Borehole Logging as an Aid in the Design of a Subsurface Pump Station

The Combined Sewer Overflow (CSO) Project in Providence, RI consists of a 16,000-foot-long TBM rock tunnel connecting to an 117x61x68-foot pump cavern 300 feet in the sedimentary rocks of the Providence Formation. An important consideration related to the pump cavern construction was the presence and potential angle of intersection of the pump cavern ceiling and/or walls with a highly deformed and friable graphitic shale unit, since this could have major impacts on worker safety and, therefore, the amount of structural support required. To gain a better understanding of bedrock conditions and discontinuities at the pump cavern location, the Project drilled and cored two deep rock boreholes for acoustic televiewer and caliper logging. Data from the borehole logging were correlated with core descriptions in order to characterize the orientations and stratigraphic positions of discontinuities in the proposed pump cavern location. Based on the results of these analyses, the pump cavern was moved from its original proposed location.