The vast and complex array of underground utility lines means that a One Call contractor’s marks may not be as accurate as your underground project demands.
What to do before you start digging
The first step is to contact your state’s One Call office and provide them with the location of the 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 Cody Mecham with McLaughlin. “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. McLaughlin 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 to determine if the One Call contractor missed something. For example, some water districts are not connected to the One Call system and potential water lines may not have been identified in the construction zone.
Types of locators
There are various types of locators on the market. Single-frequency split-box locators consist of a transmitter that is placed on the ground and induces a signal. The signal is picked up by the cable or pipe and then re-radiated back up to the receiver. The receiver is an important component of any locator. Most modern receivers 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 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 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. This 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.
Potholing is key to preventing utility strikes
While locators are becoming more accurate, it’s important to visually spot 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. Taking the extra steps to verify the One Call contractor’s markings and then potholing may seem counterproductive, but Mecham 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 Mecham. “The cost of shutting down a project for a day is sure to exceed the cost for a $3,000 locator and a little extra time.”