I10 Fort Grant TI: Deep Soil Mixing

For an Arizona Department of Transportation (ADOT) project, NCSG was subcontracted by Speedie and Associates, Inc. (SAI), to provide recommendations for the design of ground improvement under the center piers of a new bridge at the Fort Grant Road Traffic Interchange (TI) in Wilcox, AZ.

The Wilcox area has a complex geology, including Playa landforms with salt flats at the surface, and soft clay layers at depth. Some of these clay layers are under-consolidated (i.e., overconsolidation ratio, OCR < 1) with significant concern for large time-dependent settlements. Drilled shaft and spread footings were found to have unacceptable settlements at the pier location of the new bridge, and NCSG was tasked with providing ground improvement recommendations.

After evaluating several combinations of ground improvement and foundation alternatives, the combination of deep soil mixing (DSM) method and spread footing was selected. The DSM configuration and stiffness was established to help meet the stringent settlement criteria set by the ADOT bridge (structural) engineer. This configuration results in stiffened block of soils that floats on the under-consolidated clays (i.e., similar to a boat floating on water). The bridge pier was supported by a spread footing on this block. NCSG developed the design and performance specifications for DSM as well as recommendations for spread footing bearing resistance. Hayward Baker Inc. (HBI) was selected to perform the DSM work. NCSG provided construction training and oversight for ADOT including reviewing field test results and modifications during field work.

This project has been constructed and is in service.