USE OF METROWEST TUNNEL DATA TO UNDERSTAND REGIONAL GEOLOGY AND PLAN TUNNELING EXPLORATION IN NEW ENGLAND
Most of the MetroWest Tunnel lies in the outer, northwestern portions of the Late Proterozoic Milford Granite and Dedham Granodiorite, and is highly fractured with little evidence of folding. Relatively narrow fault-defined zones and several sets of faults have been mapped both in the tunnel and on the surface. Zones of water inflow and weak rock, several to a few hundred feet wide, are associated with faults and shear zones, especially at their intersection, and with adjacent hydrothermally altered rock. These data support the geology from quadrangle mapping and demonstrate that outcrop mapping by experienced field geologists can more accurately predict geologic conditions at tunnel depths. An exploration program combining the MetroWest map data, detailed surface mapping, and site-specific boring data can provide a more accurate and regionally specific model for predicting tunnel mining costs.