With mapping of a 400-foot-deep access shaft in February 2001, Hager GeoScience completed over three years of collecting and managing geologic data for the MetroWest Water Supply Tunnel (MWWST) Project. The purpose of the work has been to document existing subsurface conditions along the 17.6-mile long, deep rock tunnel and provide the design engineers with the information necessary to design the final tunnel liner. Decisions regarding grouting and liner type used data collected by HGI field personnel regarding water inflow and lithological and structural features intersected as the tunnel was advanced through the bedrock.

HGI’s integrated imaging and text database system, GeoScan-32, has proved to be an effective means for archiving and retrieving large amounts of mapping data. GeoScan-32 provides for graphical and text data entry while maintaining a link between both types of data. Scanned images of completed field maps and associated data for a specific heading or tunnel section can be collected and copied onto CD ROM to provide readily available information to the Construction Manager and Design Engineer. The GeoScan-32 system, used for project reference during the tunnel mapping, now serves as a vehicle for geological analysis of the mapping results.

Several generations of faults, shears, and intrusions in the MetroWest Tunnel reveal a complex tectonic history. Healed Proterozoic thrust faults are often reactivated by multiple episodes, including hydrothermal and “cold” events. The local effects of these tectonic features on tunneling for the MetroWest Project can be used to predict geologic conditions for future tunnel projects.