GPR case study: How we used GPR survey methods to determine the gas holder base thickness non-intrusively, with access to one side only.

Survey objectives

GPR can be used to determine slab thickness and construction detail nondestructively. This can be achieved by scanning the required area with a number of scanlines positioned as required or, for a more detailed survey, by scanning on a regular orthogonal grid.

We recently undertook an interesting variation of this type of survey, where we were required to determine the thickness and construction detail of a gas holder base at a London gasworks. This posed a number of problems, notably the sloping sides of the base.

GPR survey method

Ground Penetrating Radar (GPR) scans were collected along radial grid lines at 10m centres measured along the base circumference. The sloping sides were scanned by pulling the antenna and survey wheel (to record positional information and regulate the scan rate) up the sloping concrete.

A GSSI SIR-3000 GPR system was used with two antennas (central frequencies of 1.5 GHz and 900 Mhz), giving a range of depths and resolutions. The 1.5 GHz antenna would typically penetrate to a depth of 400 mm in concrete and suitable for detecting shallow features such as layers of reinforcement, whilst the 900 MHz antenna would typically penetrate to a depth of 0.9 m in concrete (conditions permitting), thus resolving deeper features, albeit with a loss in resolution in the data.

Survey findings

The base slab thickness was nominally 275 mm thick, varying between 180-380 mm.

Two layers of 200 mm mesh reinforcement were detected in the slab top. The top layer was at 50-90 mm depth and the second layer at 140-260 mm depth. The sloping sides were reinforced with one layer of 400 mm mesh reinforcement at 100-190 mm depth.