Efficient, economic, and safe excavation of tunnels depends on a detailed understanding of rock conditions to be encountered at the working face. Unanticipated joints, faults, or shear zones could lead to potentially hazardous conditions that may result in work stoppages and resultant claims and disputes. New methods of risk identification during site investigation and active tunnel excavation can have a significant impact on contractors competitive advantage or on overall project costs for owners. Kajima has applied a new method of seismic tomography in Japan pioneered by Kajima and NSA Engineering called True Reflection Tomography "TRT." Using seismic energy from multiple sources including the TBM cutting action, TRT creates a 3-D isometric map of the geologic structure some 100 m out and up to 30 m around the tunnel alignment. Kajima has applied TRT on twelve tunnelling projects since its introduction in February 1999 to predict rock conditions ahead of the tunnel face. The images produced by TRT have been used satisfactorily to manage the risk caused by encountering unforeseen geologic conditions.
Reflection tomography TRT methods developed by NSA process reflections produced by seismic waves generated from a number of source types and tunnelling machines commonly found in mining and tunnelling. An isometric plot of detected reflection anomalies along the tunnel alignment give a 3-D image of the geologic structure for some distance along the tunnel alignment. This information allows the site engineer and geologist to more accurately assess expected conditions for risk management and abatement. Kajima Corporation has used TRT extensively on the Fujikawa and Kanaya Tunnels in often very difficult and varied geologic conditions to manage risk for improved safety and operational management resulting in a major economic benefit to the owner. TRT data analysis is complex and requires special skills to extract accurate and complete geologic information. Data collection typically takes about four hours, including hardware installation, data collection, and teardown. The data from a particular section are then transferred to NSA via the Internet for processing. Upon receipt at NSA offices, the data are input, filtered, picked, and processed for analysis by NSA geophysicists and engineers. This process usually requires between 8 to 20 hours depending on the complexity of the geologic situation. The resultant analysis is then returned to the site ready for the next morning's work.
The 4,520 m Fujikawa Highway Tunnel in Shizuoka Prefecture, Japan, is being driven through fractured and sheared andesites with tuff breccia lenses. The objective of the TRT survey for the first section was to delineate the boundary of the gravel deposit overlying an andesite and tuff breccia formation above the alignment of the TBM excavated tunnel. The objective of the survey for the second section was to image ahead of the TBM where ground conditions had deteriorated due to frequent faulting and shear zones where the tunnel passes under the Umusegawa River flowing into the Fujikawa River and continues in a 470 m section between STA. 1041+40 to STA. 1036+70.