A New In Situ Test for the Assessment of the Rock-Burst Alarm Threshold During Tunnelling

Abstract

Rock-burst is one of the most serious risks associated with hard rock tunnelling and mining at high depths. Monitoring of acoustic emissions emitted by the rock-mass during excavation and their interpretation now permits the early assessment of failure events and makes the safe management of the construction works possible. A reliable set-up of the alarm threshold is thus fundamental for the correct implementation of the procedures planned to minimise rock-burst related risk. This paper focuses on a novel in situ test specifically developed to provide an experimental basis for a more accurate assessment of the alarm threshold during tunnelling, representative of the local geomechanical conditions. The test, thanks to the compression induced by two flat jacks at the tunnel side wall, produces an artificial failure process during which acoustic emissions are measured and correlated to the mechanical response of the rock-mass, without the typical limitations of scale that characterised the laboratory experiments. The new methodology, named the Mules method, was successfully tested during the excavation of some stretches of the Brenner Base Tunnel in the Brixner granite, affected by mild spalling episodes. The case-history is fully described in the paper to illustrate the practical application of the proposed approach.

Measurement and Classification Criteria of Strength Decrease Rate and Brittleness Indicator Index for Rockburst Proneness Evaluation of Hard Rocks

“…Several non-energy indexes of rockburst tendency in laboratory tests were put forward, e.g., brittleness indicator index B, decrease module index DMI, deformation brittleness index K u , and strength decrease rate index SDR [12], as shown in Table 1. The K u is defined as the ratio of strain corresponding to the stress at the unloading point and the terminal strain at the unloading stage, but the elastic strain at the peak is impossible to obtain, so it cannot accurately reflect the actual bursting tendency of rocks.…”

“…At present, rockburst is a global underground engineering problem; it is difficult to predict accurately, and to prevent and control effectively [10]. Once rockburst occurs, it may lead to more casualties and economic losses than other disasters [2,11,12].…”

A size‐dependent energy‐based strain burst criterion

“…Rocks around deep underground openings are under a polyaxial stress state and can exhibit violent dynamic behaviour during mining operations due to unloading caused by excavation. [1][2][3][4][5][6][7] This sudden change in boundary conditions under high in-situ confining pressure governs the violent failure of rock, commonly known as 'strain burst', due to the release of strain energy in the form of kinetic energy. It highlights the significance of the rock's strain energy storing and dissipation characteristics in its failure mechanism.…”

Rockburst prediction and early warning for a highway tunnel excavated by TBM based on microseismic monitoring

“…Furthermore, a downward shift phenomenon of frequency band energy distribution (Liang et al, 2020b), apparent stress, apparent volume, and main frequency (Xue et al, 2021) were also considered as early-warning indicators of rockburst. The MS monitoring system based on accelerometers was likewise applied in other water headrace tunnels (Xu et al, 2016;Liu Q. S. et al, 2020;Yang et al, 2023) and railway tunnels (Voza et al, 2023).…”