DOWNLOAD previous article 2. article - 2011/1next article Matevž U. Pavlič and Blaž Praznik

Detecting karstic zones during highway construction using ground-penetrating radar

Abstract

Ground-penetrating radar (GPR) has been applied to determine the subsurface karstic features during the construction of the national highway in the south-eastern part of Slovenia. The highway construction is situated mostly in the dinaric karstic region with a high density of karstic features visible on the surface. Ground-penetrating radar prospecting was done in all areas where a slope was cut into the limestone bedrock. The main purpose of the survey was to map potentially hazardous zones in the highway subsurface and to detect and characterize the karst. The ground-penetrating radar method was used because of the heterogeneous nature of the karst. With its high degree of karsticifaction and geological diversity all conventional methods failed. One of GPR’s main advantages is that, while the penetration depth is limited to several meters, the obtained resolution can be on the scale of centimeters and the measured profile is continuous. Because of the ground-penetrating radar’s limitations with respect to depth, the range surveying was done simultaneously with the road construction using 200-MHz bistatic antenna on the level of the highway plane. All the 2D radargrams were constructed in 3D models where the measurements were made in raster with 2 meters between a single GPR profile. This two-meters spacing was determined as the optimal value in which only a minimal resolution-price tradeoff was made. The gathered results were tested and compared to experimental drillings and excavations so that any anomalies and reflections were calibrated.

The drilling was conducted twice, first to calibrate the radargram reflections and secondly to check and confirm the calibration success. Altogether, over 30 boreholes were drilled at various previously selected locations. The data obtained from the drilling proved to be very helpful with the calibration since anomalies found during the drilling were almost exclusively (over 95%) a result of the propagation of radar waves from the limestone to an air void or from the limestone to a clay pocket.

Drilling test boreholes proved to be a very useful tool for the calibration of the GPR anomalies recorded in 2D radargrams. Such a process showed a near 100 % accuracy with respect to interpreting the subsurface features, with 77% correctly interpreted as caves or clay pockets and 23% wrongly interpreted, where the interpretation was a void but it was indeed partly a clay-filled and partly an air-filled void. The completed survey also showed simultaneous surveying with GPR and road construction is a very efficient and economical way to predict various karstic features and the density of the karstic forms.

Keywords

karst, ground-penetrating radar, geotechnics, cavities detection