DISSEMINATION OF GEOLOGICAL INFORMATION IN AVOIDING GEOTECHNICAL RISKS ASSOCIATED WITH TUNNEL CONSTRUCTION: LESSONS LEARNED FROM DEEP VOIDS IN MARBLE IN LOWER MANHATTAN, NEW YORK CITY

Test boring associated with the recently accomplished City Water Tunnel # 3 in Manhattan, New York City revealed moderately to highly weathered marble with insoluble silicate residues composed mostly of phologopite, chlorite, and tremolite. Apparent control on the weatherability of the marble was in response to original mineral constituents dominating this lithology. Encountered marble samples ranged between pure calcitic to mixed dolomitic/siliceous types. Fresh marble samples collected from adjacent boring locations revealed characteristic geochemical data: Lime (25-45%), Silica (4-7%), Alumina (1-3%), MgO (5-20%), and LOI (35-42%); compressive strength of unweathered marble varied between 2000-3000 Kg/cm². Relict foliations with schistose in nature were also noted in some marbles. Geotechnical characteristics such as sudden decrease in N-value (blow counts), faster drilling rate, and poor RQD (Rock Quality Designation) associated with this weathered zone also suggested incompetent and unconsolidated rock debris. Both physical and geotechnical attributes of the marble indicated the existence of dissolution-controlled subsurface conditions. Although the probability is low, deep voids in the marble in Lower Manhattan could cause problems if the tunnel boring operation intersects them. Usually, solution voids in carbonates do not occur at such depth, unless there is the presence of intricate joint or fracture systems, which continue deep within the subsurface rocks. However, faulting, in and around Canal Street, could create just such a situation. Possible occurrences of complex and perhaps interconnected fractures, enlarged by solution within the marble, may create an easy passageway for water flow. Moreover, it appears that such a void was encountered by a test boring on Bowery St. just south of Canal St. at a depth of about 200 feet. If a water filled void, which is part of an interconnected fracture system, is encountered during the tunnel boring, then water could enter the tunnel at a rate that would be difficult to control. From the test boring data, it appears that the present, proposed tunnel alignment is well west of the major occurrence of the marble. Nevertheless, it would be prudent to be alert to signs of excessive water entering the tunnel, particularly during the probe drilling operations.