Trajectory-Based Charged-Section-to-Backfill Distance Control for Boundary Longhole Blasting in a Phosphate Mine

Boundary longholes that deviate toward adjacent cemented backfill can reduce charge-to-backfill distance and increase blast-induced disturbance. This study develops an integrated, trajectory-based method for controlling charged-section distance in a backfilled phosphate mine. A Kernel Ridge (KR) model with a radial basis function (RBF) kernel was evaluated using grouped five-fold cross-validation of 647 depth-specific records and externally validated using 43 unseen boreholes comprising 96 records. High-risk holes were then remeasured, reconstructed in DIMINE, and evaluated against an LS-DYNA-derived distance criterion. KR achieved a mean absolute error (MAE) of 0.429 m and placed 72.33% of errors within 0.5 m; external validation yielded an MAE of 0.291 m and a root mean square error (RMSE) of 0.412 m. Numerical analyses at blasthole-to-backfill distances of 0.5, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, and 2.0 m showed a rapid transition within the 1.0–1.5 m interval. At 1.5 m, the maximum interface effective stress and peak particle velocity were 1.983 MPa and 9.925 cm/s, respectively, below the adopted 2.32 MPa stress reference and 12 cm/s velocity criterion. The 1.5 m criterion was applied in DIMINE, while monitoring at the 1075 m draw level of stope 1130-56 supported the engineering implementation of the integrated procedure. The method provides a spatially explicit basis for pre-blast distance control.