I10 Widening: St. Marys Road to 29th Street

Since the 1990s, the Arizona Department of Transportation (ADOT) is widening the existing Interstate 10 (I-10) going through Tucson from six to eight lanes to cope with the increased traffic demand of a growing Tucson. ADOT is performing its work sequentially in segments. ADOT retained HDR, Inc. to perform the design for segment of I-10 between St. Marys Road and 29^th Street in Downtown Tucson. NCSG performed the geotechnical investigations under subcontract to HDR.

The complete reconstruction of mainline I-10 for the segment between St. Mary’s Road and 29th Street included 6 new bridges, 17 retaining walls totaling nearly 14,000 linear feet of walls, drainage structures paralleling the corridor, and pavement design. The field investigation program conducted by NCSG included 122 borings for the various structures. A combination of drill rigs was required, including conventional truck mounted, track mounted for drilling on slopes, and difficult access rigs for the sides of steep embankment slopes.

The proximity of the project corridor to the Santa Cruz River had a strong influence on the soil stratigraphy, and corresponding weaker fluvial (river deposited) soils located near the surface creating significant design challenges. Additionally, perched groundwater was encountered in some of the deep bridge borings, which required careful consideration during the design of the deep foundations for the bridge structures. NCSG prepared a comprehensive design recommendations report for the project.

NCSG was heavily involved during the construction phase assisting HDR and ADOT in the review of MSE-wall shop drawings and calculations, review of drilled shaft integrity testing data, recommendations for fixes of defective drilled shafts, and troubleshooting issues related to retaining walls, drilled shaft foundations and other geotechnical aspects of the project.

Photo Credits: Some photos during the construction phase are courtesy of ADOT, in particular Antonio Conte. Several others are courtesy of Ted Buell with HDR, Inc.