Navigating the Underground Maze

Sept. 28, 2010

Congested, crammed, jam-packed, overcrowded, teeming: all these words seem to describe the current underground infrastructure. Locating underground lines is becoming more of a challenge; as more communication and utilities lines are being installed underground to enhance our daily lives, they create more headaches for contractors.

Congested, crammed, jam-packed, overcrowded, teeming: all these words seem to describe the current underground infrastructure. Locating underground lines is becoming more of a challenge; as more communication and utilities lines are being installed underground to enhance our daily lives, they create more headaches for contractors.

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 Dave Gasmovic, president of McLaughlin. "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."

Major Locating Challenges

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. On the other hand, a cable line lying nearby is well-grounded and produces a strong signal.

The first step before digging is to contact your state’s One Call office and provide them with the location of the proposed area to be excavated. A One Call contractor will visit the site and mark the location of the existing utilities.

"The One Call contractors do a good job of marking the location of existing underground lines and pipes," says Gasmovic. "However, due to interference, the marks may not be as accurate as needed in some cases."

Underground interference could cause the marks to be three to four inches off the actual line or pipe. 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 on 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 that 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 they hand dig or excavate 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 and generates an alternating current down the cable or pipe.

Coil clamp systems fit many locating application. 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 and eliminates the need to connect clamps or coils to the line or pipe.

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

No matter the features, Gasmovic encourages contractors to select a locator that is simple for the operator to pick up and use within a week.

Line Exposure

While locators are becoming more accurate, it’s important to see exactly where the line or pipe is located. Contractors are not allowed to dig in the safe zone, which may be from 18 inches up to three feet from either side of the marked line. The required distance varies by state. Contractors are only allowed to dig by hand or use a non-destructive method like vacuum excavators in the safe zone.

Using a vacuum excavator instead of a shovel has advantages. A shovel against a water pipe is non-destructive, but on a fiber optic line a shovel can be as destructive as a backhoe, especially in hard ground conditions.

"A lot of cable has been installed using horizontal directional drills (HDD) rather than trenchers, so you don’t have the old-fashioned ditch line like in the past," says Gasmovic.

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. This indicates the line is in close 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 and range in size from 100 to 1200 gallons (379 to 4542 L) 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 1500 psi should be used for gas and fiber lines in order to not damage the line coating. A higher pressure can be used for water lines.

Selecting the Right Unit

Vacuum excavators come in all sizes and options, so it’s important to select a unit that will best fit your intended use. Water-based units typically dig faster through a wide variety of spoil types and reduce the volume of the 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 do not cut as well in hard ground conditions, such as clay.

"I encourage contractors to look for a unit with a good-quality vacuum blower, the heart of the vacuum," say 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 four-inch hose and 1025 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 three-inch hose will suffice. The blower size also affects the amount of engine power required. So a larger blower will increase the cost of the unit.

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.

There are number of options available:

  • Controls that allow the contractor to reverse the flow of the vacuum to blow the spoil back into the hole
  • Booms that support the weight of the hose, placing less effort on the operator
  • Combo units that include a jetter to clean sewers and remove the resulting trash
  • Automatic tank clean-out systems and auxiliary hydraulic systems that allow the contractor to run a concrete saw or breaker off the unit
  • Gasmovic also encourages 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. A sharp tip may slice or puncture a 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.

The Payoff

Taking the extra steps to verify the One Call contractor’s markings and then potholing may seem like an added expense or more time, but Gasmovic stresses that safety is important.

"Hitting a gas line with a backhoe, trencher or HDD could be catastrophic. A water line hit could put a hospital out of business," says Gasmovic. "The cost of shutting down a project for a day is sure to exceed the cost for a $3000 locator and a little extra time."

Greg Ehm is a Technical Writer with Two Rivers Marketing.