Drilled Shafts in Karst Geology - I35 NEX Case History-Paper

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This case history describes the design and construction of heavily loaded, large-diameter drilled shafts for Bridge 101 on the I-35 Northeast Expansion project in San Antonio, Texas, where karst features were a concern.<br /><br />The geotechnical team reviewed historical topographic maps, geology, and existing boring records, noting a former creek and subsurface clay layers that could indicate karst. Borings were attempted at each foundation, supplemented by shear-wave and electrical-resistivity surveys. These investigations identified potential clay-filled voids and informed detailed subsurface profiles shared with the structural designers and contractor.<br /><br />To address uncertainty, shaft designs generally relied on side resistance and excluded end bearing where karst could occur. Skin friction was also disregarded within identified karst zones. Construction plans required field verification, coring at specified locations, and geotechnical review if unexpected features or weak strata were found.<br /><br />At Bent 9, a 96-inch shaft encountered extensive clay-filled karst between the expected bearing level and intact limestone. After pausing drilling, the team investigated and cored the material, then revised the bearing-stratum and shaft-tip elevations. The design was recalculated without resistance from the void zones or end bearing; the shaft was extended into intact limestone. Open voids were not found, so permanent casing was unnecessary.<br /><br />The authors attribute the project’s success to early identification and communication of karst risks, thorough site investigation, geophysical testing, site-specific design, and construction verification. Karst was encountered at several Bridge 101 bents, broadly consistent with the investigation. However, an inaccessible foundation location at Bridge 1 could not be investigated in advance; an open karst feature there was discovered only after the shaft had been poured.

Keywords

Bridge 101

I-35 Northeast Expansion

San Antonio Texas

drilled shafts

karst features

geotechnical investigation

side resistance

limestone bearing strata

electrical resistivity survey

construction verification