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

Hongran Chen,Lei Xue,Mengyang Zhai

doi:10.3390/jmse9121338

Abstract

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.

Highlights

Around 80% of Earth’s coastlines are classified as coastal cliffs [1], and the recession of the cliffs is a significant coast environmental problem worldwide that may affect30% of the world’s population [2]
We speculated that they served as the characteristic events corresponding to the volume-expansion point of the rock bridge, because they had the highest energy level in the pre-peak loading stage of the rock bridge
Along with the characteristic events, micro cracks began to grow and coalesce from the 20th step, and the cluster of micro cracks nearly spanned across the whole rock bridge at the 28th step prior to the peak-stress point of the rock bridge, as the temporal and spatial distribution of acoustic emissions (AEs) showed (Figure 7)

Summary

Introduction

Around 80% of Earth’s coastlines are classified as coastal cliffs [1], and the recession of the cliffs is a significant coast environmental problem worldwide that may affect30% of the world’s population (those living within 60 km of the coast) [2]. Owing to the impact and erosion effect of sea waves, a coastal cliff can be gradually undercut [7]. Under this circumstance, the stress state of the upper segment of the cliff is similar in nature to that of a cantilever beam; vertical tensile cracks can generate (or initiate from pre-existing joints) and propagate downward (Figure 1). Once the rock bridge ruptures, the dangerous rock mass will separate away from the cliff rapidly, causing a catastrophic collapse as well as the recession of the coastline. The rock bridge is the key internal factor controlling the evolution of cliffs, and cliff collapses cannot be directly triggered by certain external factors before rock bridges rupture. We concentrate on the damage process of rock bridges, which is the groundwork for uncovering the evolutionary mechanism of collapses

Methods

Results

Discussion

Conclusion