Non-destructive evaluation of two-component backfill grouting behind segmental tunnel linings

Abstract

Non-destructive testing (NDT) techniques, such as Ground Penetrating Radar (GPR) and Impact Echo (IE), are being considered as potential methods for ensuring complete annular-grout backfill behind pre-cast segmental tunnel liners. Current methods for ensuring grout backfill involve drilled verification holes through waterproof tunnel liners. 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 NDT 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 study of the application to two-component grouts are necessary. Laboratory testing of the two-component grout was conducted to obtain electromagnetic (EM) and elastic properties as they determine the efficacy of GPR and IE, respectively. Experiments were then conducted on actual tunnel segments to determine the most appropriate GPR antenna frequency that can penetrate through the steel-reinforced segments with 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. Implementation of each technique on installed segments within the tunnel provided further evaluation using known backfill conditions at specific locations. The ability to conduct the testing during construction was shown as space is limited within the tunnel boring machine (TBM). The findings of this research allow for direct implementation of the GPR for detection of voids behind segmental tunnel liners with two-component grout backfill with the IE as a supplementary option for secondary evaluation.