John Kemeny, Split Engineering and University of Arizona, Tucson, USA and James Donovan, University of Utah, Salt Lake City, Utah, USA. This note was first published in GE’s November 2005 issue.
At the heart of designing structures in rocks is a thorough characterisation of the rock mass prior to excavation (eg Priest, 1983). The results of rock mass characterisation go on to be used in blast and excavation design, determination of support requirements, cost analyses, numerical modelling, and many other aspects of the design process.
At a minimum, rock mass characterisation usually involves borehole logging and sampling, laboratory testing, and field mapping and data collection. Due to access problems, safety and time and cost concerns, there are many uncertainties and hazards associated with the field mapping and data collection aspects of rock characterisation projects.
New geotechnical/surveying technologies are becoming increasing important. These include GPS, digital methods for field surveying and data collection, still and video digital cameras, and GIS and associated software for data processing and visualisation.
Ground based 3D imaging is a new and emerging technology for rock mass characteris ation. This article defines 3D imaging to include ground based LIDAR surveys (also called 3D laser scanning), high resolution digital cameras, and a host of software for data processing, interpretation and visualisation.
Laser scanners work by collecting an array of high resolution laserbased position measurements. Laser scanners are capable of collecting data at rates over 20,000 points per second, with a position accuracy of less than 5mm at distances up to 800m. The output from a laser scanner survey is a “point cloud” consisting of millions of reflection points that represent the 3D surface that was scanned.
After some data cleaning, a triangulated surface can be rendered from the point cloud data, and many subsequent calculations and visualisations can be made using the 3D surface. In addition, a technique called texture mapping or photo draping can be used to overlay highresolution colour information from digital images on to the 3D surface.
The techniques are being used in a number of engineering applications, including civil and architectural design, modelling, scene reconstruction, damage and condition assessment. An example of a point cloud of a rock face is shown in Figure 1 (taken along a highway south of Ouray, Colorado). This point cloud has about 1.5M points and the scanning took about 15 minutes.