A joint application of semi-airborne and in-tunnel geophysical survey in complex limestone geology

When tunnelingin karst mountainous areas, some adverse geological conditions such as karst caves and water-rich strata are often encountered, which may induce unexpected geological hazards and represent major hidden threats to construction safety. This paper describes the successful use of integrated investigation techniques in Qinlan-1 tunnel to predict geological hazards in Guangxi, China. The study area is located in limestone strata which are significantly affected by the fault and developed karst. The comprehensive investigation technique mainly included the geological survey, semi-airborne transient electromagnetic method (SATEM), seismic ahead prospecting (SAP), and ground penetrating radar (GPR). First, SATEM was applied to delineate the distribution scope and depth of the anomalous zones of apparent resistivity in the study area before the tunnel excavation. Then, SAP and GPR were used during excavation to investigate the detailed geological condition, such as the lithological change, the fracture zone's interface, and the karst cave's location. Finally, a fracture zone and a mud-filled karst cave were successfully detected, proving the effectiveness of the comprehensive investigation system. The comprehensive investigation system improves the exploration efficiency and accuracy of the adverse geological structure of the tunnel.