Page 14 - AL811 2022 issue 4

Preventing Cross Bores Damages
By Mark H. Bruce,
President Cross Bore Safety Association EVP Hydromax USA
C Figure 1: Class 1 Cross Bore ross bores are defined
as an intersection of an existing underground utility by another utility that
compromises the integrity of either structure1. Cross bores occur from using trenchless utility installation methods. HDD, percussion moles and plows
are the primary installation tools that create cross bores. Fundamentally, the causes of creating cross bores is not knowing the location of an existing utility nor ensuring construction activities do not create an intersection. Many states have exemptions or exceptions for locating or no penalties for not locating of sewer utilities. TCA 65-31-112 appears to provide no civil penalties to state departments for failure to follow provisions of TCA 65-31. States are beginning to tighten their utility damage prevention rules to eliminate sewer utility and other exceptions. These changes are positive improvements.
Cross bores of gas lines in sewers have infrequently resulted in catastrophic explosions with resulting injury, loss of life and other damages. Resulting costs can exceed tens of millions of dollars. All cross bores are required to be prevented, however, some of the largest damages are gas lines cross bores
of existing sewers. Cross bores were recognized by NTSB per a report of Nov. 12, 1976 of an explosion in Kenosha, Wisconsin where two persons were killed and four persons were injured as a drain cleaner “.....cut a 2 inch gas main installed by boring through the bottom of the sewer tile.” General awareness
of cross bore risk did not begin until the late 1990’s when two large gas installation contractors began to
Figure 2: Class 2 Cross Bore
promote cross bore safety, subsequent to their significant incidents.
A Class 1 cross bore, such as gas line
in a sewer, can be a lingering risk, subsequently activated by a drain cleaner’s rotating root cutter. If the intersecting utility is an electric utility, a sewer drain cleaner that subsequently called to clear a blocked sewer can be electrocuted if the insulation is cut
by the cleaner’s root cutting tool. If the intersecting utility is a gas utility, gas can be released if cut, causing the structure to fill with gas. An ignition source, such as light switch or pilot light, can cause an explosion in less than a minute. Gas in sewer line cross bores are often referred to as “ticking time bombs’, waiting for the life of the utility, up to 80 years or more.
Class 2 cross bores, such as a fiber
optic installation that intersects both
a gas line and a sewer, can cause the sewer line to a structure to fill with
gas and be subject to explosion when gas comes into contact with an ignition source. A class 2 cross bore in Texas and subsequent explosion is shown in Figure 2 and Figure 3. The HDD fiber optic installation intersected first a sewer lateral, then a gas line, causing gas to flow into the house through
the sewer. No injuries resulted. The occupants left the house shortly before the explosion. The contractor personnel were far enough away to not be struck by the flying debris.
There are leading practices that
can help ensure safe construction
is performed, even when damage prevention regulations allow for incomplete utility locates. The Cross Bore Safety Association published in
Figure 3: Class 2 Cross Bore Explosion
2021 the Leading Practice for Cross Bore Risk Reduction1. The included recommendations are suggestions to consider and evaluate for the particular conditions of utility or contractor. These are in addition to standard good practices. Especially where the risk of catastrophic damages can result, such as gas lines installed by trenchless methods, pre-inspections with sondes and GPS locates and post-construction sewer inspections are a leading practice. Post inspections verify sewers do not have intersections. Daylighting at the potential intersections of utilities are frequently recommended best practices.
Leaving the daylight excavation open
to record a photo or video of both the utilities showing the safe crossing can verify and ensure the procedure was followed. For both sewer inspections and daylighting locations, high accuracy GPS locations allow confidence the photos and inspection work were performed at the correct location.
GPS accuracy levels of 12” are typical, often 4” accuracy is achieved. The GPS coordinates, photos and videos allow for the risk reduction efforts to be verified and provide high confidence results. The data is usually kept in a GIS mapping system for easy accessibility and long-term storage of the data. Retaining the data for the life of the asset is recommended.
Utilities should take the lead to specify in contract documents the requirements that ensure damage prevention, including immediate and long-term risks from cross bores. Detailed specifications levels the playing field with safety conscious contractors not being undercut by bids that do not have cost included for appropriate levels
12 • Alabama 811 2022, Issue 4