Monitoring of spallation processes in rocks by continuous measurements of surface deformation and wave parameters

Spallation is a type of surface rock failure under uniaxial and biaxial compression manifested by successive production and ejection of spalls/fragments. This type of failure is observed in laboratory experiments on uniaxial/biaxial compression of rocks and mortar as well as in rock masses. In the latter case spallation is seen in slopes and in the walls of underground openings. In its unstable phase the spallation can lead to such a dangerous phenomenon as strain rockburst.

Spallation is caused by formation and extensive growth of wing cracks parallel to a free surface (e.g., excavation wall) under the applied compressive load. Their growth amplified by the strong interaction with the surface leads to separation of thin layers whose subsequent buckling produces the spalls and opens a new surface. This produces new wing cracks extensively growing parallel to the new surface, thus enabling the process that repeats itself, e.g. [1].

A critical role in this mechanism is played by the interaction of the wing crack with the free surface. The interaction is the stronger the closer the wing crack to the free surface. The closeness to the free surface is limited by the sizes of the largest pre-existing defects seeding the wing cracks. Therefore, the wing cracks inducing each step of spallation are approximately coplanar. Subsequently, the layer separated from the bulk of the rock can be considered as a plate connected to the main part of the rock by bridges formed by intact rock sections remaining between the wing cracks. In the first approximation the effect of bridges can be modelled by Winkler layer [2]. The cracks are assumed to be disc-like; the interaction with the free surface is computed using the beam asymptotics [3].

The velocity of flexural wave propagation depends upon the Winkler layer stiffness and the frequency of oscillations. There exists a minimum frequency, below which the wave does not propagate.  Both parameters depend upon the average crack radius and the number of wing cracks. If the monitoring of the wave velocities and the minimum frequency is complemented by monitoring of the average surface deformation (for instance using non-contact methods such as the digital image correlation) the parameters of the spallation process can be determined, and the approaching buckling phase identified. Results of this research will be instrumental in developing methods of monitoring and predicting strain rockbursts.

1. Wang H, A.V. Dyskin, E. Pasternak, P. Dight and B. Jeffcoat-Sacco, 2022. Fracture mechanics of spallation. Engineering Fracture Mechanics, 260:108186.

2. He, J., Pasternak, E. and A.V. Dyskin, 2020. Bridges outside fracture process zone: Their existence and effect. Engineering Fracture Mechanics, 225, 106453.

3. Dyskin, A.V., L.N. Germanovich and K.B. Ustinov, 2000. Asymptotic analysis of crack interaction with free boundary. J. Solids Structures, 37, 857-886.

4. Lloyd J.R. and Miklowitz, 1962. Wave Propagation in an Elastic Beam or Plate on an Elastic Foundation. J. Applied Mechanics, 459-464.

Acknowledgement. The authors acknowledge support from the Australian Research Council through project DP210102224.