A mixed-use project off Abercorn Street hit groundwater at six feet, turning what looked like a straightforward three-level excavation into a complex shoring problem. The developer had assumed stiff clay based on a nearby site near Forsyth Park, but here we found loose silty sands and organic lenses typical of the Pleistocene terrace deposits. We redesigned the retaining wall system with cantilever walls combined with an underdrain network, because without active drainage the hydrostatic pressure would have overwhelmed any conventional gravity structure. This scenario repeats across Savannah, where the subtle variations in the coastal plain stratigraphy demand a geotechnical approach that goes beyond textbook solutions. For deeper cuts near the river, we often pair the wall design with CPT testing to profile the soft zones continuously, and in areas where liquefiable layers appear we integrate liquefaction analysis directly into the wall's seismic performance assessment.
In Savannah's coastal plain, a retaining wall without a properly designed subdrainage system is a temporary structure, regardless of the reinforcement.
Method and coverage
Regional considerations
The most common mistake we see in Savannah is the assumption that a 3,000 psi gravity wall with a standard twelve-inch gravel backfill will perform adequately without a continuous subdrain. We investigated a failure on Wilmington Island where a segmental block wall rotated outward after just eighteen months of service. The contractor had omitted the drain pipe and had backfilled directly with on-site silty sand, which lost its shear strength during the summer thunderstorm cycle. The repair required complete excavation behind the wall, installation of a chimney drain, and reconstruction of the reinforced zone, costing the owner nearly three times the original wall budget. Another persistent issue involves ignoring the long-term settlement of the foundation soil beneath the wall heel: we have measured differential settlements exceeding two inches in walls founded on the compressible Hawthorne clays, which cracked the stem and exposed the reinforcement to the brackish groundwater. A pre-construction settlement analysis using consolidation data from incremental load tests, combined with a properly sized key, prevents these expensive post-construction interventions in Savannah's variable subsurface.
Process video
Standards that apply
IBC 2021, Section 1807 – Retaining Walls, ASCE 7-22, Chapter 3 – Dead, Live, and Earth Pressure Loads, AASHTO LRFD Bridge Design Specifications, 9th Edition (2020), Section 11 – Abutments, Piers, and Walls, ASTM D2487-17e1 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), FHWA-NHI-10-024 – Earth Retaining Structures (Reference Manual)
Complementary services
Cantilever and Gravity Wall Design
Complete structural and geotechnical design of reinforced concrete cantilever walls and MSE gravity walls for commercial and residential developments in Chatham County. We perform global stability analysis using Spencer's method, bearing capacity checks on the underlying Cooper Marl or Hawthorne Group strata, and sliding and overturning verification per AASHTO LRFD. Each submittal package includes construction drawings with drainage details, backfill specifications, and a staged inspection schedule.
Temporary Excavation Support and Shoring
Design of soldier pile and lagging systems, sheet pile walls, and soil nail walls for deep excavations near existing structures in Savannah's Historic District. We model the excavation sequence in PLAXIS 2D to predict lateral deflections and ground surface settlements, and we prescribe pre-loading of tiebacks where adjacent buildings are sensitive to movement. The shoring design accounts for the high water table through dewatering and cutoff considerations.
Typical parameters
Q&A
How deep a retaining wall can be built in Savannah's coastal soils without tiebacks?
In our experience across Chatham County, a cantilever reinforced concrete wall can reach twelve to fourteen feet of exposed height when founded on the Cooper Marl formation, provided the embedment is adequate and a continuous subdrain is installed. Beyond that height, the base width and reinforcement quantities become uneconomical, and we typically transition to a mechanically stabilized earth (MSE) wall with geogrid reinforcement or to a soldier pile system with tiebacks. The final decision depends on the proximity of property lines and the results of the subsurface exploration.
What is the typical cost range for a retaining wall design in Savannah?
For a standard cantilever retaining wall in the Savannah area, the design fee ranges from US$1,020 to US$4,740, depending on the wall height, the complexity of the site geometry, and the number of required design iterations. This includes the geotechnical report, structural calculations, and signed and sealed construction drawings. The construction cost of the wall itself is separate and varies with the chosen system and access conditions.
How do you handle the high groundwater table in Savannah for retaining wall design?
We assume the hydrostatic pressure acts full-height behind the wall unless a properly designed drainage system is specified and maintained. Our standard detail includes a twelve-inch thick granular chimney drain wrapped in a non-woven geotextile, connected to a six-inch perforated HDPE collector pipe at the base that outlets to daylight or to a sump pump. We also specify the backfill to be free-draining No. 57 stone for the entire reinforced zone, and we verify the outfall capacity for a 25-year storm event to prevent water buildup during Savannah's intense summer rainfall.
Do retaining walls in Savannah need to be designed for seismic loads?
Yes. Chatham County is assigned a spectral acceleration Ss of 0.35g for Risk Category II structures per the IBC seismic hazard maps, and the site class—often D or E in our local soils—can amplify the ground motion. We incorporate the seismic earth pressure increment using the Mononobe-Okabe method as referenced in AASHTO LRFD, and we check the wall for sliding and overturning under the combined static plus seismic load combination. For walls exceeding twenty feet in height, we also perform a Newmark sliding block analysis to estimate the permanent displacement during the design earthquake.