Cai, Zheng-YiZheng-YiCaiLi, Hung-HuiHung-HuiLiLe, Hoang-KhanhHoang-KhanhLeMENG-CHIA WENGKu, Chia-HaoChia-HaoKuChien, Chih-ChunChih-ChunChien2026-03-242026-03-2420269789819542628https://www.scopus.com/record/display.uri?eid=2-s2.0-105031530533&origin=resultslisthttps://scholars.lib.ntu.edu.tw/handle/123456789/736753Rock masses often exhibit intricate weakness planes that often interconnect to form an impenetrable rock bridge, potentially impacting the stability of the rock mass. To understand how fractures develop under sustained shear stress and whether changes in the wave propagation characteristics can reflect the mechanical behavior of fractured rock, this study integrates direct shear tests on fractured rock specimens with ultrasonic measurements. By analyzing the shear stress–shear displacement curves and wave velocity variations obtained from the mechanical tests, preliminary insights into the relationship between initial failure behavior and crack development before reaching the peak shear strength of the rock mass have been gained. The following findings are obtained: (1) The initial failure shear strength (τi) was approximately 0.5–0.7 times the peak shear strength (τp), and the initial failure shear displacement (si) was approximately 0.5–0.6 times the peak shear displacement (sp). (2) As the length of the rock bridge increased, the initial failure shear strength (τi), the initial failure shear displacement (si), and the peak shear displacement (sp) also increased. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2026.Direct shear testRock bridgeUltrasonic velocityconference paper10.1007/978-981-95-4263-5_52-s2.0-105031530533