Pad foundation location survey, Waterloo

GPR pad foundation location survey

Ground Penetrating Radar (GPR) provides an excellent means of locating and determining foundation detail, in particular the plan view geometry. It is a Non-Destructive Testing (NDT) survey technique, although sometimes hand probing and shallow test pits may be required to confirm the detail.

On a 40 x 28m site in Waterloo, the previous building 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 purpose of the GPR survey was 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 300MHz and 800MHz was used for the survey. The higher frequency antenna (800MHz) provides high resolution data to approximately 0.9m depth, while the lower frequency antenna (300MHz) 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 1,120m2 site was scanned on an orthogonal grid with scan lines at 0.5 centres.

Survey findings

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 170-340mm depth from the scan surface.

GPR radargram showing detected pad foundations
Radargram showing detected pad foundations

 

GPR also detected areas of buried reinforced concrete and linear features likely to be services.