Measurement of Rock Joint Surfaces by Using Smartphone Structure from Motion (SfM) Photogrammetry

Area of Science:

• Geotechnical Engineering
• Rock Mechanics
• Photogrammetry

Background:

• Accurate measurement of rock joint surface topography is critical for estimating rock discontinuity shear strength in rock engineering.
• Existing methods like profilometers and laser scanners are often inaccurate or prohibitively expensive.
• A need exists for a high-speed, cost-effective, and precise technique for rock joint surface topography acquisition.

Purpose of the Study:

• To present and evaluate a smartphone-based Structure from Motion (SfM) photogrammetric solution for measuring rock joint surfaces.
• To assess the accuracy and feasibility of using common smartphones for detailed rock surface reconstruction.

Main Methods:

• Acquired image datasets of two rock joint specimens using iPhone 6s, Pixel 2, and a T329t scanner.
• Processed image data through SfM software to generate 3D models of the rock joint surfaces.
• Evaluated the technique's accuracy using root mean square error (RMSE) of cloud-to-cloud distance and mean error of the Joint Roughness Coefficient (JRC).

Main Results:

• Smartphone SfM achieved RMSE values below 0.08 mm for iPhone 6s and Pixel 2.
• Mean errors for JRC were -7.54% and -5.27% at 0.25 mm and 1.0 mm point intervals, respectively.
• The smartphone SfM method demonstrated accuracy comparable to 3D laser scanning for laboratory-scale rock joint surfaces.

Conclusions:

• Smartphone Structure from Motion (SfM) photogrammetry is a viable, accurate, and cost-effective alternative for measuring rock joint surfaces.
• This method has the potential to become a widely adopted technique in rock engineering for surface characterization.
• The study validates the use of readily available smartphone technology for precise geotechnical measurements.