Mapping shotcrete thickness using LiDAR and photogrammetry data: Correcting for over-calculation due to rockmass convergence

• Identification of a limitation associated with shotcrete thickness mapping from 3d data.
• Explanation of the effect of longitudinal displacement profiles on underground surveying.
• Solutions to limit effects of longitudinal displacement on shotcrete thickness mapping.

The benchmark method of measuring shotcrete thickness from 3D LiDAR and photogrammetry data involves scanning sequential blast rounds, aligning the data in a 3D environment, and calculating the spatial difference between the two models. The calculated difference between the two 3D surface models is measured as the thickness of the sprayed concrete. This methodology does not account for the convergence of the rockmass that naturally occurs between the scanning protocols, nor is it included in the difference equation, and resultantly the user over-measures the thickness of the sprayed concrete. The over-measurement can be corrected through changing the time of scanning with respect to the excavation sequence or calibrating the solution based on known rockmass convergence rates or numerical modelling. The use of 3D imaging data for the calculation of shotcrete thickness will remain a useful tool for geotechnical engineers, but corrections must made to the state-of-practice methodology in order to achieve accurate results.

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