Energy Characteristics of Acoustic Emission at the Volume-Expansion Point of a Rock Bridge: A New Insight into the Evolutionary Mechanism of Coastal Cliff Collapse

The recession of a coast can destabilize coastal cliffs. The stability of a cliff is controlled by a rock bridge. Identifying the volume-expansion point of rock bridges is crucial to assess cliff stability, but currently there are few identifying methods. Using a numerical analytical tool, we investigate the acoustic emission characteristics during shear tests on rock bridges. Acoustic emission events with a high energy level, i.e., characteristic events which occur at the volume-expansion point of rock bridges, can indicate this point. The characteristic events, the mainshock (the maximum event corresponding to rock-bridge rupture), and the smaller events between them constitute a special activity pattern, as the micro-seismicity during the evolutionary process of a coastal cliff collapse in Mesnil-Val, NW France showed. This pattern arises in rock bridges with different mechanical properties and geometry, or under different loading conditions. Although the energy level of characteristic events and mainshocks changes with the variation of the conditions, the difference of their energy level is approximately constant. The spatial distribution of characteristic events and mainshocks can indicate the location of rock bridges. These findings help to better understand the evolutionary mechanism of collapses and provide guidelines for monitoring the stability of coastal cliffs.

Cascade effect of rock bridge failure in planar rock slides: numerical test with a distinct element code

Plane failure along inclined joints is a classical mechanism involved in rock slope movements. It is known that the number, size and position of rock bridges along the potential failure plane are of prime importance when assessing slope stability. However, the rock bridge failure phenomenology itself has not been comprehensively understood up to now. In this study, the propagation cascade effect of rock bridge failure leading to catastrophic block sliding is studied and the influence of rock bridge position in regard to the rockfall failure mode (shear or tension) is highlighted. Numerical modelling using the distinct element method (UDEC, Itasca) is undertaken in order to assess the stability of a 10 m3 rock block lying on an inclined joint with a dip angle of 40 or 80∘. The progressive failure of rock bridges is simulated assuming a Mohr–Coulomb failure criterion and considering stress transfers from a failed bridge to the surrounding ones. Two phases of the failure process are described: (1) a stable propagation of the rock bridge failures along the joint and (2) an unstable propagation (cascade effect) of rock bridge failures until the block slides down. Additionally, the most critical position of rock bridges has been identified. It corresponds to the top of the rock block for a dip angle of 40∘ and to its bottom for an angle of 80∘.

Experimental Study on Shear Mechanism of Rock-Like Material Containing a Single Non-Persistent Rough Joint

The geometrical and mechanical properties of non-persistent joints as well as the mechanical behavior of intact rock (rock bridges) are significantly effective in the shear strength of weakness planes containing non-persistent joints. Therefore, comprehensive knowledge of the shear mechanism of both joints and rock bridges is required to assess the shear strength of the planes. In this study, the shear behavior of specimens containing a single non-persistent rough joint is investigated. A novel procedure was used to prepare cast specimens embedding a non-persistent (disc-shaped) rough joint using 3D printing and casting technology, and the shear strength of the specimens was examined through an extensive direct shear testing program under constant normal load (CNL) condition. Three levels for three different variables of the joint roughness, rock bridge ratio, and normal stress were considered, and the effects of these factors on the shear behavior of prepared samples were tested. The experimental results show a clear influence of the three variables on the shear strength of the specimens. The results show that the normal stress applied to the jointed zone of weakness planes is considerable, and thus joint friction contribution should be taken into account during shear strength evaluation. Furthermore, the dilation mechanism of the specimens before and after failure was investigated through a digital image correlation analysis. Finally, a camcorder was used to analyze the location and sequence of the initiated cracks.

Numerical Study on Anisotropic Influence of Joint Spacing on Mechanical Behavior of Rock Mass Models under Uniaxial Compression

Mechanical properties of rock masses are dominated by the nonlinear response of joints and their arrangement. In this paper, combined influences of joint spacing (s) and joint inclination angle (β) on mechanical behavior of rock mass models with large open joints under uniaxial compression were investigated by PFC modeling. With a large amount of local measurement circles placed along the pre-defined measurement lines (ML), stresses and joint response parameters at different scales (the measurement circles, the MLs and the whole specimen) were defined and calculated. It was found that macroscopic behaviors of the jointed specimens, such as four types of deformation behaviors, four failure modes, strength, deformability modulus and ductility index, are dominated by nonlinear response of the joint system, especially the interaction between the joints and rock bridges. The joints may experience three stages, i.e., starting to close, closed and opening again. On the joint plane, the peak stresses of the rock bridges and those of the joints may not be reached at the same time; i.e., joint strength mobilization happens with the loss of the rock bridges’ resistance. The influence of s on specimen behavior is little for β = 90°, obvious for β = 0° or 30° and significant for β = 45° or 60°, and this can be related to their different microscopic damage mechanisms.

Cascade effect of rock bridge failure in planar rock slides: explicit numerical modelling with a distinct element code

Plane failure along inclined joints is a classical mechanism involved in rock slopes movements. It is known that the number, size and position of rock bridges along the potential failure plane are of main importance when assessing slope stability. However, the rock bridges failure phenomenology itself has not been comprehensively understood up to now. In this study, the propagation cascade effect of rock bridges failure leading to catastrophic block sliding is studied and the influence of rock bridges position in regard to the rockfall failure mode (shear or tensile) is highlighted. Numerical modelling using the distinct element method (UDEC-ITASCA) is undertaken in order to assess the stability of a 10 m3 rock block lying on an inclined joint with a dip angle of 40° or 80°. The progressive failure of rock bridges is simulated assuming a Mohr–Coulomb failure criterion and considering stress transfers from a failed bridge to the surrounding ones. Two phases of the failure process are described: (1) a stable propagation of the rock bridge failures along the joint and (2) an unstable propagation (cascade effect) of rock bridges failures until the block slides down. Additionally, the most critical position of rock bridges has been identified. It corresponds to the top of the rock block for a dip angle of 40° and to its bottom for an angle of 80°.

Seismic noise monitoring of a small rock block collapse test

SUMMARY We tested the capability of seismic noise to monitor the stability conditions of a small rock block that we forced to fail in four following stages. Ambient vibrations were recorded with a broad-band 3C seismometer placed on top of the block and were processed to analyse their spectral and polarization characteristics with diverse algorithms. To analyse the spectral content of the records, we applied the multitaper method while seismic noise polarization features were investigated by means of the singular value decomposition of the Hermitian spectral density matrix. Numerical modelling was found to add limited value because of the uncertainty in estimating correctly spatial and mechanical features of the rock bridges between the block and the rock mass. Nevertheless, a modelling exercise we performed is in agreement with previous post-failure observations according to which unstable rocks may be coupled to the stable rock mass by rock bridges covering only a few per cent of the total surface of the fractures. Our analyses confirm that, when approaching final collapse, there is a trend of the block eigenmodes towards lower frequencies and show that polarized bands become narrower.

Analysis of Progressive Failure Mechanism of Rock Slope with Locked Section Based on Energy Theory

Progressive failure in rock bridges along pre-existing discontinuities is one of the predominant destruction modes of rock slopes. The monitoring and prediction of the impending progressive failure is of great significance to ensure the stability of the rock structures and the safety of the workers. The deformation and fracture of rocks are complex processes with energy evolution between rocks and the external environment. Regarding the whole slope as a system, an energy evolution equation of rock slope systems during progressive failure was established by an energy method of systemic stability. Then, considering the weakening effect of joints and the locking effect of rock bridges, a method for calculating the safety factor of rock slopes with a locked section was proposed. Finally, the energy evolution equation and the calculation method of safety factor are verified by a case study. The results show that when the energy dissipated in the progressive failure process of rock bridges is less than the energy accumulated by itself, the deformation energy stored in the slope system can make the locked section deform continuously until the damage occurs. The system energy equal to zero can be used as the critical criterion for the dynamic instability of the rock slope with locked section. The accumulated deformation energy in the slope system can promote the development of the cracks in the locked section, and the residual energy in the critical sliding state is finally released in the form of kinetic energy, which is the main reason for the progressive dynamic instability of rock slopes.

Geostructural and Geomechanical Study of the Piastrone Quarry (Seravezza, Italy) Supported by Photogrammetry to Assess Failure Mode

The use of non-contact-techniques for rock mass characterization has been growing significantly over the last decade. However, their application to stability assessment of ornamental stone has not yet received much attention from researchers. This study utilizes rock mass data both in terms of slope orientations and degree of fracturing obtained from a point cloud, a set of three-dimensional (3D) points representing a rock mass surface, to (1) investigate the influence of geostructures at different scales and (2) assess quarry stability by determining areas susceptible to different failure types. Multi-resolution point clouds are obtained through several photogrammetric survey techniques to identify important structural elements of the site. By integrating orientation data of discontinuity planes, obtained with a traditional survey, and of traces, outlined on point clouds, several joint sets were identified. Kinematic tests revealed various potential failure modes of the rock slope. Moreover, an analysis of the influence of the discontinuity strength determined by the presence of rock bridges was carried out. The study revealed that the strength of the quarry face is governed by the presence of rock bridges that act to improve the stability condition of the rock fronts.