Bridges in Appalachian-Type Karst Geotechnical and Foundation Design Concerns

摘要

The subsurface peculiarities of karst are seldom recognized by project planners. This becomes readily apparent when the bottom falls out, the sinkhole is elevated to disaster status, and the event makes spectacular news headlines. Areas of karst terrane have long been recognized through historical accounts and data compiled by state and federal agencies, professional organizations, and municipalities. Obtaining background data is simple. LIDAR imagery is another tool that can strip away the veneer of urban development and identify topographic features indicative of karst. A site reconnaissance should complement the aerial review. Indirect methods, such as geophysics, may be of use if carefully planned and performed with an understanding of the "vagaries" of karst., and the results integrated with the data. Bridge foundation types must reach competent material, which in a karst environment generally means unweathered bedrock. Competent rock should be expected to be at variable depths and must be considered in any investigation program. Likewise, any deep foundation design must be flexible enough to include provisions for field changes in foundation element lengths. Grouting, both high- and low-mobility, is often used in karst terrane for foundation improvements. The paper examples two sites located less than 10 miles apart in a known karst area. One of the bridges has performed satisfactorily, the other, newer bridge (replacing one that was a victim of karst-related problems) is showing evidence of continuing settlement and is being actively monitored.