Geotechnical Challenges in Freeway Widening Project—Case Study

Geotechnical challenges encountered in a freeway widening project are presented along with approaches used to tackle these challenges. Approximately 44 miles of freeway widening along with 42 bridge widenings and 8 tie-back walls were studied. Geotechnical challenges included nearby active earthquake surface faulting and commensurate seismic shaking, liquefaction potential in stream crossings, very hard granite rock, poor subgrade drainage in shallow rock, coarse alluvium with boulders and cobbles, foundation design with scour on the order of 25 feet, and pavement section design in expansive, collapsible and corrosive soils. To account for seismic shaking, acceleration response spectra were developed for each new structure location, in accordance with Caltrans Seismic Design Criteria. Deep and shallow foundation recommendations were also presented to the design team, carefully considering existing bridge foundations. Fifty, stand alone, Structure Preliminary Geotechnical Reports (SPGRs) were prepared for these structure improvements including tie-back walls. One summary report was prepared to present pavement recommendations for on- and off-ramps and main lanes for this alignment. All these reports were reviewed by the Riverside County Transportation Commission (RCTC) and Caltrans. All these geotechnical challenges were studied. Feasible, safe and economical geotechnical recommendations were then presented to handle those challenges.