Geotechnical investigations and engineering programs are the cornerstone of structural bridge design, and it is paramount to gather reliable information during field investigations. However, geotechnical engineering and the geologic sciences are characterized by uncertainty and railroads often encounter variable geologic conditions during foundation installation. This was the case with a CSX Transportation (CSXT) bridge crossing over Buck Creek between the cities of Helena and Pelham, Alabama. The project is located in an area that is underlain by karst bedrock consisting of limestone and dolomite. Both types of bedrock present are subject to dissolution from mildly acidic groundwater penetrating the bedrock through joints or discontinuities. The dissolution process produces an irregular bedrock surface that may contain clay-filled, open discontinuities in the bedrock. The differential weathering process isolates remnant rock units, leaving weathered-in-place boulders within a soil matrix. The constructed Helena CSXT Bridge is nearly 600 feet in total length and consists of five (5) evenly spaced spans supported by a combination of driven HP14x89 steel piles and micropiles installed within competent bedrock. This paper will provide background to the geotechnical design and challenges faced during installation of foundation elements within karst bedrock. The design team, including geotechnical and structural engineers, worked together to find successful solutions to minimize the impacts of the variable bedrock line while maintaining the construction progress. Continuous engagement by the geotechnical design engineers throughout the foundation construction phase was key to a successful project completion.
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