Throughout the country, cross bores continue to be a cause of concern as more and more stakeholders are addressing the problem and associated risks. Cross bores, unintended intersections of utilities, pose a threat to public safety and industry workers alike. All types of cross bores can cause damage, however the intersection of natural gas lines in sewer pipes are considered the highest threats. Effective risk mitigation starts with planning and a holistic view of the problem.
One of the first documented cases of a cross bore was identified in a 1976 National Transportation
Safety Board (NTSB) report. In that report, the “investigation disclosed that the gas main had been installed by boring through the bottom of the sewer tile,” which ultimately resulted in two deaths, four persons injured and damages to adjacent houses. For nearly three decades, effective technologies have been slow to evolve in supporting effective cross bore prevention, particularly for sewer systems. In 1999, the Kentucky Utilities Commission issued one of the key rulings that initiated a lot of activity in regards to cross bore mitigation. In this case, a sewer utility filed a claim that the local gas utility was causing significant damages by boring gas pipes through their sewer infrastructure. The gas utility claimed that sewers were not located and marked ahead of gas installations. In short, the Commission ruled that the gas utility must avoid damaging sewers and the sewer utility must locate their assets.
As a response to the demonstrated risk posed by gas cross bores, Hydromax USA has been involved
with the elements of cross bore investigation since our beginnings in the early 2000s. As technologies improved and industry stakeholders began seriously addressing the risks of cross bores, comprehensive programs began taking shape in some parts of the country around 2012. A comprehensive program begins with the development of a risk model to quantify the scale of the potential problem. For gas companies, this includes a thorough review of all historical installation records; this step is crucial due to the inconsistency of historical records since the early 1980s when Horizontal Directional Drilling (HDD) became the widely-accepted preferred utility installation
method. In order to build a risk model, attributes such as installation year, pipe diameter and material, installation method, and high occupancy structure form the building blocks of the risk model, thereby identifying areas of highest risk. Higher occupancy structures, such as hospitals and schools, should rank higher on the priority list due to the consequence of an incident.
Once a risk model is developed, the scoping phase of the cross bore program begins. Guiding this phase are the following considerations: legacy (prior installations) inspections and/or support of current HDD construction activities. Consideration should be given as to whether or not sewers should be located prior to or following boring to eliminate the risk of cross bores. Because of the potential for both gas main and gas services to cross bore through existing sewers, sewer inspections should be completed to the foundation of the structure. Failure to inspect the entire length of sewer laterals leaves a gap in program coverage because gas service installation via trenchless technologies cannot be verified to be free of cross bores. For example, one gas company has identified hundreds of cross bores since the late 1990’s. The compiled data showed that nearly 80% of those identified cross bores were polyethylene (PE) gas services bored through sewer laterals. If the sewer inspections had not gone to the foundation, many of these cross bores would not have been identified.
Another key component of scoping is knowledge of federal, state and local regulations, which can have a profound impact on specific field activities. For example, a gas utility that operates across multiple states will have different regulations, particularly in regards to Operator Qualifications (OQ). Some elements of cross bore programs may require OQ in some states, whereas the same activity does not require OQ in other states. At the completion of the scoping step, it is important to quantify the volume of work, get budgets to support the program and set expectations for all stakeholders. Large gas systems can take more than 10 years to complete a full legacy program.
After the scope and budget have been determined, it’s time to plan for the execution of the working elements of the program. A comprehensive work plan and flow chart should be drafted to guide field efforts. If the limits of inspection are to the foundation of structures, then what technologies
will be necessary to ensure a complete inspection? There is no silver bullet when it comes to the tools used to eliminate and identify cross bores. Hydromax USA uses many technologies to provide valid and reliable data and has capitalized on the advancements in lateral launch CCTV systems, which form the backbone of our field data collection. However, lateral launch systems have limitations as well, particularly in regards to sewer pipe sizes, distances and conditions (i.e., offset joints, encrustation, bends, roots, etc.). Push camera systems may be needed, which require access to private property. Notification of local residents and appointment scheduling can be expensive and time consuming, yet a necessary component of a comprehensive cross bore program.
In terms of developing the framework to support data collection, ideally, the GIS data sets from both the gas and sewer utilities should be compiled in a central location. The overlaid gas and sewer data provide a reasonable starting point whereby a new GIS layer can be created for the elements of the cross bore program. Due to the tremendous amount of potential field-collected data, verifiable geo-referenced data is a critical element for any program to be sustainable. Hydromax USA captures GPS points in the field to update GIS data sets and mapping systems. All data can be time stamped to ensure verification in support of quality assurance and auditing. The advent of tablets and software innovations support better field data collection, which leads to better, more timely and efficient presentation of the results.
Trenchless installation techniques are widely used as minimally invasive alternatives to digging open
trenches. There is no doubt in regards to the time and economic efficiencies that can be gained from
trenchless methods; however, utility cross bores are a known unintended consequence that must be addressed. Technologies have evolved and processes have been developed to build comprehensive
programs to eliminate these risks. It all starts with open dialogue among key stakeholders.
Jeffrey Griffiths is Director of Hydromax USA. He can be reached at email@example.com. Hilary Skinner is a Project Manager for Hydromax USA and can be reached at firstname.lastname@example.org.