Locating & Marking Systems

It is a common paradox in underground utility work that the time to locate a line, valve or other buried component may be substantially greater than the time it takes to do the actual maintenance. While field work varies, there is a common interest among utilities in rapid and accurate locating to minimize digging effort, avoid unintentional damage, and maximize service to the customer.

The integrity of buried utilities – whether gas, water, wastewater, power, telephone or cable TV – is of vital importance. Users have come to expect that when any of these services are interrupted, they will be restored quickly, regardless of circumstances.

Passive underground markers have been adopted in a number of municipalities over the past few years with measurable improvements in locating efficiency compared to other methods. These electronic markers are supplied in various frequencies so that each utility has its own unique electronic identification to avoid confusion.

According to Mike Cannon, project engineer for Bount Sikes & Associates, an Orlando, FL-based public works consulting engineering firm, the challenge of fast and accurate location is often complicated by a concentration of utilities within a small right-of-way, inaccurate as-built plans and unmarked facilities. "We've found that most locating equipment gives only an approximation of position and depth" said Cannon. "When there are high-power lines overhead, a high water table, or pipes and metal objects in close proximity, it can be very difficult to pinpoint specific buried facilities."

The Florida engineer notes that until a few years ago, most utilities using plastic pipe relied solely on tracer wire with an applied signal to find underground facilities. The tracer wire process is imprecise and suffers from inaccurate placement of wires, frequent construction damage, and ground shifts that break the wires. In other cases, workers simply dig by hand at an approximated location or use a probe rod or water jet probe to try to locate a buried component. Using such methods, can take hours to zero in on the desired location.

Cannon specified passive markers from 3M, the only major manufacturer of this type of marker, for a 1993 project involving the South Seminole County and North Orange County, FL, Wastewater Transmission Authority. Based on positive experience with that process, Seminole County soon revised its Water & Sewer Standards and the Land Development Code to specify the ScotchMark brand passive marker system for county wide water and sewer locating. Neighboring Orange County eventually followed suit, according to Cannon, and is now using passive markers on water and sewer installations.

The 3M electronic markers are sold in near-surface, mid-range, and full-range models. The small 3-inch, near-surface units are designed for use at 2 feet below grade. Two mid-range models are available for 4 to 6 feet depths. Large 15-inch diameter markers allow location of facilities that are as far as 8 feet below grade.

These devices are completely passive and function indefinitely because they require no external source of power and resist moisture, chemicals, minerals and temperature extremes. The markers are color coded and tuned to identify various services in accordance with American Public Works Association guidelines.

A portable, hand-held locator transmits a pulsed radio frequency signal to a buried marker, and the signal is reflected back, giving the operator both a visual reading and an audible tone. The process is not affected by nearby metal objects, electrical power fields, or passive markers that are tuned to other frequencies.

Pin-point accuracy
Seminole County survey party chief Glen Denman said, "Most locating equipment gives only an approximation, but with this system we can pinpoint a spot accurately and sort out water and sewer facilities from other objects."

Denman explains that the Seminole County Public Works Department places passive markers along water and sewer routes on major and secondary roadways and inside subdivisions. Markers are buried every 100 feet on long straightaways, at 90, 45, and 22 degree turns, and at all tees, wyes, valves and hydrant valves, and the terminal end of a service lead. In some cases, the county's facilities share trench space with private users, but with the passive marker system, there is no question as to which pipes are part of the water and sewer network.

Independent contractors working in this Florida county also are required to use marker balls, placing them two feet below natural ground level on all utility construction. New Seminole County as-built plans show the location of each marker ball, and workers find that they can use a posthole digger to quickly reach the buried markers. Cannon estimates that use of passive markers has cut locating time by 80 percent and reduced labor costs in the county by thousands of dollars annually.

Natural gas utilities also are finding passive markers useful for efficient digging. Gene Booth, southwest regional supervisor for Southern Cross Corporation, provides field services to gas companies in Louisiana, Texas, New Mexico, Oklahoma and Arkansas. He reports that passive markers are replacing less effective means for locating buried facilities in the gas industry.

"The traditional way to mark service taps and stub-outs on a natural gas line in a new subdivision has been to bury tracer wire and tie the ends of wires to a wood or plastic stake," Booth explains. "This theoretically allows the contractor to find the stub-out and make the gas line connection to each property. Unfortunately, the wires are often broken at multiple points, and cannot be located."

Near surface markers
According to Booth, gas utilities in his region are adopting ScotchMark near-surface markers instead of stakes and tracer wire to spot gas line service taps and stub-outs. Buried up to two feet below the surface, the 3-inch long reusable colored plastic markers are seldom accidentally damaged and can be easily located. The near surface markers are used to mark gas valves on toad widening jobs.

"Construction subcontractors have their own electronic marker system locators, which are inexpensive and very simple to operate," Booth said. "They are pleased with this method for finding buried facilities because it saves them a substantial amount of time and work when connecting to gas mains."

Booth convinced Lone Star Gas in Dallas, TX, of the benefits of passive marking by meeting his own challenge. He gave the company 10 marker balls, which they buried at random to a depth of 4 feet. The only qualification of the test was that he be given their general location within 50 feet. Booth quickly found all 10 markers, demonstrating the strong advantage of passive marking over other locating methods. Lone Star Gas adopted the passive marking system.

3M Corrosion Protection Products
6801 River Place Blvd A130-3N-54
Austin, Texas 78726-9000
United States
Phone: 800-722-6721
Fax: 651-604-0214