Deep and Shallow Soil Mixing for the SR-83/US-331 Choctawhatchee Bay Bridge Project in Santa Rosa Beach, Florida-Paper

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The SR-83/US-331 highway in Florida serves as a major commercial route and the primary hurricane evacuation corridor for southern Walton County. To address increasing traffic and long-standing settlement caused by loose, variable foundation soils, FDOT expanded the highway from two lanes to four and selected soil mixing for ground improvement.<br /><br />The design combined a 10-foot-thick platform of Shallow Mixing Method (SMM) elements with Deep Mixing Method (DMM) columns extending to deeper bearing layers. In total, 9,845 columns were installed across the South, Center, and North Causeways. Bench-scale testing and field trials established suitable grout mixes and installation parameters, while automated monitoring and an extensive quality-control program tracked construction.<br /><br />Construction faced congested work areas, tighter-than-expected sands, and substantial changes in soil conditions over short distances. These challenges reduced productivity, increased equipment wear and cement use, and sometimes made DMM tool removal exceptionally difficult. A second field trial supported use of a higher water-to-cement ratio at the Center Causeway, improving constructability while meeting strength requirements.<br /><br />Despite delays and difficult ground conditions, installation was completed in September 2016 without remedial work. All cored DMM elements met the specified minimum strength. Three SMM wet-grab samples initially failed strength criteria, but verification coring confirmed adequate strength. Load testing showed that the system satisfied the project’s requirement of less than two inches of differential settlement over any 1,000-foot stretch.<br /><br />The project demonstrates that soil mixing can create a consistent, large-scale soil-cement foundation under highly variable conditions. The authors emphasize the value of thorough site investigation, rigorous quality control, and proactive collaboration among project teams.

Keywords

SR-83

US-331

Florida highway expansion

soil mixing

Shallow Mixing Method

Deep Mixing Method

ground improvement

hurricane evacuation corridor

differential settlement

quality control