Underground contractors must safely and efficiently identify existing infrastructure. This task has become complex and time-consuming. The introduction of the nationwide one-call system promises to make identifying infrastructure easier. However, the vast and complex array of underground lines means that one-call contractor’s marks may not be as accurate as your underground project demands.

“The biggest challenge is that there is so much more pipe and cable in the ground today,” says McLaughlin president Dave Gasmovic. “These lines give off more interference when you’re trying to locate them. Since the area is more congested, extra care is needed when you expose the lines.”

Anytime you use an underground locator to find a cable or line, you induce an alternating current or magnetic field onto a cable. The receiver retrieves the signal coming off the cable you are locating. As the signal radiates off the cable, it not only goes up, but also radiates off other lines in the vicinity. The signal reradiates off the cables and goes up to the ground, creating a false signal.

“People assume it’s the phone line, start digging, and it ends up being a gas or water line,” says Gasmovic.
The accuracy of your locator signal really depends on how well the product you are trying to locate is grounded. A cast iron water main is difficult to locate since the gaskets between the pipe joints are not a good conductor; however, a cable line lying nearby is well-grounded, producing a strong signal.

Step one before digging is to contact your state’s one-call office and provide it with the location of the proposed area to be excavated. A one-call contractor will visit the site to mark the location of the existing utilities. Underground interference could cause the marks to be 3 to 4 in. off the actual line or pipe, Gasmovic notes. If you use a shovel to visually locate the line, it may take some time to find it. Gasmovic encourages contractors to purchase their own locator to double-check the accuracy of the original markings.

Underground contractors can make a sweep of the area and might pick up something that the one-call contractor may have missed. Some water districts are not connected to the one-call system, therefore, potential water lines may not have been identified in the construction zone. It’s all a matter of preventing damage in today’s maze of underground lines.

“The old philosophy of I’ll cut, fix it and go on are over,” says Gasmovic. “You can be putting homes and businesses out of services and shutting down your project for a day. Using a locator and a vacuum in combination will help you locate the cable and get to your main excavation work in a timely manner.”

Selecting a Locator

There are various types of locators on the market. Single-frequency split-box locators have been around for decades. These systems consist of a transmitter that is placed on the ground and induce a signal. The signal is picked up by the cable or pipe and then re-radiated back up to the receiver. Single-frequency systems work well on lines and pipes in non-congested easements but put a high frequency into the ground and they light up everything underground and may produce a distorted signal.

Since lines and pipes have different grounding systems that may locate better at a higher or lower frequency, multi-frequency systems allow you to tune the frequency you are putting into the ground to the type of line or pipe you are trying to locate. The lower the frequency, the better it will stay on the cable you are trying to locate.

The receiver is an important component of any locator. Single-frequency systems have a single receiving antenna and work well where there’s not a lot of congestion. In more congested areas, multiple receiving antennas help filter out interference.

Most modern receivers today can estimate the depth of the line or pipe at the push of a button. The locator measures signal strength and uses an algorithm to convert this information into an estimated depth. However, interference can distort the depth estimate. Despite the possible inaccuracy, the estimated depth gives the contractor an idea of the location of the line or pipe as he or she hand digs or excavates the area with a vacuum.

There are also different ways to put the locator signal on the line or pipe. Modern locators have a direct connection method. This allows workers to connect cable leads, similar to jumper cables, to the line or pipe to be located, generating an alternating current down the cable or pipe.

Coil clamp systems fit many locating applications. While many clamp systems must go around the cable or pipe and touch, some locators feature an inducing coil that sits on the pipe and induces the signal into the product. The inductive method requires the user to set the transmitter on the ground. A signal radiates down through the ground onto the cable, eliminating the need to connect clamps or coils to the line or pipe.

Some locators offer a Current Measurement Index (CMI), which measures the current you are putting onto the cable. This helps ensure you are staying on the original line to be located and not jumping to other lines in the area.

Line Exposure

Contractors are not allowed to dig in the safe zone, which may be 18 in. to 3 ft from either side of the marked line. Contractors are only allowed to hand dig or use a non-destructive method like vacuum excavators in the safe zone.

When lines are installed using a trencher or backhoe, a lighter material, like sand, is placed around the line. As a contractor digs, the ground gets softer, indicating the line is in proximity. Lines installed using HDD don’t disturb the ground or leave a ditch line, so the ground is the same hardness and it is difficult to know if you are getting close to the line or cable. Since the ground may be hard, you can easily cut a cable line with a shovel. Using a vacuum with air or water at a non-damaging pressure will safely expose the line.

Vacuum Excavator 101

Vacuum excavators are self-contained units that use pressurized air or water to displace soil and create a dry or wet spoil. The displaced dry or wet spoil is removed from the area through a hose using high-velocity suction and stored in a holding tank on the vacuum. Vacuum excavators can be mounted to a trailer or the back of a truck; they range in size from 100 to 1,200 gals of capacity.

Since vacuum excavators use low-pressure air or water to remove spoil, they are perfect for potholing or identifying existing utilities during underground construction projects.

“Damaging existing utilities can be costly in terms of project downtime and potential contractor fines,” says Gasmovic. “The low-pressure water and air will not damage existing utilities like a backhoe, compact excavator or shovel. In fact, the air and water move around the existing utilities, giving the operator a clear view.”

Operators can select the amount of air or water pressure, depending on the utility. A lower pressure of 1,500 psi should be used for gas and fiber lines so as not to damage the line coating; a higher pressure can be used for water lines.

Selecting the Right Unit

Water-based units typically dig faster through a wide variety of spoil types, reducing the volume of material. These units move more displaced wet spoil into a holding tank than on an air system. However, the displaced spoil is wet and cannot be returned to the site immediately without drying. While spoil from air systems can be directly returned to the site, these systems don’t cut as well in hard ground conditions.

“I encourage contractors to look for a unit with a good-quality vacuum blower, the heart of the vacuum,” says Gasmovic. “They should also select a tank that has the capacity to hold a half-day’s or day’s worth of spoil. This will reduce the number of trips you need to make to dump the holding tank.”

If you are working in areas with cobble rock, then a unit with a 4-in. hose and 1,025-cfm blower unit will be more productive. Cobble soils will require a larger blower to effectively remove the spoil. The larger diameter hose will help reduce the potential for clogging. In areas without rocks, a 575-cfm system and 3-in. hose will suffice. The blower size also affects the amount of engine power required.

Gasmovic recommends that contractors pay special attention to the filtration system and select a system that will filter the spoil and avoid clogging. Finally, be sure to select a strong trailer frame that will support the weight of the unit and a full tank of spoil.

Gasmovic also urges contractors to think about safety when operating a vacuum excavator. It’s important that the air knife or water lance have a covered tip so as not to damage the cable or line. When exposing around underground electrical lines, operators should wear dielectric boots and gloves. The electrical line could have a fault and leak electrical current down the line. Eye protection is a must when using either system, as well as proper traffic control equipment when working along a street.

Greg Ehm is a technical writer, based in Des Moines, Iowa.

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