Construction is always dangerous. Much of our concern is focused on the workers, such as the issue of high-powered fiber amplifiers discussed last month in “Something New to Worry About.” Damage to infrastructure is also a concern, especially the threat to buried utilities during underground construction.
Directional boring is especially hazardous. The boring tool packs a lot of energy to work its way through the ground and can severely damage objects in its way. One contractor installing fiber in Nashville, Tenn., punctured seven water mains in a year and two in the same week, which flooded neighborhoods.
Horror stories of contractors puncturing high-pressure gas mains often make the news because they can lead to explosions and fires, causing injuries or deaths. Last year, one punctured a high-pressure gas main about 10 blocks from our home, and disaster was averted only through quick action by the city and utilities to block off the area and shut down the electricity.
Recently, Scott Landes, an activist in the “Call Before You Dig” movement and publisher at dp-Pro magazine, which covers underground damage prevention, told me about cross boring. It’s another problem becoming more common involving undetected damage to underground utilities.
Generally, the damage is to storm or sanitary sewer pipes. Since the sewer pipe has no pressure, unlike a water or gas main, there may be little evidence of damage. And since it is usually a large pipe, it’s a big target. Sewer systems have many connections to buildings along the road, and storm sewers have catch basins along the curbs that may be near where construction is planned.
Much of the concern is over gas line installation that penetrates the sewer, but the damage is not detected. If the gas line blocks the sewer line enough to cause a backup, a cleaning may be necessary. The tools used to clean blockages in the sewer can damage the gas line, resulting in leakage into the sewer and consequently into local buildings connected by the sewer. This indirect gas leak can cause fire or explosion just like a direct leak.
If the cross-bore penetration occurs while installing a fiber optic cable, it leaves the cable in the same situation if the sewer becomes clogged and mechanical clearing is done. Instead of a broken gas line, one has a broken fiber optic cable with no evidence of construction that caused damage.
The optical time-domain reflectometer comes in handy here for troubleshooting, as it can locate the damage to the cable. However, exposing the problem requires considerable digging to actually locate the break in the cable and determine how to fix it.
Landes asked me if fiber optic contractors were aware of the problem. My response was that I had never heard any mention of such a problem causing a fiber optic cable failure. I did several searches on the internet and found nothing about failures such as this. It probably is a rare occurrence, so it is certainly an avoidable one.
Landes also told me about the Cross Bore Safety Association (CBSA), an organization aimed at educating utility owners and contractors about the problem.
According to CBSA’s “Leading Practices for Cross Bore Risk Reduction” (available at www.crossboresafety.org), a big problem with sewers is location. Many sanitary and storm sewers are very old and often were constructed before good records were kept. I remember while living in downtown Boston during the “Big Dig” when workers uncovered some sewers still in service in our neighborhood that were made from wood! (Some of the water pipes were made of lead, too.)
What complicates the problem is that many of the connections to sanitary and storm sewers are not documented. Some are not even metallic pipes that can be found by simple magnetic locating instruments. Sometimes ground penetrating radar can be used to locate sewer pipes, but few contractors have such sophisticated equipment available.
One possible method of finding cross bores is to use a pull-back camera on the end of the tool. After boring, as the tool is pulled back, the camera can be used to find voids in the bored hole, indicating that a pipe may be penetrated and more investigation is needed.
Knowing about a potential problem is a good place to start in finding solutions. In this case, better documentation is one part of the solution, which requires city and county departments to work together with contractors to update records when undocumented pipes are found during construction.