The Impact of Fracture Persistence and Intact Rock Bridge Failure on the In Situ Block Area Distribution

The in situ block size distribution is an essential characteristic of fractured rock masses and impacts the assessment of rockfall hazards and other fields of rock mechanics. The block size distribution can be estimated rather easily for fully persistent fractures, but it is a challenge to determine this parameter when non-persistent fractures in a rock mass should be considered. In many approaches, the block size distribution is estimated by assuming that the fractures are fully persistent, resulting in an underestimation of the block sizes for many fracture geometries. In addition, the block size distribution is influenced by intact rock bridge failure, especially in rock masses with non-persistent fractures, either in a short-term perspective during a slope failure event when the rock mass increasingly disintegrates or in a long-term view when the rock mass progressively weakens. The quantification of intact rock bridge failure in a rock mass is highly complex, comprising fracture coalescence and crack growth driven by time-dependent changes of the in situ stresses due to thermal, freezing-thawing, and pore water pressure fluctuations. This contribution presents stochastic analyses of the two-dimensional in situ block area distribution and the mean block area of non-persistent fracture networks. The applied 2D discrete fracture network approach takes into account the potential failure of intact rock bridges based on a pre-defined threshold length and relies on input parameters that can be easily measured in the field by classical discontinuity mapping methods (e.g., scanline mapping). In addition, on the basis of these discrete fracture network analyses, an empirical relationship was determined between (i) the mean block area for persistent fractures, (ii) the mean block area for non-persistent fractures, and (iii) the mean interconnectivity factor. The further adaptation of this 2D approach to 3D block geometries is discussed on the basis of general considerations. The calculations carried out in this contribution highlight the large impact of non-persistent fractures and intact rock bridge failure for rock mass characterization, e.g., rockfall assessment.

Assessing the effect of lithological setting, block characteristics and slope topography on the runout length of rockfalls in the Alps and on the island of La Réunion

In four study areas within different lithological settings and rockfall activity, lidar data were applied for a morphometric analysis of block sizes, block shapes and talus cone characteristics. This information was used to investigate the dependencies between block size, block shape and lithology on the one hand and runout distances on the other hand. In our study, we were able to show that lithology seems to have an influence on block size and shape and that gravitational sorting did not occur on all of the studied debris cones but that other parameters apparently control the runout length of boulders. Such a parameter seems to be the block shape, as it plays the role of a moderating parameter in two of the four study sites, while we could not confirm this for our other study sites. We also investigated the influence of terrain parameters such as slope inclination, profile curvature and roughness. The derived roughness values show a clear difference between the four study sites and seem to be a good proxy for block size distribution on the talus cones and thus could be used in further studies to analyse a larger sample of block size distribution on talus cones with different lithologies.

The RockModels Project: Rockfalls Characterization and Modelling

<p>A rockfall is a rapid mass movement generated by the detachment of a rock volume from the slope that falls, rolls and bounces during its propagation downhill. Rockfalls have great destructive potential due to the high kinetic and impact energies that may reach during the propagation. Rockfalls are frequent instability processes in road cuts, open pit mines and quarries, steep slopes and cliffs. The initial mobilized mass can be either a single massive block or a set of blocks defined by the joints present in the massif. During the propagation, the block or blocks detached may break when impacts against the terrain, producing a distribution of fragments with independent trajectories. Knowledge of the size and trajectory of the blocks resulting from fragmentation is critical for the assessment of the potential damage and the design of protective structures.</p><p>In this contribution, we summarise the main achievements of the RockModels project (BIA2016-75668-P, AEI/FEDER,UE). This project aims at quantifying the risk induced by fragmental rockfalls, by developing quantitative risk assessment methodologies and providing tools to improve its prevention and mitigation. It has three general objectives: i) Explicit identification of unstable rock volumes and stability assessment; ii)Development and validation of a fragmentation model, iii) Rockfall propagation analysis by means of the development of a 3D simulator tool and its calibration.</p><p>The use of geomatic techniques such as terrestrial photogrammetry or from UAV allow the generation of high-resolution 3D models of cliffs and the joint system characterization based on 3D point clouds. The orientation and persistence of joints within the rock mass define the kinematically unstable rock volumes and determine the initial block size distribution.  We inventoried fragmental rockfalls occurred in Spain by obtaining a 3D model, the orthophoto, specific cartographies and detailed volumes measurements to obtain the block size distribution in the deposits of each event. The fragmental rockfalls inventory have been collected in a spatial database using PostGIS and following the INSPIRE directive for natural hazards. This data can be consulted at different scales with a developed Web Map Service (WMS) (https://rockdb.upc.edu/). The inventory is the empirical data used to developed, calibrate and validate the Rockfall Fractal Fragmentation Model proposed, as well as the 3D trajectory simulator RockGIS that incorporates the fragmentation module.</p><p>More empirical data has been obtained by performing 4 real scale fragmentation test in a quarry. The impact of each block and trajectories of the fragments were recorded by several high speed cameras from different points of view. A program has been implemented to measure the kinematics of each tested block using the high-speed videos. The obtained kinematic parameters have been used for the calibration of the RockGIS simulator. An additional essay was carry out at laboratory to study the effect of the comminution among blocks. The distribution of fragments obtained confirms that the blocks undergoing greater confinement generate a greater number of fragments decreasing their maximum volume.</p>