The Importance of Geologic Characterization of Fault-Controlled Karst at a Raw Water Intake Site

Due to access, only one deep boring could be obtained for the geotechnical investigation for a raw water intake structure in Cadiz, Kentucky to be set into a limestone bluff and founded in limestone bedrock 21 feet below river stage. The rock core indicated high recovery and rock quality designation (RQD) values. A small cave was noted in the bluff, suggesting that karst could be encountered during excavation. Unfilled and clay-filled caves were encountered during excavation, along with vertical and low-angle fault zones and fault-controlled karst. A lower boundary of cave development was observed above the planned foundation bearing level. Once below river stage, water inflow entered the excavation from the sidewall and floor along fault- or karst-controlled discontinuities. Shallow, karst-enhanced faults dipped into the cuts made for the permanent terrace and access road. Stability analyses suggested that block failure onto the terrace could occur in the design earthquake. The extent and magnitude of karst development were not revealed by the single deep boring. This project underscored the need to obtain adequate geotechnical information, including sufficient borings, during the project design phase. We emphasize the need for understanding the geologic history and structural geology of a project site, including the use of topographic and geologic maps and the capability of recognizing and mapping structural features such as faults, joints, slickensides, and shattered rock zones. Review of rock core alone may not reveal local evidence of faulting, if subsequent karst development removes the evidence of movement.