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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 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.
Hydrometer analysis was used as the main method to determine soil particle size and, ultimately, soil texture. Sieve analysis results were used to examine the particle size distribution of larger grained material, and to distinguish soil from gravel. The laboratory tests do not reveal data for the soil in isolation, but rather provide results for the entire sample tested. Since the aim was to prove that soil conditions have an impact on the performance of ground-penetrating radar, the soil portion was isolated from the entire sample by only considering particle sizes less than 2 mm. The gravel portion of each sample was therefore excluded. The relative proportions of clay, silt and sand were then expressed as a percentage of the soil portion of the total sample. These percentages were then used to classify the soil texture in terms of the soil texture triangle of the United States Department of Agriculture (USDA).
The Leica DS-2000 utility detection radar device was used for the groundpenetrating radar testing. This device transmits two different electromagnetic waves (250 and 700 MHz). The higher-frequency electromagnetic waves allow for better resolution in detecting shallower objects. The lowerfrequency electromagnetic waves allow for deeper penetration, but the imaging resolution is notably lower for ground-penetrating radar testing equipment.
The equipment was tested at three testing sites: the Montecasino Office Precinct, the Sandton Gate Development and the Rustenburg Effluent Treatment Plant.
RESULTS
Montecasino Office Precinct: This site is situated in Magaliessig Extension 64, Johannesburg. The soil texture was classified as sandy loam according to the USDA’s soil texture triangle. Ground-penetrating radar was effective under this soil texture, with accurate depth measurements. The response of the ground-penetrating radar was different, and better than that obtained at the other two test sites.
Sandton Gate Development: This site is situated in Glenadrienne, Sandton, Johannesburg. The soil texture was classified as clay loam according to the USDA’s soil texture triangle. Ground-penetrating radar was reasonably effective under this soil texture. However, the radar depth measurements were inaccurate and constantly shallower than the actual depths measured after exposing the services. The response of the ground-penetrating radar differed from that obtained at the other two test sites. The performance of the radar was not as good as at the first site, but better than at the third site.
Rustenburg Effluent Treatment Plant: This site is situated on Brons Street, Rustenburg. The soil texture was classified as clay according to the USDA’s soil texture triangle. Ground-penetrating radar was totally ineffective under this soil texture, and failed to detect any services. The device used was unable to detect a 75 mm-diameter PVC pipe installed as shallow as 345 mm below the natural ground level. The response of the groundpenetrating radar differed from that obtained at the other two sites, and was of no use.
Conclusions
It could be concluded that ground-penetrating radar performs well under sandy loam conditions with accurate depth measurements. Ground-penetrating radar was reasonably effective under clay loam soil. However, depth measurements were inaccurate. Ground-penetrating radar was ineffective under clay soil conditions. It was proven that different soil textures have an impact on the response of the ground-penetrating radar. It could also tentatively be concluded that the response of the ground-penetrating radar weakens as the size of the soil particle decreases.
Reference
United States Department of Agriculture (USDA), 2018. Natural Resource Conservation Service. https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/?cid=nrcs142p2_054167.
