GEOPHYSICAL BOREHOLE LOGGING AS AN EXPLORATION TOOL FOR TUNNEL DESIGN

Since 1997 Hager GeoScience, Inc. has been performing shaft and tunnel mapping for a 17.6-mile-long deep-rock water supply tunnel outside Boston, Massachusetts. The exploration phase included drilling a series of boreholes from the surface through the proposed tunnel alignment and collecting rock core; the core was logged and lithology and structure, including fractures, described. However, no oriented core was taken or geophysical borehole logging performed. Thus, fractures and other discontinuities could not be completely characterized from the borehole descriptions, and the orientation of fracture sets that might produce problems during tunnel construction could not be determined.

We have used our tunnel mapping data to show how geophysical borehole logging during the preliminary investigation could have benefited design of this tunnel and can benefit the design of future tunnels as well. A suite of logs including temperature, caliper, and acoustic televiewer (ATV) logs, combined with the information gathered during coring, would have better enabled the contractor to predict which fracture sets might create problems during tunnel construction. Acoustic televiewer logging is more accurate and less expensive than oriented core, and can be performed in a borehole as small as 3 inches.