A Case Study, Design Change From Spread Footing to Drilled Shafts-Paper

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The paper describes a design change to the foundation of Pier 2 on a five-span bridge over the Lehigh River in Allentown, Pennsylvania. The original design called for a spread footing founded on dolomite bedrock. During construction, however, irregular bedrock pinnacles and weathered rock prevented the cofferdam sheet piles from sealing properly. Water also entered through fractures and possible solution openings in the karstic dolomite. Despite repeated efforts, crews could not keep the excavation dry or safely reach and inspect the footing subgrade.<br /><br />The design-build team collaboratively replaced the spread footing with three drilled shafts. Additional borings were completed to investigate the variable bedrock conditions. Each shaft was designed with a minimum 25-foot rock socket, relying conservatively on skin friction and disregarding tip resistance.<br /><br />Construction involved placing an approximately 14-foot-thick tremie-concrete seal to resist water pressure, then drilling the shafts through the seal and into the rock. The shafts were excavated and concreted underwater using the tremie method. Base cleanliness was checked by video inspection, and concrete integrity was assessed with Crosshole Sonic Logging. All three shafts met quality-control requirements, and Pier 2 was completed successfully.<br /><br />The case highlights the value of collaboration in design-build projects, where foundation designs can be adapted to actual site conditions. It also cautions that spread footings that appear economical may prove difficult to construct in river settings underlain by karst. Designers should consider construction risks and include specifications for remediation if karst features are encountered.

Keywords

Pier 2

Allentown Pennsylvania

Lehigh River bridge

foundation redesign

drilled shafts

karst dolomite

tremie concrete seal

rock socket

design-build collaboration

crosshole sonic logging