Peering into the Void

Given the localized congestion of utilities under our city streets and highways, locating and mapping their positions can be challenging. When the utilities fail, it’s important to determine the location of the failure, as well as the location of any nearby utilities, before any repairs can be made.

Such was the case of slumping ground along a section of Ontario highway southeast of Ottawa. Two existing large steel culverts were experiencing inflow of sediment. During site preparation for the restoration slip-lining program, the construction crews started to see localized sinkholes in the shoulders of the highway. The integrity of the subsurface was a concern, but so were the consequences for the highway’s surface. If holes opened up on the asphalt road surface, the implications for the speeding highway traffic could be disastrous.

The local multiVIEW Locates Inc. operations centre was deployed with a Noggin 250 ground penetrating radar (GPR) system manufactured by Sensors & Software Inc. The GPR pulses Fmfrequency radio waves into the ground surface and reflects changes in the electrical properties from the underlying geology. This reflection makes it possible to map the thickness of the road asphalt; the continuity of the underlying stratified engineered road bed; and areas where this bedding has been altered. It also lets technicians profile the location of buried utilities within the exploration limit of the GPR system.

With the help of a crash truck and coordinated lane closures, workers collected GPR data across both east and westbound lanes, in the grassy median and beyond both highway shoulders. This information was analyzed for inference of subsurface anomalies, which could infer the presence of voids immediately beneath the surface.

Additional analysis was completed by software data manipulation to produce time-slice images of the data at specific reflection time intervals-like viewing a layer cake from above and successively removing the overlying and underlying layers to uniquely view the contents of the first, second, or third layer, and so on.

Comprehensive review of the GPR data indicated three anomalous zones which could represent possible voids. These targets were subsequently field investigated and determined not to represent voids, but other soil features. The application of the GPR technology permitted the slip-lining restoration process to confidently proceed while the contractor watched the ground surface for signs of additional soil distress.

With files from John E. Scaife.