GeoView utilizes Ground penetrating radar (GPR)
Alex Pommier
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GeoView utilizes Ground penetrating radar (GPR)

The initial purpose of the geophysical investigation was to determine the lateral extent of voids associated with the failure of a shallow storm water pipeline. The pipeline passed under a concrete slab and near an adjacent apartment building. Observable structural damage was observed in both the concrete slab and the building. Ground penetrating radar (GPR) using a variety of antenna frequencies was used for the investigation. The first part of the GPR study was performed using high frequency antennas specifically designed to identify shallow sub-slab voids and/or low-density soils beneath concrete. The GPR successfully delineated the lateral extent of the subsurface voids and determined that voids extended up to 5 to 10 ft away from the pipeline. The voids extended underneath the concrete-covered breezeway and the southeast corner of the apartment building. During the evaluation of the GPR data it was observed that soils underneath the pipeline appeared to be disturbed. If the formation of voids and subsequent damage were only associated with the failure of the pipeline, then soils beneath the pipe should have been unaffected. Because of the observed disturbance of soils below the depth of the pipeline, the survey was repeated using a low frequency GPR antenna. This data provided a much greater depth of penetration and allowed for an assessment of geological conditions below the pipeline. Two potential sinkhole-related geological features were identified during this phase of the investigation. The presence of the sinkhole-related geological features suggested that the failure of the pipeline and subsequent formation of voids was not-related to a simple deficiency in pipeline construction, but rather a failure due to the loss of underlying soil support attributable to sinkhole activity. Based on the results of the GPR study a follow-up geotechnical investigation was performed and confirmed the presence of active sinkhole-related conditions in the area of pipeline failure. Because of the identification of unstable soil conditions below the pipeline, a compaction-grouting program was completed in order to stabilize the soils below the pipeline prior to repair. A figure with the report findings and examples of the GPR output are provided below.

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