Providing Structural Support and Reducing Long-Term Settlement in the Soft Silts and Clays above the Cooper Marl. Ashley Phosphate Road and Route 52 Flyover, Charleston, SC

摘要

The project sites for the two bridge constructed at Ashley Phosphate Road and US52 Flyover were underlain by variable upper sediments ranging from silty sand soils to clayey sand soils and silty clay material. The variable subsoil conditions in the North Charleston area have presented an admirable challenge for the engineering team to design a ground improvement program to meet the performance requirement of Transportation Department of South Carolina. A total of three ground improvement techniques were employed to provide additional bearing support and to minimize both static and post seismic induced settlement and liquefaction potential of the underlying subsoils. Vibro stone columns and vibro concrete columns were designed to improve the soils and compaction grouting was used to supplement the vibro stone column work and in some cases replacing the vibro concrete columns and stone columns. Differential settlement between grade supported structures (pavement) and deep foundation supported structures (bridges) is a problem around the Charleston, SC area. Resulting settlement forces continual and costly repaving of many approach ramps. The Ashley Phosphate/I-26 Interchange project was designed for total static post-construction settlement to be 1 inch or less in areas improved by vibro-replacement and 2 inches or less in areas improved with wick drains. This would be beneath embankments reaching 35 feet in height. Construction would follow a period of 3-6 months after placing embankment fill. During this time monitoring of the embankment fill was conducted to verify the initial settlement due to the surcharge leveling off. Less than expected initial settlement was recorded. Less than 1 inch of post-construction settlement has occurred after bridges and approaches have been up one year at portions of the work. Compaction grouting was a viable alternative to vibro-replacement in areas where subsurface obstructions were encountered. This caused for a modified design, but seemed to ensure that schedule was maintained with only a small effect on cost. When difficulties arise on the project, especially with respect to constructability issues, it is important that the owner, engineer, construction manager, general contractor and specialty subcontractor work quickly to mitigate the effect to construction cost and schedule impact delays while maintaining the intent of the ground improvement design. Those involved at this project partnered to quickly identify the problems, present solutions and provide timely turnaround to protect the project's integrity and maintain schedule.