GPR as part of SUE and Damage Prevention

Transportation infrastructure maintenance and capacity is a hot political topic right now. The U.S. economy depends on its network of roads, rails, ports and airports to move people and goods both nationally and internationally.

In New England, an international airport planned to install a cooling tower as part of an expansion of the existing mechanical system. The aviation organization contracted KCI to conduct an underground geophysical investigation of the existing infrastructure in the area, as part of the design phase. Knowing the precise location of utilities helps prevent damage to them and allows the engineering team to design around them.

To depict and define the existing utilities, we utilize the subsurface utility engineering (SUE) process, in accordance with the American Society of Civil Engineers ASCE 38-02. The standard guideline outlines the process for the depiction of existing subsurface utility data at various quality levels. In this case, the client needed to identify the approximate horizontal position, Quality Level B (QL-B), of existing, unknown and/or unmapped utility structures. Prior to the field work, the team completed a Quality Level D (QL-D) records research investigation. As-built plans indicated the presence of non-conductive/non-metallic utilities within the project area—in this case, a reinforced concrete pipe storm drain system. These types of utilities are prevalent throughout the Northeast USA. Although electromagnetic (EM) locating equipment cannot detect this infrastructure, crews utilized a combination of records research and field experience to identify utilities that don’t generate an electromagnetic tone.

To depict the approximate horizontal positioning of this underground infrastructure, field technicians deployed Ground Penetrating Radar (GPR) in conjunction with alternative surface geophysical equipment.

GPR graphic

A Sensors & Software, Inc. LMX200TM GPR unit offered the ability to provide high-quality real-time data for field interpretation, and then store and refine the data by post-processing methods. The SUE field technician was able to depict and correctly interpret the existence of the non-conductive storm line indicated by the hyperbolic shape with green reference dot.

GPR display
Verification of the non-conductive storm line location, indicated by the green dot at the apex of the hyperbola.

The horizontal positioning was then confirmed by utilizing a fiberglass detectable fish tape. The tone from this metallic wire, which was fished through the pipe, was then heard using the EM equipment. Technicians then painted horizontal reference points on the surface along the alignment that would later be surveyed (QL-B). During the same field work, additional target utilities, some conductive in nature, and a possible dense composition layer were also observed while operating the LMX200.

For this and many similar projects, the combination of various SUE quality levels provide clients with an accuracy of the existing underground infrastructure to help mitigate potential redesign and reduce damage costs once in construction. Our team interpreted the existing underground infrastructure record drawings (QL-D) and utilized standard geophysical locating techniques alongside an LMX200 GPR system (QL-B) to delineate the known and unknown utilities and anomalies.

Cases like the one described show the importance of SUE and its direct impact on the design and construction components of a project. To help determine the level of SUE investigation required for a project, consult a professional SUE provider for assistance.