Discontinuities are an intrinsic characteristic of rock masses, and they appear at every scale of a technical survey. Issues related to the procedure’s duration and the operator’s safety during the survey have encouraged the development of non-contact methods: images and digital models serve as representations of the rock mass for surveying the geometric features of discontinuities. In particular, various automatic and semi-automatic trace sampling methods were developed in the last decade, focused on estimating persistence and the degree of fracturing. A review of these methods is presented in this paper, highlighting their strengths and disadvantages.
Discontinuities are an intrinsic characteristic of rock masses. They appear as traces on rock exposures, such as natural outcrops, tunnel fronts, quarry faces. Natural outcrops can have an infinite variety of shapes and dimensions, but a common characteristic is their non-planar surface, on which traces can be clearly recognized. The surface of tunnel fronts, on the contrary, is affected by the excavation method used. Traces can be masked somehow.
Moreover, elements that do not represent actual traces could be created in the excavation process. Therefore, particular attention should be paid to the detection of traces on tunnel fronts. Quarry faces pose similar issues. On perfectly smooth surfaces like the one in the picture, traces are linear elements that must be distinguished from veins and other textural features.
Trace length plays a role in different parameters used to describe rock mass behavior in a quantitative way. The first and most common one is persistence.
In 1978, the ISRM commission [1] defined persistence as the areal extent or size of a discontinuity within a plane. Persistence can be crudely quantified by observing the trace lengths of discontinuities on exposed surfaces. It is one of the most important rock mass parameters but one of the most difficult to determine, too. Other parameters have been defined to quantify fracture abundance in a rock mass. They are all defined as the ratio between the dimension of the considered feature and the sampling region’s size ([2], [3]). In particular, the areal intensity P21 is defined as the length of traces per unit sampling area. The volumetric intensity P32 is instead the area of fractures per unit volume of rock mass: even if not explicitly mentioned, we will see later that also P32 can be inferred based on trace sampling.