Effective methods used for a utilities verification and marking include:
1. Field walk-downs of excavation site by approved subject matter experts.
2. Testing to validate identified utilities.
3. Testing to identify utilities, not yet identified:
a. Ground penetrating radar method.
b. Passive/active frequency method.
4. Survey Paint Markings and/or Survey Flags:
a. should be durable and timed to effectively coincide with the work group mobilization;
b. should be installed no more than two or three days prior to the actual excavation work; and
c. should be verified immediately prior to the beginning of the excavation work.
5. Utilities color coding:
a. Same American Public Works Association (APWA) color coding as municipalities use.
10.3.1 Field Location of Excavation Boundaries
Accurate identification of excavation boundaries is necessary. It accurately communicates those boundaries to all workers and supervisors. It also sets the excavation limits, based on the research and locator testing provided for that particular excavation. Elements of the identification of locations and excavation boundaries include:
1. Excavation maps that can be used to identify exact excavation boundaries (field markings can easily be transferred from these maps);
2. Excavation maps that can be attached to the excavation permits for additional communication assurance; and
3. Appropriately marked field excavation boundaries.
10.3.2 Locator Equipment Selection and Limitations
It is important to note that no locator technology should be relied upon as the sole source for identifying buried or embedded utilities. Configuration management is the most effective identification method.
The majority of underground utility locators being used today apply a signal to the underground system being located. This signal may either be of an audio or radio frequency. An active signal is applied to the underground utility by various methods, with the signal being generated from the transmitter unit of the location system. Proper setup of the transmitter increases the accuracy of the receiver unit. Proper use of the receiver unit dictates that the antennas be moved in a straight path and not in a swinging motion. Accurate depth readings are gained from experience.
Electromagnetic (EM) pipe and cable locators feature microprocessor-controlled transmitters and receivers capable of detecting power lines, telephone cables, and metal piping at depths up to 20 feet. These detection devices operate with multiple discrete broadband frequencies, antenna configurations, and grounding capabilities.
10.3.2.1 Direct or Conductive Method
One method of applying a signal to a known utility is the inductive clamp or coupler, where the signal is induced to the utility by means of a jawed clamp placed around the utility access point without the grounding system being disturbed. Inductive couplers and clamps all apply a signal to the utility in basically the same manner. The signal is induced onto the utility to be located by an electromagnetic field created by the coupler and clamp. It is necessary that the utility have grounds in place at both ends of the section to be located. Missing bonds across insulated sections of the utility prevents this method from being used along the entire length of the utility.
Underground metal pipes and cables should not be joined with conductive materials, thus making them short, separated pieces of a broadcast antenna. There could be multiple utilities bonded together (i.e., cathodic protection and common grounding), making their individual resolution difficult. These and other problems create the need for variety within the EM method itself. Couplers and clamps should be positioned below the electrical grounding point on CATV, electrical, and telephone cables. Systems that do not use earth ground, such as railway signal cables, cannot be located with couplers or clamps. Gas meters with insulated couplings should not be bonded. They should be direct-connected.
10.3.2.2 Inductive or Indirect Method
Another method is the inductive or indirect mode. With this method, the transmitter is placed on the surface of the ground above the known utility. The signal from the transmitter is induced onto the utility, making location of the utility possible. Inductive transmitters should only be used when access points for the utilities are not available. This method can lead to serious tracing errors, especially if other utilities are buried/embedded in the same area.
10.3.2.3 Detector Frequency and Power Considerations
Choosing the appropriate locator technology and methods greatly improves the chances of success. As an operator attempts to detect the location of underground/embedded utilities, the
frequency and power capabilities of the instrument being used should be understood. In many detectors, as the frequency increases, the available power decreases.
Frequency selection can affect depth of penetration, distance of travel, resonance efficiency, and resolution issues, as well as other operating characteristics. Most locator manufacturers preset frequencies, and there is an optimum frequency to use for a particular utility system.
These are some of the most frequently used:
1. ELF (Extremely Low-Frequency) operates at below 300 Hz. It is typically used for:
power cables that are energized and carrying a load; water pipes grounded to a power system that are energized and carrying load; and deep, very conductive, long-length utilities.
2. VLF (Very Low-Frequency) operates between 3kHz and 30 kHz. It is typically used for very long, continuous conductors, and deep conductive long length utilities.
3. LF (Low-Frequency) operates between 30kHz and 300kHz. It is typically used for shallow (8 feet or less) conductors of medium length.
4. HF (High-Frequency) operates between 3MHz and 30MHz. It is typically used for cables; shallow short conductors.
Not all utilities are metallic or of sufficient length in the ground. Therefore, resonant EM methods are not always the answer. The current family of surface geophysical methods available for utility and other near-surface structure or hazard detection is extensive. Other EM techniques such as measuring eddy currents, differential heat, and thermal reflection are also available. Below are three types presently in use:
1. Terrain Conductivity uses the VLF range, which uses Eddy Currents to measure differences in ground conductivity. It has proven useful in locating very deep or short metallic utilities and air/gas-filled utilities.
2. Ground-Penetrating Radar which uses 1-100 gigahertz microwaves to measure reflections due to dielectric differences in subsurface materials. It measures the strength and amount of time necessary to bounce a signal off different layers. This technology is effective only in resistant soils. Utilities should be of sufficient size to be detected. The deeper the utility, the larger it should be in order to be detected. Highly different soil/utility materials give the best results.
3. Thermal measures heat output. When the amount of heat from the utility is different from that of the surrounding soil, it might be detectable. Some utilities produce their own exothermic heat or retain heat longer than the surrounding soil.
10.3.3 Locator Operator Training
Each piece of locating equipment is unique. It is very important that operators be trained to use the equipment before applying it in a field application that determines safe boundaries for excavation workers. Some of the ground penetrating radar equipment is very dependent on operator interpretation of the characterization profiles generated from the locator equipment.
Operators unfamiliar with the equipment specifications and operating instructions can make interpretive mistakes that can lead to serious injury to the excavation workers.
Proper selection of available techniques and the use and interpretation of data produced by this equipment is essential to the accurate and comprehensive detection of underground utilities.
Regular calibrations and maintenance of locator equipment should be established.
10.3.4 Field Marking of Identified Utilities
Paints/Surface Markings are used at many jobsites. Care should be taken to ensure the markings are clearly present and identifiable at the time the excavation workers arrive at the excavation site. Many cases of faint markings and/or markings washed away by rain have been documented at jobsites. The recent use of biodegradable survey paints has increased this possibility. They are not as durable as former types of survey paints used. Lawn mowing equipment can erase or diminish the effectiveness of survey paints in outside areas.
Stakes or Flags are used to mark identified utilities at some job sites. These should be durable, and able to withstand the environments they are exposed to. Lawn mowing equipment can erase all traces of survey flags in very short order. Incidents resulting from missing and even relocated stakes or flags have been documented.
Utilities Color-Coding is used at most work locations. The standard utilities color-coding is well understood by most excavation subcontractors, and serves as a very effective communications tool for the facility owner.
Plastic Utilities Ribbon Tape is used in most facilities, as well as commercial and industrial applications. It is installed directly above the utility, but below the surface of the ground so an excavator can dig it up, and thereby indicate the presence of a utility, before the utility is damaged.
Metallic Utilities Ribbon Tape is used in many facilities. It is buried at a specified depth above the utility. This ribbon can be detected and its route traced by a metal detector.