Drilled Shafts in Karst Geology - I35 NEX Case History-PPT Presentation

PPT Presentation

Pdf Summary

The I-35 NEX project in San Antonio, Texas, includes 9.5 miles of elevated roadway. At the I-35/I-410 interchange, the combination of Pecan Gap limestone, nearby faulting, historic stream crossings, and an existing boring indicated potential karst conditions.<br /><br />For Bridge 101, investigation included borings and geophysical testing. Rock cores and seismic refraction identified a possible void, and subsequent shaft excavation at Bent 9 encountered discontinuous clay-filled karst features between elevations 670 and 643 feet. Intact limestone was found at elevation 643 feet and confirmed by coring and shaft inspection.<br /><br />Following a construction request for information, the geotechnical engineer determined that permanent casing was unnecessary because no open voids were present. The bearing elevation was lowered to 643 feet and the shaft tip to 634 feet. The structural engineer confirmed that the revised tip met axial resistance requirements and that the karst features did not affect lateral stability. The design used an 8-foot-diameter shaft and accounted for reduced resistance due to non-redundancy.<br /><br />The case highlights the importance of detailed site reconnaissance and data review, borings at each bent, geophysical methods such as MASW and ERI, well-developed subsurface profiles, and site-specific calculations that account for rock strength and karst features.

Keywords

I-35 NEX

San Antonio

Texas

elevated roadway

karst conditions

Pecan Gap limestone

geophysical testing

shaft excavation

axial resistance

lateral stability