Extreme Karst: Investigation at PA State Route 33

摘要

In January 2004, a sinkhole opened beneath the northbound span of PA State Route 33 over the Bushkill Creek. Both the northbound and southbound spans eventually had to be replaced. Shortly after completion, both new spans indicated signs of movement and distress. As a result a deep investigation of the karst conditions was initiated. The investigation included deep borings (over 500 feet), and a comprehensive hydrogeologic study. Water quality soundings, including continuous temperature and conductivity readings, were conducted for every boring. A deep (over 400 feet) heavily karstified zone was found to lie beneath the creek, in the area of SR33. High groundwater velocities were suspected due to a combination of: 1) problems experienced during grouting of micropiles for the replacement structure foundations, 2) the presence of large sinkholes in the creek just downstream of SR33, and 3) high pumping requirements of quarry not far upstream. It was further suspected that these high groundwater velocities were a result of a conduit like connection through the karst, between the stream and the quarry. This condition was thought to be resulting in the slow erosion of subsurface soil supporting the structure foundations A brine tracer study was conducted to confirm the suspected flow pattern and high groundwater velocities. Based upon the findings of the investigation, several options were developed to stabilize the foundations. The options were found to either be very costly, to have a low prospect for success, or result in other highly undesirable consequences. The structures will continued to be monitored, and further subsurface investigation will be conducted. It is intended to delineate the limits of the highly karstified zone, so as to be prepared should monitoring indicate that the existing structures may become unserviceable.