GPR-Based Active Tracing of Nano-Barium Titanate-Modified Cement Grout for Underground Grouting Detection

Accurate identification of grout diffusion extents and reinforcement boundaries in deep-buried, water-rich, or complex underground engineering is critical for surrounding rock stabilization, seepage prevention, and overall engineering safety. However, due to the concealed nature of underground structures and the marginal dielectric contrast between conventional cement grout and surrounding saturated formations, traditional ground penetrating radar (GPR) detection is often severely hampered by weak reflection signals and poor interface identifiability. To address this critical engineering challenge, a novel active electromagnetic tracing strategy based on nano-barium titanate (BTO) high-dielectric admixtures was proposed to deliberately modulate the dielectric properties of cement grout and amplify the electromagnetic impedance mismatch at the grouting boundary. Cement grout systems with different BTO dosages (0%, 5%, and 10%) were systematically investigated through 24 h continuous time-lapse GPR experiments and finite-difference time-domain (FDTD)-based gprMax numerical simulations. The results indicate that nano-BTO significantly increases the initial dielectric constant of the grout from 22.14 (0% BTO) to 42.90 (5% BTO) and 49.70 (10% BTO). At the 24 h terminal node, the dielectric constants remain at 24.06 and 35.52 for the 5% and 10% BTO groups, maintaining notable increments of 43.90% and 112.44% over the pure cement group (16.72). Consequently, the enhanced dielectric constant of the grout significantly boosts the interface reflection intensity in physical experiments; compared to the pure cement control group, the reflection amplitudes of the 5% and 10% BTO groups exhibit initial enhancements of 36.66% and 46.21%, respectively, and maintain terminal enhancements of 40.54% and 51.00% at the 24 h node, effectively securing long-term boundary clarity in GPR profiles. Both numerical simulations and physical experiments quantitatively confirm this reflection enhancement, validating the feasibility of the active tracing strategy. This study provides a novel technical paradigm and theoretical foundation for the non-destructive boundary identification, diffusion monitoring, and quality evaluation of concealed grouting bodies under complex underground environments, effectively serving underground engineering safety and disaster prevention.