Case study: How we used GPR to locate and determine pad foundation detail, particularly the plan view geometry, on a site in Waterloo.

Survey objectives

The previous building on a 40 x 28 m site in Waterloo had been demolished for redevelopment, but the existing basement slab and foundations remained. Historic drawings indicated that the building previously on the site was of steel column construction with mass concrete pad foundations.

It was hoped to reuse the existing pad foundations, but first, the old foundations needed to be located and their condition determined. The GPR survey aimed to establish their location, size and depth below the surface.

GPR survey method

A GSSI DF system with a digital dual-frequency antenna with central frequencies of 300 MHz and 800 MHz was used for the survey. The higher-frequency antenna (800 MHz) provides high-resolution data to approximately 0.9 m depth, while the lower-frequency antenna (300 MHz) provides greater depth penetration. Data can be merged to display both sets of data in a combined file, making data analysis and interpretation easier and more efficient.

The 1,120 m2 site was scanned on an orthogonal grid with scan lines at 0.5 m centres.

Pad Foundation survey results

GPR was successful in detecting the presence and extent of the buried pad foundations within the scanned area.

The nature and strength of the metallic-type reflections suggest that the pad foundations are most likely to be steel grillages. The tops of the steel grillages were detected at 170-340 mm depth from the scan surface.

GPR also detected areas of buried reinforced concrete and linear features, probably services.

It should be noted that the results of this survey were clear. This is not always the case. Although GPR is a Non-Destructive Testing (NDT) survey technique, sometimes hand probing and shallow test pits may be required to confirm the interpretation and the nature of the features detected.