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Subsurface Utility Mapping

Geophysics Identifies Buried Utilities and Other Subsurface Obstructions Prior to Construction of Wide-Area Solar Carports


Installation of overhead solar panels across large, uncovered parking lots in U.S. Southwest to provide electricity for the associated facilities and power for the new plug-in vehicle charging stations constructed in the parking lots.  In addition, the solar panel arrays were designed as “solar carports” to provide protective shade against the blistering desert sun for employee and customer vehicles.


Because the installation work was performed at existing facilities, there was concern that underground utilities and other substructures may be encountered unexpectedly during construction, which entailed drilling to install piers and footings for the large overhead solar panel arrays, and backhoe trenching to lay the cable runs that would bring electrical current from the solar arrays to the facility itself.  In addition, there were concerns that other problematic underground features, like underground storage tanks (USTs), old foundation remnants, and pockets of buried refuse, may also be present.  Such underground “surprises” could lead to cost overruns if they caused delays or damages during construction, or if their presence required additional design work in the middle of the project.

Although some “as-built” drawings of the facilities were available, they were deemed to be unreliable because they were out-of-date and did not show all of the surface features and underground utilities; moreover, many of the drawings were of a “general arrangement,” or diagrammatic, nature and did not show utility networks accurately enough for design or construction purposes.  For the project to move forward, a complete and accurate facility map, one that included underground utility locations, was needed.


AGS used a multi-method approach to locate buried utilities and other substructures of potential concern.  AGS scanned the proposed trench alignments and pier and footing locations with a combination of a pipe and cable locating system, ground penetrating radar, and a simple hand-held “junk-finder” type shallow-looking metal detector.  AGS marked the location of detected utilities on the ground surface with colored spray paint according to the American Public Works Association (APWA) guidelines.  The utility locations identified by AGS were then surveyed by the land survey crew for incorporation into the facility map.

To search for substructures and buried refuse, which are not readily detected with a standard pipe and cable locating system, AGS used a high-sensitivity electromagnetic (EM) metal detector (the Geonics EM61) in combination with an EM Terrain Conductivity Meter (the Geonics EM31).  In general, the EM61 was used to look for localized metal masses like USTs, reinforced concrete foundations, and buried metal debris, while the EM31 was used to look non-metallic targets such as pockets of domestic waste-fill and backfilled sumps where hazardous liquids may have been disposed.  The EM surveys were performed in conjunction with a Global Positioning System (GPS), which provided horizontal positioning information for the EM data and facilitated rapid data processing and analysis.


The locations of underground utilities detected by AGS’ were incorporated into the facility maps prepared by the land surveyors.  In addition, AGS downloaded and processed EM data in the field to produce data profiles and contour maps that showed EM anomalies indicative of suspicions subsurface conditions.  The geo-referenced EM anomaly maps were incorporated into the facility maps prepared by the land surveying company, allowing the anomaly locations to be referenced to the facility so that they could be avoided or investigated in the future.  The final maps were used by the design team to position the solar panel footings and route the cables to minimize conflict with existing substructures.  This process helped control projects costs by minimizing delays and damages that can occur if buried utilities and other substructures are encountered unexpectedly during construction.