one call notification to 811. Since there is no open trench and the boring can be 100 feet or more underground, it is possible to intersect another underground line.

Why can a Cross Bore be a problem?

In rare cases, a gas, electric or communications line may cross through a sewer pipe. If a plumber unknowingly strikes one of these lines while cutting through a sewer blockage, the line could be broken. In the case of a gas line, natural gas could enter the sewer system, possibly causing an explosion. In the case of an electric line, serious injury could occur to the operator of the equipment.

Suggestions to help prevent damage because of a Cross Bore

Homeowners
If you have a blockage or are scheduled to have your sewer or septic pipe cleaned, make sure the natural gas utility serving the area is contacted first (either by you or your sewer/septic cleaner) to determine if an in-line camera inspection is necessary.

Plumbers
Always assume that an obstruction involves a cross bore. Mechanical purging equipment (rooters) have the ability to damage underground utility lines. Collapsed sewer walls can also cause the rooter equipment to be misdirected outside of the sewer, bringing the equipment dangerously close to other buried utility lines.

Some information courtesy of Peoples Natural Gas and the Cross Bore Safety Association.

You can find out more about Cross Bores at www.peoples-gas.com/CrossBores.aspx and www.crossboresafety.org.



Downside to HDD Excavation: Cross Bores.



Cross Bore: An intersection of one underground utility or structure by a second utility resulting in a direct contact between the utilities.

 

A cross bore compromises the structural integrity of either the utility or the underground structure. A Cross bores is created when some utility is mistakenly and unknowingly installed to pierce or pass through another utility, e.g., when a gas or electric line installed by a mini-HDD, impact moling or plowing ends up passing through an existing sewer lateral.

Although HDD excavation to install underground utilities has many advantages (cost, installation time, reduced restoration of the area to its original state), there are dangers.  One danger is a cross bore:  when an installed utility compromises the integrity of a second and existing utility within the area of excavation.  The above photo depicts a sewer lateral compromised with the installation of a natural gas service line that has been installed with HDD equipment and techniques.

The danger in the installation depicted may not be in the compromised sewer lateral. The sewer lateral appears as if it will still function. The real danger is not today, but tomorrow or next week or 5 years from now, when the sewer lateral eventually becomes clogged and needs to be cleaned. 

The plumber who cleans this sewer lateral will typically use a rotating cutting tool snaked through the sewer lateral, to cut or grind through the blockage. The cutting tool can easily cut through a plastic gas line compromising its integrity, causing the escape of natural gas. Because natural gas is lighter than air, it will migrate upwards, or back into the house. The accumulation of natural gas in the house can be catastrophic when it finds an ignition point, such as a water heater pilot light or a light switch.

Prevention.

Prevention of a cross bore during the installation of new underground facilities via HDD technologies is critical, and mostly involves common sense planning, observation and practices.

  1. Call 811 in advance of your excavation. State and federal laws obligate excavators to notify underground utility companies of their intent to excavate. The national 8-1-1 three digit number is reserved for this use. Laws vary, but in general excavators are obligated to place an excavation notification 48 hours (Washington DC), two days (West Virginia), two business days (Delaware, Maryland, Virginia) or three business days (Pennsylvania and New Jersey) in advance of the start of work. 
  2. Identify every facility near or across the proposed excavation path. With the work site marked, every utility and service lateral must be identified and accounted for. HDD excavation does not follow an exact planned path, and it is critical to know what’s below, above, around and across the proposed excavation path in the event the cutter head deviates from plan. Facility owners are a resource during the identification process – helping an excavator avoid damage is always safer and less expensive than repairing a damaged facility.
  3. Expose every facility near or across the proposed excavation path. Vacuum excavation and hand digging (potholing) are used to expose every facility that may cross or is near the proposed excavation path. Without exposing the facility, the excavator will not be able to visually confirm that the new installed facility does not compromise and existing facility in the ground. Some facility owners may insist on having a representative onsite during the planning and excavation, to ensure their facilities are not damaged and that backfilling occurs according to their specifications.
  4. Adjust the plan as necessary. The path or depth may need to be adjusted based on the location and depth of existing facilities within the planned excavation path. Some facility owners may have clearance minimums between their facility and anything installed in the right of way near their facilities. When in doubt, a telephone call or face-to-face meeting with the facility owner is prudent.
  5. Use a spotter. When the actual excavation takes place, a spotter should be used when the cutting head is anywhere near an existing facility. Visually check the drill head as it passes through potholes, entrances and exit pits.. The spotter should be empowered to halt the excavation at any time.

Inspect existing underground facilities. After excavation is complete and the new facility is installed, inspect the existing facilities before backfilling. If any facilities have been damaged (such as a nick, or a hole, or a crack, or a cross bore), the appropriate facility owner must be contacted for inspection and repair before backfilling is completed.

Resources.

There are a variety of resources available to assist excavators in learning how to safely use HDD excavation equipment in the installation of new underground facilities.

HDD Consortium, “Horizontal Directional Drilling Good Practices Guidelines”, ISBN 1-928984-13-4. This guide covers all aspects of Horizontal Directional Drilling, and is incorporated by reference in the PA One Call law. The third edition is available at https://www.nastt.org/products/13.

“Guidelines for Use of Mini-Horizontal Directional Drilling for Placement of High Density Polyethylene Pipe”, Dr. Larry Slavin, The Plastics Pipe Institute. https://plasticpipe.org/pdf/tr-46-hdd-guidelines.pdf.

The Mid Atlantic Society for Trenchless Technology (www.mastt.org) conducts seminars on trenchless technology. For a list of seminar locations, dates and cost, please see http://www.mastt.org.

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Dan Lucarelli is the Director – Marketing and Education for Pennsylvania One Call System, Inc. He can be reached at djlucarelli@pa1call.org.

What You Need to Know
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