Unexpected Shoring Wall Constraints in Savannah, Georgia

A new multi-story student housing structure required parking below existing grade. The general contractor requested temporary sheet piling for the majority of the project, with a small portion to be permanent sheet piling. Typical vibration restrictions are 1.0 in./s (25.4 mm/s); however, in the historic district the limits were reduced to 0.25 in./s (6.35 mm/s). During the installation of the sheet piling, vibrations were monitored and found to be 0.08 in./s (2.03 mm/s), well below the strict criteria. However, an existing historic brick-and-mortar structure built between 1873 and 1902 was located directly across the street, and there was noticeable impact to the mortar from the vibrations. The project was placed on hold, and a new solution was required to salvage the $90M development. Time was critical; the facility had to be completed before fall semester began. Spacing was critical; the temporary sheet piling was to be placed in the right-of-way with a cast in place wall poured in front. Many alternative solutions were considered, and we concluded a permanent secant wall constructed with drilled shafts and augercast piles was the best solution. The cast in place concrete building wall was eliminated and replaced with the secant wall. Larger diameter and deeper elements were constructed at each column location with smaller diameter secant piles in between. While the secant wall was considerably more expensive, vibrations were eliminated, spacing limitations were resolved, and the entire system was redesigned and constructed to meet the overall schedule.