Blast overbreak measurement by light sectioning

Summary In this new method for measuring the cross-section of tunnels and other excavations, the opening is outlined by a plane of light projected from a conical mirror. The image is recorded on videotape, enhanced, then measured by microcomputer. The measured profile is compared automatically with the specified profile to give values for overbreak and underbreak.Trials in Mexican tunnels and at an underground mine in Canada have evaluated the technique in relation to traditional mechanical, photographic, and surveying alternatives. Results indicate that the light sectioning method requires less than a minute per profile and no surveying skills. Costs are low, and the measurements are accurate to within a centimetre or two. Using the same photoanalysis technique and software, rock quality can be measured at the same time and place as overbreak, which helps the engineer to decide whether overbreak is caused by geological conditions or by deficiencies in blasting.