This paper describes how digital face mapping can be employed to construct fracture trace maps from rock exposures. This information is subsequently used to characterize and classify rock masses using a series of geomechanical parameters. The methodology has been developed to take into consideration the limited size of rock faces recorded by photographs. A case study from a Quebec underground mine is used to illustrate the data gathering process, the construction of trace maps and characterization results. This paper further addresses the number of photographs necessary to minimize associated characterization errors.
Rock mass characterization requires the collection, analysis and interpretation of structural data. In order to achieve a statistically representative characterization it is necessary to sample a sufficiently large number of fractures. Priest & Hudson (1976) suggest that for a typical site it is necessary to sample between 1000 and 2000 fractures. In a production environment there IS limited time to undertake a comprehensive field mapping program. This is often compounded in moderate or poor quality ground by the early installation of surface support such as shotcrete liners or mesh. It follows that there is a need to maximise data collection while at the same time minimizing the time allotted for this task. This has led to variations of traditional mapping techniques (e.g. Mathis 1988) and use of photographic images (Maerz 1990, Lerny & Hadjigeorgiou 2003). Important considerations include the skill and time required for data processing, as well as the cost, complexity, weight and size of mapping equipment.
Previous work by the authors led to the development of a digital face mapping methodology that speeds up data collection and employs image processing algorithms to construct fracture trace maps. The performance of this system is greatly enhanced in the underground environment by judicial use directional illumination.
This paper addresses sampling limitations of digital face mapping in development openings in underground mines. The sampled area is limited by the height of the excavation, usually less than 3m and the width of the excavation, usually dictated by the width of underground equipment. These practical constraints limit the area that can be covered by a photograph. This is of some concern given that several fracture parameters (spacing and size) are influenced by biases associated to the size of the sampling area.
The authors employ a portable and relatively inexpensive digital face mapping system that uses a digital camera, a distance meter, a laser pointer and two spirit levels placed orthogonal to one another. These are attached to a steel plate fixed on a tripod, Figure I. The levels and the laser pointer are used to set the camera normal to the rock exposure. A portable light is used to enhance the representation of structural features in the photographs.
(Figure in full paper)
A procedure for taking photographs of rock exposures is illustrated in Figure2. A picture is taken with a flashlight normal to the rock face, and further pictures with the rock face illuminated with different oblique lightings.