MINING IN MANHATTAN: PART 1- CLASSIFICATION OF RECENTLY MAPPED GEOLOGIC STRUCTURES IN LOWER MANHATTAN TUNNELS

New York City is experiencing one of the most prolific periods in its history of tunnel construction. Presently, five tunnels are under construction using tunnel boring machines. The authors have mapped the most extensive of the five and are presently mapping others. The mapped tunnel is nine miles in length, and runs down the West side of Manhattan from Central Park to lower Manhattan, where it crosses to the East side and then turns north, terminating near Midtown. The 12.5 feet diameter tunnel, which was mined at depths in rock ranging from 360 feet to 560 feet below the surface, also includes ten new vertical shafts that will bring water to near-surface distribution chambers. The alignment of the tunnel and locations of the vertical shafts provided a unique opportunity to trace the most dominant, persistent rock structures throughout lower Manhattan. Maps were prepared every 100 feet following industry standards for tunnel mapping. Maps included lithologic contacts, joint sets, foliation, folds, faults, shear zones, as well as descriptions of engineering parameters. A number of faults were identified and classified based on their orientation, geotechnical properties, relative age, and influence on mining production. Some types posed no concern; but others, especially at points of intersection, required installation of steel support to maintain the structural integrity of the rock mass. This paper presents the geotechnical findings of the New York City Department of Environmental Protection (NYCDEP) Water Tunnel No. 3, Stage 2- South Leg in lower Manhattan. A description of field mapping techniques, tools, and methods for tunnel and shaft mapping is provided. A comprehensive set of tables and stereonet projections are included for joint sets, foliation, folds and faults. Scenarios regarding the implications on future tunnel design in Manhattan are described. A geotechnical map that includes cross-sections and areas with tunneling risk are depicted in GIS.