Geophysics Consultant: Vector Engineers, Inc., Shelbyville, Kentucky
A large international airport suspected problems with subgrade degradation under a section of runway. To minimize disruption to air service, Vector Engineers in conjunction with HNTB decided to utilize geophysical methods to evaluate the possibility of voids beneath the concrete runway slab. Ground penetrating radar was chosen. Ground penetration radar utilizes an electromagnetic pulse transmitted into the ground by the transmitter antenna. When the pulse encounters an interface between two materials where the two materials possess different dielectric constants (dielectric constant is a measure of electrical capacitance), part of the pulse is reflected back to the ground surface, where it is detected by the receiver antenna. By looking at variations in two-way travel time (i.e. the time it takes for the pulse to travel from the transmitter antenna to the reflector and back to the receiver antenna) and reflection amplitude along a profile, anomalous zones indicative of voids and other subsurface features are readily identifiable. The results from our geophysical survey indicated several unknown voids that were eventually grouted.