# Research on Grouting Dynamic Monitoring Based on Borehole-Tunnel Joint Resistivity Method
## 4 Results and Discussion  
The grouting horizons of Boreholes 1, 2, and 3 are located 30 m below the tunnel. A grouting horizon plane was selected for analysis, as shown in Figure 4. Prior to grouting, the grouting horizon exhibited only minor low-resistivity anomalies near the tunnel, likely attributable to inherent differences in the formation's electrical properties. After injecting 783 tons, 1515 tons, and 593 tons of grout into Boreholes 1, 2, and 3, respectively, post-grouting monitoring (Figure (b)b) revealed a distinct \(\mathrm{Y}\)-shaped low-resistivity anomaly near the boreholes. The injected grout, rich in free water during fracture filling, exhibitedsignificantly lower resistivity compared to the surrounding rock. This process created interconnected conductive pathways through fracture networks, causing a rapid resistivity decline. Comparative inversion results show that the average resistivity of the grouting horizon dropped from a background value of 200 \(\Omega\)-m to 180 \(\Omega\)-m (approximately a 10% reduction), reflecting differential fracture development along the F1 fault. Under pressure-driven diffusion, the grout migrated preferentially along dominant fracture pathways, resulting in extensive resistivity reduction. Arrows in Figure 6b indicate high-connectivity fractures that facilitated grout propagation.  
To clarify grout diffusion pathways and coverage, resistivity difference maps were generated by subtracting pre- and post-grouting inversion results (Figure 6c). Analysis reveals that grout from Borehole 1 likely propagated from the mid-section toward the borehole base, diffusing bilaterally with a left-upward bias. For Boreholes 2 and 3, grout migrated from the mid-sections (where basal fractures were underdeveloped), spreading laterally and concentrating between the two boreholes. The grout breached the structural block of the F1 fault and extended to Tunnel 1. Grout diffusion predominantly aligned with the maximum principal stress axis, forming angles of 15\({}^{\circ}\) and \(-\)20\({}^{\circ}\) relative to the tunnel axis, consistent with anisotropic fracture-controlled grout behavior.  
Post-grouting resistivity recovery was observed over time (Figure 6d). As free water dissipated and cement hydration products formed denser structures, conductive pathways degraded, leading to gradual resistivity rebound. Thirty days after grouting, the average resistivity rose from a minimum of 180 \(\Omega\)-m to approximately 220 \(\Omega\)-m. However, recovery exhibited spatial heterogeneity: most areas (Y-shaped zone) returned to background or slightly higher resistivity due to complete grout solidification, while residual unset grout in marginal zones or areas influenced by adjacent boreholes maintained lower resistivity.  
Real-time monitoring data from Tunnel Survey Lines 1 and 2 provided insights into lateral grout diffusion but lacked vertical resolution. The newly deployed tunnel-borehole 3D cross-line survey (Figure 7) addresses this gap. Revised analysis shows grout from Borehole 1 primarily diffused from the mid-section (50-180 m depth), accumulating mini  
Figure 6: Resistivity plan view of grouting horizons (30 m below tunnel) for Boreholes 1 and 2.  
mally at the borehole base and migrating upward. This suggests fault-induced fractures are shallow, with no connectivity to deeper aquifers, see Table 3.  
Integrated results from Figures 4 and 5 indicate maximum lateral and vertical grout displacements of 100 m and 40 m, respectively, highlighting the anisotropic nature of fracture development.