Verification of countermeasures to prevent spalling of shaft walls during deep shaft excavation; A Case study at the Horonobe Underground Research Center, Japan

Nago, Makito*; Tsusaka, Kimikazu*; Aoyagi, Kazuhei  ; Sugawara, Kentaro*; Kodama, Junichi*

This study examines the effects of rock spalling on support elements in order to prevent cracking and to ensure the integrity of the support system and safety during shaft sinking. When the ventilation shaft at the Horonobe Underground Research Center reached a depth of more than 250 m, severe rock spalling occurred, and cracks developed in the concrete lining immediately above the spalling zone. Therefore, a three-dimensional numerical analysis of the shaft was conducted to estimate changes in the stress distribution within the concrete lining caused by the spalling. The simulation results indicated that the vertical tensile stress in the concrete lining increased as the spalling progressed. By integrating the analytical results with field observations and considering various support patterns to prevent further rock spalling, a flowchart for selecting the optimal support pattern was developed. Shaft sinking was subsequently completed to a depth of 500 m without significant damage to the concrete lining or excessive spalling. The flowchart developed in this study will contribute to the selection of optimal support patterns for future shaft sinking projects.