I-25 TREX Project: O-Cell Tests and Drilled Shaft Design Recommendations

The TREX project is a $1.6B Design/Build underway in Denver, Colorado. The 17 mile long project is a combination of highway improvements to I-25 and I-225 and a new Light Rail Transit (LRT) line within the highway rights-of-way. The Contract required full-scale load tests on three drilled shafts. These were performed using the Osterberg Cell (O-Cell) method. The end-bearing and side shear measured in competent bedrock were well above design parameters determined using traditional Denver Area design methods. However, the test shafts in weathered bedrock had less than acceptable factors of safety (FS). Using the O-Cell tests results and data obtained as part of a Colorado Department of Transportation (CDOT) research study, the traditional Denver Area drilled shaft design method was modified for the weathered bedrock to meet AASHTO factor of safety requirements. The modified method was used to design the project drilled shaft foundations.