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This paper presents a case history of structures along the I-35 Northeast Expansion (NEX) Central project, which is a $1.59 billion design-build project for the Texas Department of Transportation (TxDOT). The project was awarded to Alamo NEX Construction (ANC, a joint venture led by Ferrovial Construction and Webber LLC) in August 2021. The project is 9.5 miles of highway improvements that include design, construction, and maintenance of elevated non-tolled managed lanes. The elevated lanes are separate northbound and southbound bridges that are being constructed above the existing interstate. The majority of the foundations are large-diameter drilled shafts, with a single shaft supporting each bridge bent. This case history focuses on the foundations for bridge bents at the interchange of I-35 and I-410, where karst features were encountered. The site investigation included soil borings/rock cores at each bridge bent. The field investigation included geophysical testing using multi-channel seismic wave imaging (MASW) and electrical resistivity imaging (ERI). The paper includes a comparison of the geophysical findings. The design analysis methods and final design are described, followed with a discussion of the features encountered during construction.