Overbreak
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Tunnel Profilometry By Light Sectioning:

ob1.jpg (36815 bytes) Overbreak, and underbreak are problematic when driving tunnels, as in this 16 m diversion tunnel in Aqua Milpa, Mexico.  These mistakes need to be rectified, usually at no small cost.  How much is the damage?  Who pays for it?
ob2.jpg (15849 bytes) The light sectioning method was developed to quantify the volumes of rock involved.   The method makes use of a radially projected then beam of light.
ob3.jpg (21177 bytes) When projected in a tunnel, and imaged from a distance, the beams highlights the profile of this drift in a Canadian mine.
ob4.jpg (11755 bytes) Image processing techniques can be used to calculate the volume of excavated material.
ob5.jpg (14949 bytes) Image processing techniques can be used to measure overbreak, as in this example from the Mexican tunnels.  The measured tunnel profile is overlain onto the design profile. Overbreak  (blue) and underbreak (yellow) are defined outside of the of the specified tolerance (green) of the design..

In the case of case of the Agua Milpa tunnels, overbreak and underbreak were quantified, and compared to rock mass quality (Q) and blasting factors (ppf).  Statistical analysis of all results indicated that rock mass quality was more significant in causing overbreak, while perimeter powder factor was more significant in causing underbreak.

ob6.jpg (44876 bytes)
REFERENCES:

Maerz, N. H., Ibarra, J. A., and Franklin, J. A., 1996. Overbreak and underbreak in underground openings. Part 1: Measurement using the light sectioning method and digital image processing. Journal of Geotechnical and Geological Engineering 14, pp. 307-323.

Ibarra, J. A., Maerz, N. H., and Franklin, J. A., 1996. Overbreak and underbreak in underground openings. Part 2: Causes and implications. Journal of Geotechnical and Geological Engineering 14, pp. 325-340.

Franklin, J. A., Ibarra, J., and Maerz, N. H., 1990. Blast overbreak measurement by light sectioning. Int. J. of Min. and Geol. Eng., v 7, pp. 323-331.