Fragmented Rockfall Volume Distribution from Photogrammetry-Based Structural Mapping and Discrete Fracture Networks

Abstract

The design of rockfall protection structures requires information about the falling block volumes. Computational tools for rockfall trajectory simulation are now capable of modeling block fragmentation, requiring the fragmented volume-relative frequency distribution of rockfalls as input. This can be challenging at locations with scarce or nonexistent rockfall records and where block surveys are not feasible. The work in this paper shows that simple discrete fracture network realizations from structural mapping based on photogrammetric techniques can be used to reliably estimate rock fall block volumes. These estimates can be used for dimensioning rockfall protection structures in cases where data is scarce or not available. The methodology is tested at two sites in the Canadian Cordillera where limestone outcrops have been the source of recurrent rockfalls. The results suggest that fragmentation will largely tend to occur through weak planes and expansion of non-persistent discontinuities, while other block breakage mechanisms exert less influence in the fragmented volume-relative frequency distribution of rockfalls. Therefore, block volume distribution can be estimated using a simple discrete fracture network (DFN) with fully persistent discontinuities. Limitations of the methods are also discussed, as well as potential future research to address such limitations.

Remote Sensing Applications for Landslide Monitoring and Investigation in Western Canada

“…The analyses at Tornado Mountain and S042 Site are an example of how this information can also be used to calculate rock fall hazards in terms of volume-frequency curves. The method and results at Tornado Mountain and the S042 Site are detailed in [54] and [56]. A similar approach is described in [57] and tested for a rock fall site in Italy.…”

“…These results show that the approach could be used to estimate the fragmented rock fall volume-frequency relationships in the absence of rock fall records or block surveys along rock fall trajectories, as part of rock fall hazard and risk analyses. The approach presented in [54] and [56] was based on the hypothesis that fragmentation in strong rocks (particularly limestones characterized by Unconfined Compressive Strengths over 50 MPa) will tend to occur through weak planes and by extension of non-persistent discontinuities as the in situ blocks detach, fall, bounce, and roll. In this regard the method should not be applied to other, weaker rocks, without validation.…”

“…The distance between the camera and the slope were between 500 m and 600 m at the Tornado Mountain site, and between 200 m and 600 m at Site S042. The setup described rendered ground pixel sizes between 0.7 cm and 2 cm [56].…”

“…The dip, dip direction and spacing of the sets of discontinuities were fitted to probability distributions and used to develop multiple iterations of discrete fracture networks (DFN) of persistent discontinuities for both sites using Wolfram's Mathematica software package (https://www.wolfram.com). These DFNs were used to calculate the fragmented rock fall volume-frequency distributions through numerical integration [56].…”

“…One iteration of the DFN for the Tornado Mountain site (c). fragmented rock fall volume cumulative distribution based on the DFN realizations compared to block volumes surveyed along the rock fall trajectories, DFN volumes are truncated to the minimum range of surveyed block volumes for accurate comparison (lack of survey of small fraction of blocks) (d).Adapted from[55,56].…”