Examining underground utilities with ground-penetrating radar Louis Germishuys Prof Chris Cloete
Unintended damage to subsurface utilities during construction excavation is a major cause of disruption in electricity supply, telecommunications, water supply and other essential public services. Utility strikes are also a leading cause of hazardous liquid and natural gas accidents and cost billions of dollars each year. Research conducted in the Department of Construction Economics investigated the feasibility of using ground-penetrating radar to examine underground utilities.
Several non-destructive technologies are available for the examination of underground services. However, groundpenetrating radar is currently the preferred method. Groundpenetrating radar can detect non-metallic objects, which is its key advantage over other nondestructive technologies. The depth of utilities can be estimated using processing methods such as wave-speed estimation. Ground-penetrating radar has a higher resolution than other non-destructive technologies. The integration of ground-penetrating radar and global positioning system (GPS) technology ensures a high accuracy level in locating subsurface utilities in three dimensions. Ground-penetrating radar is a geophysical instrument with a diverse range of applications. It has been widely used in locating underground services due to its advantages, such as fast data
acquisition, cost efficiency when mapping large areas and highresolution imagery for improved interpretation. However, the accuracy of subsurface mapping using ground-penetrating radar has often been overlooked due to a lack of understanding of the physical basis on which it operates, a lack of a standard methodology for data collection and a lack of reliable accuracy assessments. Ground-penetrating radar is used to “see through� the ground, either to establish the structure of the soil or to find buried objects such as utilities made of metal, plastic or concrete. The variables of importance in using this technology are primarily the electromagnetic soil properties of relative permitivity, electrical conductivity and magnetic permeability, which affect how electromagnetic waves propagate and reflect in the subsurface. The aforementioned electromagnetic variables are
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influenced by the physical soil properties, such as saturation, mineralogy, porosity and soil texture. The effectiveness of groundpenetrating radar was therefore tested under three different soil textures. Grain size is classified as clay if the particle diameter is less than 0.002 mm, as silt if it is between 0.002 and 0.06 mm, and as sand if it is between 0.06 and 2 mm. Soil texture refers to the relative proportions of clay, silt and sand particle sizes, irrespective of the mineralogical or chemical composition of the material. The purpose of this study was not to quantify the impact or accurately define the relevant soil characteristics, but rather to prove that an impact exists, and to illustrate the practical problems associated with ground-penetrating radar when operating under different soil conditions.
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