Void detection in two-component annulus grout behind a pre-cast segmental tunnel liner using Ground Penetrating Radar

Ground Penetrating Radar (GPR) is being considered as a potential non-destructive technique for ensuring complete annulus grout backfill behind pre-cast segmental tunnel liners. Current methods for ensuring grout backfill involve drilled verification holes through the waterproof liner. The Rondout bypass tunnel, in Newburgh, NY, where void formation behind the tunnel liner is possible due to high groundwater pressures, provided an example project to evaluate GPR capabilities. While the application of GPR for void detection in one component grouts behind tunnel liners has been well-documented, site specific calibration of the GPR equipment and the application to two-component grouts is necessary. Experiments were conducted on actual tunnel segments to determine the most appropriate antenna frequency that can penetrate through the steel-reinforced segments and produce the best resolution. Mock air and water voids were embedded within areas of complete grout behind the segment to demonstrate the GPR signatures. Steel reflectors buried within the grout provided indirect measurements of the electromagnetic (EM) properties of the two-component grout. All field results were compared against analytical evaluation and numerical models to validate void detection. The findings of this research allow for direct implementation of the GPR technique for detection of voids behind segmental tunnel liners with two-component grout backfill