Joint orientations for TBM performance analysis using borehole geophysics to orient rock cores

摘要

A borehole televiewer was used to orient rock core for joint analysis as part of the Atlanta CSO Tunnel Geotechnical Investigation. The Atlanta CSO tunnels are about 30 feet in diameter and will be bored through medium grade gneiss and schist. The televiewer is a wireline geophysical tool that uses sonic waves to map the traces of individual joints around the inside of the borehole wall. From these traces, the televiewer software calculates the strike, dip, and aperture of each joint. The televiewer was used in 51 core holes averaging 300 feet in depth, and was much less expensive, faster, and more accurate than using oriented core. The televiewer provided joint characteristics, an acoustic velocity log to indicate areas of weathered or blocky ground where core recovery is typically poor, and a graphical picture of the borehole that can be used to orient the core for more detailed analysis. Each joint in the televiewer data was correlated to the core and classified in terms of RMR and Q parameters. Machine breaks and core damage were easily recognized because they occurred only in the core, but were not read by the televiewer in the borehole wall. A stereoplot of the joint data was made for each borehole and for the project as a whole. These stereoplots were used for three purposes: (1) kinematic wedge analysis and support design; (2) classifying the ground into different baseline types; and (3) estimating the potential benefits of the fractures on TBM performance using the Norwegian Fracturing Factor criteria.