Preventing Damage to Underground Infrastructure Takes More Than the Right Equipment
Constructing and repairing underground outside plant infrastructure are complex tasks that involve multiple factors, one of the most critical being to avoid damaging existing buried pipes, conduits, and cables.
The Common Ground Alliance’s Best Practices Guide is recognized as the most comprehensive guideline for preventing damage to underground facilities; however, despite progress made in preventing damage to buried infrastructure, accidental utility hits continue to happen on a daily basis.1
These incidents can cause significant damage, resulting in disruption of essential services, property damage, costly delays in construction, and serious injuries, even death. Their aftermath often includes litigation with costly legal fees, sometimes with judgments that can force a contractor out of business.
Identifying the presence of utilities on job sites during the planning stages is an important first step in preventing utility hits. The One-Call process is designed to coordinate the locating and marking of buried pipe and cable prior to excavation or boring.
Whether or not the crew that arrives on a utility construction site had a role in the One-Call notification process, you must not assume that paint and flags marking buried infrastructure are accurate. Instead, supervisors and crew members must have what we call "underground awareness": making themselves aware of what is underground and out of sight.
Awareness of the underground means nothing can be taken for granted.
• Locations of nearby utilities may be marked, but are the markings accurate?
• Are they complete?
• Are there utilities present that were not marked?
Failure to consider these possibilities can have serious consequences. Never begin excavating, trenching, boring, or drilling without knowing locations of existing utilities.
Though not ideal, there are times that marking is not completed before a construction crew arrives. Even if that’s the case, a careful visual inspection of the job site may reveal indications that other utilities are present. Gas meters, electrical boxes, and communications pedestals could mean there are buried lines.
Even when locations are confirmed, it is necessary to pothole (physically digging a small hole to visibly confirm the exact location and depth of a pipe or cable) when adjacent utilities parallel or cross the path of the new installation. And while you may want to assume the flag or paint markings are accurate, it’s critical to understand that all utility providers are not One-Call members, and, in many areas, water and sewer lines may not be marked.
Also, on-site personnel should be aware of tolerances required for depths of new installations and distances they should be away from existing facilities. Local codes and ordinances may be applicable. Depending on local requirements, new installations must be either 18 or 24 inches from the edge of a utility already in place, assuming the width is represented accurately by marks or the size of utility is provided and that single painted lines represent the center of the utility.
When a project is outside public right-of-way or the work location is on private property (educational institutions, government complexes, business parks, to name a few), One-Call may not make locates. Therefore, the responsibility falls on the property owner, primary contractor, and sometimes the utility contractor.
Regardless of who locates and marks buried facilities, many contractors have found it’s worth the time and effort to confirm the locations with their own personnel and locating equipment. When locating responsibilities fall on them, they must have the equipment to do the job or hire a locating specialist.
There are basic locating options in terms of the equipment needed for underground locating:
Option #1: Electromagnetic Locators
Electromagnetic locators are the primary locating tool used by utilities, contract locators, and underground construction contractors. Electromagnetic locators are relatively easy to operate, and when correctly used are accurate. An electromagnetic system consists of a handheld receiver and small transmitter.
The operator walks above where utilities are expected to be, and the receiver locates underground pipe and cable by detecting magnetic fields created by electrical current passing through the lines. Information is displayed on a window at the top of the receiver.
For communications cable and metallic pipe, the small transmitter is connected to cable or pipe, and sends current through the line to create a signal which is detected by the receiver. For plastic pipe with tracer wire, the wire is energized by the transmitter to provide a signal that the receiver can pick up.
Electromagnetic equipment has advanced significantly in the past several years. For example, the Subsite® Electronics Utiliguard® locator has an Ambient Interference Measurement (AIM®) feature that scans the surrounding area for noise that could interfere with the locating signal, and recommends the best frequencies to make the fastest, most accurate locate, and it reports both horizontal position and depth of the line being located.
The primary shortcoming of electromagnetic locators is the necessity for the presence of current flow to make the locate.
Option #2: Ground Penetrating Radar Locators
Ground penetrating radar (GPR) utility locators avoid the need for electrical current to find buried utilities. The GPR locating component, mounted on a wheel platform, is pushed across the work area as radio waves are generated downward into the earth. These signals bounce off buried objects, reflecting back to a receiving antenna, which the GPR unit converts to a graphic representation of the underground that is displayed on a screen mounted on the operator’s handle.
While GPR equipment can perform in situations that are beyond the capabilities of conventional electromagnetic locators, they also find rocks, chunks of concrete, other debris, and tree roots, making interpretation of the screen sometimes challenging.
The most significant limitation to GPR locators is they don’t work in dense, highly compacted or conductive soils, directly linking their effectiveness to soil conditions that has limited their use for utility locating. However, recent models are addressing that issue.
Option #3: Potholing
The best way to be absolutely certain of the location of a buried utility is to actually see it. Uncovering buried lines is nothing new, and for years it was done with a backhoe, compact locator, or by hand with shovels. However, mechanical methods run the risk of damaging the utilities they seek to protect.
Vacuum excavators have changed the potholing process. Using "soft" excavation technology employing either high-pressure water or air, a vacuum excavator can dig a 1-foot square, 5-feet deep pothole in about 20 minutes without the risk of damaging the pipe or cable being located. As easements become more crowded with buried utilities, many project owners, primary contractors, and regulatory agencies mandate potholing any time a new installation crosses or closely parallels an existing utility.
Most vacuum excavators on utility projects are compact trailer models, although truck-mounted models also are available.
In addition to potholing, dual-purpose vacuum excavators have become the primary method of keeping directional drilling work sites clear of drilling fluids escaping the bore hole.
Don’t Take the Hit
A first step to preventing damage to underground facilities is to understand how to avoid accident hits, and the 2016 version of CGA’s Best Practices 13.0 is available for download on the Common Ground Alliance (CGA) website.1 Carefully following CGA guidelines will reduce the risk of damaging utility infrastructure, and at the same time reduce the risk of injury to construction personnel and the general public.
Constant awareness of the underground and what it contains by everyone on the work site provides motivation to follow accepted guidelines and procedures, and to never take shortcuts.
Endnotes
1. http://commongroundalliance.com/best-practices-guide
Established in 2000, CGA is committed to saving lives and preventing damage to underground infrastructure by promoting effective damage prevention practices. The Mission of the CGA is to provide clear and tangible value to their stakeholders by helping to reduce damages to North America’s underground infrastructure. The CGA works cooperatively, fostering a sense of shared responsibility to enhance safety and protect underground facilities by: identifying and disseminating the stakeholder best practices; developing and conducting public awareness and education programs; sharing and disseminating damage prevention tools and technology; and serving as the premier resource for damage and one call center data collection, analysis and dissemination. For more information, please visit http://www.commongroundalliance.com.