Infrasense recently completed a utility and rebar detection project in the Ted Williams Tunnel in Boston. Working as a subcontractor to J.F. White Contracting Co., Infrasense used ground penetrating radar (GPR) to mark out rebar and utility lines embedded in the concrete ceiling of the tunnel.
As a part of the routine inspection process, new temporary supports must be attached to the tunnel ceiling to allow load testing of a representative sample of the existing supports. To prevent reinforcing steel and utility lines from being damaged during this attachment process, and to ensure the safety of the drilling crew, J.F. White selected Infrasense to locate the conduits and reinforcement at each of 120 locations where drilling was scheduled to take place.
The tunnel work presented a unique set of challenges for Infrasense engineers due to the dark and confined work spaces. In order to navigate the narrow plenums that provide ventilation above the main tunnel opening, Infrasense engineers used a new handheld GPR system that is easy to carry and use where there is limited mobility. Using a high-resolution 1.6 GHz frequency ground-coupled antenna, Infrasense engineers were able to survey all 120 locations, spanning more than 1 mile, in just four nights.
After each location was surveyed, Infrasense marked the conduits and rebar on the concrete surface in order to provide accurate locations for drilling crews to use. The effectiveness of the GPR system was tested and verified by Infrasense engineers by scanning and marking utility locations outside unopened junction boxes in the tunnel wall. After these locations were marked by the Infrasense crew, personnel from J.F. White and the Massachusetts Department of Transportation opened the junction box to determine the true location of the utility lines at that location; the Infrasense markings corresponded perfectly to the true utility locations at the junction box.
Ground penetrating radar is a nondestructive evaluation technique that operates by transmitting short pulses of electromagnetic energy into the concrete, using a boxed antenna attached to a survey vehicle or rolled along the slab. These pulses are reflected back to the antenna, providing a record of the properties and thicknesses of the layers within the slab. GPR can detect the depth and spacing of reinforcing steel or other metallic objects, as well as the thickness of concrete slabs. It is also able to detect subsurface deterioration of a slab through changes in the radar signal through the concrete. GPR is an especially useful tool for projects such as the utility survey performed in the Ted Williams Tunnel because the variety of GPR configurations and settings provide more flexibility and capabilities to the engineers doing the work.